difference between double dispatch and the method explains here

north = Direction( 0, -1)
east  = Direction( 1,  0)
south = Direction( 0,  1)
west  = Direction(-1,  0)



oke, then time to figure out how to change my functions because I have the feeling that I still miss
some of the pieces: 

Robot >> move: aInstruction
	aInstruction = $R
		ifTrue: [ ^ self direction: (PositioningSystem new turnRight: direction) ].
	aInstruction = $L
		ifTrue: [ ^ self direction: (PositioningSystem new turnLeft: direction) ].
	^ self
		position:
			(PositioningSystem new moveForWard: position direction: self direction)

On 06/04/19 4:49 PM, Roelof Wobben wrote:

Hello,

I just learned double dispatch.
And now for the Robot challenge of exercism Tim has pointed me to this article(https://blog.metaobject.com/2019/04/accessors-have-message-obsession.html)
but I fail to see how the move method looks like in that article.
I had a conversation with Tim in the exercism channel and the way he explains it, it looks like double dispatch for me.

Am I on the right track or do I oversee something here.

unary methods like moveRight perform specific ops and are not parametric, so only a single dispatch, depending on the receiver, is needed.

If you change it to move: aDistanceOrAngle, then performing requests like "move: 3 cms" or "move: 30 degrees" will depend not only on the receiver but also on the class of the argument. This would need double dispatch (aka multiple polymorphism). The first dispatch would be based on the receiver and the receiver's method would then dispatch it based on the class of the argument (i.e. Distance>>move or Angle>>move )

HTH .. Subbu

It would really REALLY **REALLY** help if we knew what

the heck you were trying to do.  There is an excellent

chance that it is MUCH simpler than you think.  If you

cannot show us the Smalltalk version of the problem,

can you show us the version for some other language?

On Sun, 7 Apr 2019 at 20:15, Roelof Wobben <[hidden email]> wrote:

Op 6-4-2019 om 15:15 schreef K K Subbu:
> On 06/04/19 4:49 PM, Roelof Wobben wrote:
>> Hello,
>>
>> I just learned double dispatch.
>> And now for the Robot challenge of exercism Tim has pointed me to
>> this
>> article(https://blog.metaobject.com/2019/04/accessors-have-message-obsession.html)
>>
>> but I fail to see how the move method looks like in that article.
>> I had a conversation with Tim in the exercism channel and the way he
>> explains it, it looks like double dispatch for me.
>>
>> Am I on the right track or do I oversee something here.
> unary methods like moveRight perform specific ops and are not
> parametric, so only a single dispatch, depending on the receiver, is
> needed.
>
> If you change it to move: aDistanceOrAngle, then performing requests
> like "move: 3 cms" or "move: 30 degrees" will depend not only on the
> receiver but also on the class of the argument. This would need double
> dispatch (aka multiple polymorphism). The first dispatch would be
> based on the receiver and the receiver's method would then dispatch it
> based on the class of the argument (i.e. Distance>>move or Angle>>move )
>
> HTH .. Subbu
>
>


hmm, still stuck

I have now a class Direction with as instance variables north, south,
east, west
and made the accessors.

then I thought I need a initialize like this :

initialize
    north = Direction( 0, -1).
    east  = Direction( 1,  0).
    south = Direction( 0,  1).
    west  = Direction(-1,  0).

but the Direction (0,-1)  is a problem . the compiler does not like the
(0,-1) part

to give you the big picture. I have a Robot which can turnRight ,
turnLeft and moveForward and I try to understand how the page would work
in my case.

So I have a object Direction as described above and a Object MoveForward
which is a subobject of Direction.
MoveForward has only 1 method :

IsMove
    ^  'A'

Roelof

One thing I have often seen and lamented is students

writing excessively complicated code with way too many

classes.  There is a huge difference between

  "A Robot knows its position and direction."

and

  "A Robot has-a Position and has-a Direction."

The first is the important one.  The second is

an over-commitment to too many classses.  For a

problem like this, you really really do not want

a Direction class, and you certainly have no use

for double dispatch.

A position can be represented by a pair of integers

x, y.  It could also be represented by a Point with

integer components.

A direction could be represented by a pair of integers

dx, dy such that |dx|+|dy| = 1.  It could also be

represented by a Point with integer components.

For movement, you need to be able to add the direction

to the location, which could be simply

x := x + dx.  y := y + dy.

or it could be

position := position + direction.

For turning, you need to be able to rotate a direction

vector by ninety degrees.  Now it so happens that

Point has methods #leftRotated and #rightRotated.

So we can do the following:

   a Robot has position (a Point) and direction (aPoint)

   position := 0 @ 0.

   direction := 0 @ 1.

To move forward without turning:

   position := position + direction.

To turn left without moving:

   direction := direction leftRotated.

To turn right without moving:

   direction := direction rightRotated.

To obey a sequence of characters, commands:

   commands do: [:each |

      each caseOf: {

         [$A] -> [--move forward--].

         [$L] -> [--turn left--].

         [$R] -> [--turn right--]

      }].

One of the key ideas in extreme programming is

"You Ain't Gonna Need It", abbreviated to YAGNI!

The idea is *DON'T* generalise beyond your immediate

needs.  In this case, for example, the likelihood of

*this* program needing to deal with more general

kinds of movement is ZERO.  And the only reason for

using Point here instead of just using a few simple

assignment statements is that Point already exists,

so costs nothing to write, and as a familiar class,

code using it should be easy to read.

If someone challenges you to do something counter-productive,

refuse the challenge.

On Mon, 8 Apr 2019 at 17:21, Roelof Wobben <[hidden email]> wrote:

I can try to explain what I trying to solve.

I have a Robot which can turn left,  turn right or moveForward.

now I have a string like 'LAR'

that means the robot needs to turn left (l) , move forward one place (A) and turn left.
and I have to keep track to which direction the robot is facing and on which coordinate it stands.

so to summarize with the above string

lets say the robot is facing north on coordinate (0,0)
then it has to turn left , so its facing east and still on coordinate (0,0)
then it has to move forward, so its still  facing east but are on coordinate(0,1)
then it has to turn right, so its facing north and on coordinate (0,1)

and TimMacKinnon has challenged me to do this with double dispatch.

So I think now I need a object Direction, a sub object North and a sub - sub object TurnLeft, turnRight and moveForward.

So I can use double dispath first the direction North, East, South, West
and then use double dispatch to find the right move.

Roelof





Op 8-4-2019 om 06:50 schreef Richard O'Keefe:

It would really REALLY **REALLY** help if we knew what

the heck you were trying to do.  There is an excellent

chance that it is MUCH simpler than you think.  If you

cannot show us the Smalltalk version of the problem,

can you show us the version for some other language?

On Sun, 7 Apr 2019 at 20:15, Roelof Wobben <[hidden email]> wrote:

Op 6-4-2019 om 15:15 schreef K K Subbu:
> On 06/04/19 4:49 PM, Roelof Wobben wrote:
>> Hello,
>>
>> I just learned double dispatch.
>> And now for the Robot challenge of exercism Tim has pointed me to
>> this
>> article(https://blog.metaobject.com/2019/04/accessors-have-message-obsession.html)
>>
>> but I fail to see how the move method looks like in that article.
>> I had a conversation with Tim in the exercism channel and the way he
>> explains it, it looks like double dispatch for me.
>>
>> Am I on the right track or do I oversee something here.
> unary methods like moveRight perform specific ops and are not
> parametric, so only a single dispatch, depending on the receiver, is
> needed.
>
> If you change it to move: aDistanceOrAngle, then performing requests
> like "move: 3 cms" or "move: 30 degrees" will depend not only on the
> receiver but also on the class of the argument. This would need double
> dispatch (aka multiple polymorphism). The first dispatch would be
> based on the receiver and the receiver's method would then dispatch it
> based on the class of the argument (i.e. Distance>>move or Angle>>move )
>
> HTH .. Subbu
>
>


hmm, still stuck

I have now a class Direction with as instance variables north, south,
east, west
and made the accessors.

then I thought I need a initialize like this :

initialize
    north = Direction( 0, -1).
    east  = Direction( 1,  0).
    south = Direction( 0,  1).
    west  = Direction(-1,  0).

but the Direction (0,-1)  is a problem . the compiler does not like the
(0,-1) part

to give you the big picture. I have a Robot which can turnRight ,
turnLeft and moveForward and I try to understand how the page would work
in my case.

So I have a object Direction as described above and a Object MoveForward
which is a subobject of Direction.
MoveForward has only 1 method :

IsMove
    ^  'A'

Roelof

One thing I have often seen and lamented is students

writing excessively complicated code with way too many

classes.  There is a huge difference between

  "A Robot knows its position and direction."

and

  "A Robot has-a Position and has-a Direction."

The first is the important one.  The second is

an over-commitment to too many classses.  For a

problem like this, you really really do not want

a Direction class, and you certainly have no use

for double dispatch.

A position can be represented by a pair of integers

x, y.  It could also be represented by a Point with

integer components.

A direction could be represented by a pair of integers

dx, dy such that |dx|+|dy| = 1.  It could also be

represented by a Point with integer components.

For movement, you need to be able to add the direction

to the location, which could be simply

x := x + dx.  y := y + dy.

or it could be

position := position + direction.

For turning, you need to be able to rotate a direction

vector by ninety degrees.  Now it so happens that

Point has methods #leftRotated and #rightRotated.

So we can do the following:

   a Robot has position (a Point) and direction (aPoint)

   position := 0 @ 0.

   direction := 0 @ 1.

To move forward without turning:

   position := position + direction.

To turn left without moving:

   direction := direction leftRotated.

To turn right without moving:

   direction := direction rightRotated.

To obey a sequence of characters, commands:

   commands do: [:each |

      each caseOf: {

         [$A] -> [--move forward--].

         [$L] -> [--turn left--].

         [$R] -> [--turn right--]

      }].

One of the key ideas in extreme programming is

"You Ain't Gonna Need It", abbreviated to YAGNI!

The idea is *DON'T* generalise beyond your immediate

needs.  In this case, for example, the likelihood of

*this* program needing to deal with more general

kinds of movement is ZERO.  And the only reason for

using Point here instead of just using a few simple

assignment statements is that Point already exists,

so costs nothing to write, and as a familiar class,

code using it should be easy to read.

If someone challenges you to do something counter-productive,

refuse the challenge.

On Mon, 8 Apr 2019 at 17:21, Roelof Wobben <[hidden email]> wrote:

I can try to explain what I trying to solve.

I have a Robot which can turn left,  turn right or moveForward.

now I have a string like 'LAR'

that means the robot needs to turn left (l) , move forward one place (A) and turn left.
and I have to keep track to which direction the robot is facing and on which coordinate it stands.

so to summarize with the above string

lets say the robot is facing north on coordinate (0,0)
then it has to turn left , so its facing east and still on coordinate (0,0)
then it has to move forward, so its still  facing east but are on coordinate(0,1)
then it has to turn right, so its facing north and on coordinate (0,1)

and TimMacKinnon has challenged me to do this with double dispatch.

So I think now I need a object Direction, a sub object North and a sub - sub object TurnLeft, turnRight and moveForward.

So I can use double dispath first the direction North, East, South, West
and then use double dispatch to find the right move.

Roelof





Op 8-4-2019 om 06:50 schreef Richard O'Keefe:

It would really REALLY **REALLY** help if we knew what

the heck you were trying to do.  There is an excellent

chance that it is MUCH simpler than you think.  If you

cannot show us the Smalltalk version of the problem,

can you show us the version for some other language?

On Sun, 7 Apr 2019 at 20:15, Roelof Wobben <[hidden email]> wrote:

Op 6-4-2019 om 15:15 schreef K K Subbu:
> On 06/04/19 4:49 PM, Roelof Wobben wrote:
>> Hello,
>>
>> I just learned double dispatch.
>> And now for the Robot challenge of exercism Tim has pointed me to
>> this
>> article(https://blog.metaobject.com/2019/04/accessors-have-message-obsession.html)
>>
>> but I fail to see how the move method looks like in that article.
>> I had a conversation with Tim in the exercism channel and the way he
>> explains it, it looks like double dispatch for me.
>>
>> Am I on the right track or do I oversee something here.
> unary methods like moveRight perform specific ops and are not
> parametric, so only a single dispatch, depending on the receiver, is
> needed.
>
> If you change it to move: aDistanceOrAngle, then performing requests
> like "move: 3 cms" or "move: 30 degrees" will depend not only on the
> receiver but also on the class of the argument. This would need double
> dispatch (aka multiple polymorphism). The first dispatch would be
> based on the receiver and the receiver's method would then dispatch it
> based on the class of the argument (i.e. Distance>>move or Angle>>move )
>
> HTH .. Subbu
>
>


hmm, still stuck

I have now a class Direction with as instance variables north, south,
east, west
and made the accessors.

then I thought I need a initialize like this :

initialize
    north = Direction( 0, -1).
    east  = Direction( 1,  0).
    south = Direction( 0,  1).
    west  = Direction(-1,  0).

but the Direction (0,-1)  is a problem . the compiler does not like the
(0,-1) part

to give you the big picture. I have a Robot which can turnRight ,
turnLeft and moveForward and I try to understand how the page would work
in my case.

So I have a object Direction as described above and a Object MoveForward
which is a subobject of Direction.
MoveForward has only 1 method :

IsMove
    ^  'A'

Roelof

The basic issue here is abstraction.

An instance of "Robot" in your program is not a

physical object.  How could it possibly point North,

South, or Nor-nor-west?  It cannot.

Its location and direction are abstract values

*metaphorically* related to real world notions

like position vectors and velocity vectors.

"North" in this program is not a real thing,

it is an *idea* which could be represented by

'North', 'north', #North, #north, $N, $n,

'Raki',  'raki',  #Raki,  #raki,  $R, $r,

137, (0@ -1), a picture of the star Polaris,

the colour red (the conventional colour for

that end of a compass needle which points north),

a sound recording of a lecture by Alfred North

Whitehead, or anything you please, as long as,

inside the program, it *acts* the way *you* want

"north" to act (which is not necessarily the way

the physical direction North acts, and in fact in

this case it most certainly is not).

Locations and movements in a 2D space are, in Smalltalk,

commonly represented by Points.  "Represented by."

As for this method:

test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth
    | result |
    result := robotSimulatorCalculator
        moveDirection: 'north'
        position:
            (Dictionary new
                add: 'x' -> 0;
                add: 'y' -> 0;
                yourself)
        instructions: 'A'.
    self
        assert: result
        equals:
            (Dictionary new
                add: 'direction' -> 'north';
                add:
                    'position'
                        ->
                            (Dictionary new
                                add: 'x' -> 0;
                                add: 'y' -> 1;
                                yourself);
                yourself)

PLEASE tell me that is not what they are actually using.

Let's start with

  (Dictionary new)

     add: k1 -> v1;

     ...

     add: kn -> vn;

     yourself

Did you know that sending #add: to a dictionary is not

portable?  Storing actual Association objects inside

Dictionaries was originally an encapsulation error and

remains a performance error, so there are Smalltalks

that do not make that mistake.  The *portable* way to

make a Dictionary is

    (Dictionary new)

       at: k1 put: v1;

       ...

       at: kn put: vn;

       yourself.

And why in the name of sanity are the keys *strings*

instead of *symbols*?  This is not Smalltalk.  It is

Javascript in drag.

Now exercism.io has a habit of insisting on particular

implementations.  For example, I completed the SML track,

and found that the test code ONLY worked with Poly and

not with any of the three SML implementations I already

had on my machine.  Since you are doing this in Pharo,

I take it that exercism.io will insist on the Smalltalk

track being done in Pharo, and in that case it is

*nauseating* to use a Dictionary when you could use a

Point.  Old-fashioned Smalltalk style would have been

to return something like

   #(<direction> <x> <y>)

e.g. #(north 1 0), and I still prefer that.

In fact *good* Smalltalk style for something like this

would be

test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth

  robotSimulatorCalculator

    moveTo: 0@0;

    head: #north;

    obey: 'A'.

  self assert: robotSimulatorCalculator heading equals: #north.

  self assert: robotSimulatorCalculator location equals: 0@1.

-- We're starting to get the idea that identifiers like

robotSimulatorCalculator are not a very good idea when

simulatedRobot would do the job as well or better.

(This is also pointing us towards Betrand Meyer's

Command/Query Separation principle, but we shan't

go there today.)

This is important feedback to give to the exercism.io

people.  The test code should use a SMALLTALK interface,

not a warmed-over JAVASCRIPT interface.

Now, how do we map between direction *names* and

direction *points*?  Well, we have to start by

laying down clearly what we *mean* by the directions.

To move North one step is to add 1 to y and 0 to x.

(We know that from the appalling test case above.)

To move South one step is to add -1 to y and 0 to x.

(South is the opposite of North.)

To move East one step, oh we have a problem.

THIS NEEDS SPELLING OUT.  And one of the things the

exercism.io exercises are HORRIBLY BAD AT is specifying

the problem.  Nearly every single exercise I have tried,

I have been unable to tell what the problem is without

examining the test cases, and that is not the way

exercises are supposed to work.  (Yeah, that's why I'm

screaming about it.  I've taught a class using exercises

like this that were not of my writing and vague specifications

really upset the students.  People who had taken the class

under someone else several years before were still angry

about it.)

The geometric classes in Smalltalk were written to support

graphic user interfaces.  And in user interfaces, the y

coordinate increases DOWN.  So if we take the compass rose

and rotate it so that North is DOWN, it follows that

West is right and East is left.  So

To move East one step is to add -1 to x and 0 to y.

To move West one step is to add 1 to x and 0 to y.

The chances are excellent that the problem specification

is inconsistent with this.  Sigh.  Let's proceed, though.

North  0@1

South  0@ -1

East   -1@0

West   1@0

pointToName: aPoint

  ^aPoint x isZero

     ifTrue:  [aPoint y > 0 ifTrue: [#north] ifFalse: [#south]]

     ifFalse: [aPoint x > 0 ifTrue: [#west ] ifFalse: [#east ]]

nameToPoint: aSymbol

  aSymbol = #north ifTrue: [^0 @ 1].

  aSymbol = #south ifTrue: [^0 @ -1].

  aSymbol = #west  ifTrue: [^1 @ 0].

  aSymbol = #east  ifTrue: [^-1 @ 0].

  aSymbol error: 'not a compass direction in lower case'.

Another problem I had with exercism was a "Space-Age"

exercise where the README.md capitalised the planet names

but test_Space-Age.<whatever> insisted on lower case.

That might well happen here.

Just for grins,

Dictionary>>

asPoint

  ^(self at: 'x') @ (self at: 'y')

Point>>

asDictionary

  ^(Dictionary new)

     at: 'x' put: self x;

     at: 'y' put: self y;

     yourself

On Mon, 8 Apr 2019 at 22:15, Roelof Wobben <[hidden email]> wrote:

Richard thanks.

One thing I do not see direct.

you said :

A direction could be represented by a pair of integers

dx, dy such that |dx|+|dy| = 1.  It could also be

represented by a Point with integer components.

for me a direction is the direction the robot is facing so something like north or east.

the challenge also wants a output like this :

test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth
    | result |
    result := robotSimulatorCalculator
        moveDirection: 'north'
        position:
            (Dictionary new
                add: 'x' -> 0;
                add: 'y' -> 0;
                yourself)
        instructions: 'A'.
    self
        assert: result
        equals:
            (Dictionary new
                add: 'direction' -> 'north';
                add:
                    'position'
                        ->
                            (Dictionary new
                                add: 'x' -> 0;
                                add: 'y' -> 1;
                                yourself);
                yourself)

so how do I "convert" the point you are using to the text.

Or do I misunderstood you somewhere wrong.

Roelof

Op 8-4-2019 om 10:57 schreef Richard O'Keefe:

One thing I have often seen and lamented is students

writing excessively complicated code with way too many

classes.  There is a huge difference between

  "A Robot knows its position and direction."

and

  "A Robot has-a Position and has-a Direction."

The first is the important one.  The second is

an over-commitment to too many classses.  For a

problem like this, you really really do not want

a Direction class, and you certainly have no use

for double dispatch.

A position can be represented by a pair of integers

x, y.  It could also be represented by a Point with

integer components.

A direction could be represented by a pair of integers

dx, dy such that |dx|+|dy| = 1.  It could also be

represented by a Point with integer components.

For movement, you need to be able to add the direction

to the location, which could be simply

x := x + dx.  y := y + dy.

or it could be

position := position + direction.

For turning, you need to be able to rotate a direction

vector by ninety degrees.  Now it so happens that

Point has methods #leftRotated and #rightRotated.

So we can do the following:

   a Robot has position (a Point) and direction (aPoint)

   position := 0 @ 0.

   direction := 0 @ 1.

To move forward without turning:

   position := position + direction.

To turn left without moving:

   direction := direction leftRotated.

To turn right without moving:

   direction := direction rightRotated.

To obey a sequence of characters, commands:

   commands do: [:each |

      each caseOf: {

         [$A] -> [--move forward--].

         [$L] -> [--turn left--].

         [$R] -> [--turn right--]

      }].

One of the key ideas in extreme programming is

"You Ain't Gonna Need It", abbreviated to YAGNI!

The idea is *DON'T* generalise beyond your immediate

needs.  In this case, for example, the likelihood of

*this* program needing to deal with more general

kinds of movement is ZERO.  And the only reason for

using Point here instead of just using a few simple

assignment statements is that Point already exists,

so costs nothing to write, and as a familiar class,

code using it should be easy to read.

If someone challenges you to do something counter-productive,

refuse the challenge.

On Mon, 8 Apr 2019 at 17:21, Roelof Wobben <[hidden email]> wrote:

I can try to explain what I trying to solve.

I have a Robot which can turn left,  turn right or moveForward.

now I have a string like 'LAR'

that means the robot needs to turn left (l) , move forward one place (A) and turn left.
and I have to keep track to which direction the robot is facing and on which coordinate it stands.

so to summarize with the above string

lets say the robot is facing north on coordinate (0,0)
then it has to turn left , so its facing east and still on coordinate (0,0)
then it has to move forward, so its still  facing east but are on coordinate(0,1)
then it has to turn right, so its facing north and on coordinate (0,1)

and TimMacKinnon has challenged me to do this with double dispatch.

So I think now I need a object Direction, a sub object North and a sub - sub object TurnLeft, turnRight and moveForward.

So I can use double dispath first the direction North, East, South, West
and then use double dispatch to find the right move.

Roelof





Op 8-4-2019 om 06:50 schreef Richard O'Keefe:

It would really REALLY **REALLY** help if we knew what

the heck you were trying to do.  There is an excellent

chance that it is MUCH simpler than you think.  If you

cannot show us the Smalltalk version of the problem,

can you show us the version for some other language?

On Sun, 7 Apr 2019 at 20:15, Roelof Wobben <[hidden email]> wrote:

Op 6-4-2019 om 15:15 schreef K K Subbu:
> On 06/04/19 4:49 PM, Roelof Wobben wrote:
>> Hello,
>>
>> I just learned double dispatch.
>> And now for the Robot challenge of exercism Tim has pointed me to
>> this
>> article(https://blog.metaobject.com/2019/04/accessors-have-message-obsession.html)
>>
>> but I fail to see how the move method looks like in that article.
>> I had a conversation with Tim in the exercism channel and the way he
>> explains it, it looks like double dispatch for me.
>>
>> Am I on the right track or do I oversee something here.
> unary methods like moveRight perform specific ops and are not
> parametric, so only a single dispatch, depending on the receiver, is
> needed.
>
> If you change it to move: aDistanceOrAngle, then performing requests
> like "move: 3 cms" or "move: 30 degrees" will depend not only on the
> receiver but also on the class of the argument. This would need double
> dispatch (aka multiple polymorphism). The first dispatch would be
> based on the receiver and the receiver's method would then dispatch it
> based on the class of the argument (i.e. Distance>>move or Angle>>move )
>
> HTH .. Subbu
>
>


hmm, still stuck

I have now a class Direction with as instance variables north, south,
east, west
and made the accessors.

then I thought I need a initialize like this :

initialize
    north = Direction( 0, -1).
    east  = Direction( 1,  0).
    south = Direction( 0,  1).
    west  = Direction(-1,  0).

but the Direction (0,-1)  is a problem . the compiler does not like the
(0,-1) part

to give you the big picture. I have a Robot which can turnRight ,
turnLeft and moveForward and I try to understand how the page would work
in my case.

So I have a object Direction as described above and a Object MoveForward
which is a subobject of Direction.
MoveForward has only 1 method :

IsMove
    ^  'A'

Roelof

You are expected to use my code fragments for *ideas*,

not to incorporate them *literally* in your code.  As

I explained, *without seeing the specification*, I have

no way to tell whether the specification uses a left-handed

or right-handed coordinate system.

For what it's worth, here's a complete program in my

Smalltalk dialect.  It doesn't plug into the exercism

testing framework because I can do not know what it

looks like.  But if it makes the code more complicated

that this, it's doing it wrong.

require: 'geometry.st'  "Point"
require: 'print.st'     "OutputStream>>print:"
   
Object subclass: #Robot
  instanceVariableNames: 'position direction'
  poolDirectionaries:    'FileStream'

  methods for: 'initialising'
    pvtPostNew
      position  := 0@0.
      direction := 1@0.

  methods for: 'accessing'
    direction
      ^direction copy

    location
      ^location copy

    obey: commands
      commands do: [:each |
        each caseOf: {
          [$A] -> [position  := position  + direction].
          [$L] -> [direction := direction leftRotated].
          [$R] -> [direction := direction rightRotated]
        }].

  class methods for: 'main'
    start
      [StdIn atEnd] whileFalse: [
         |robot|
         robot := Robot new.
         Robot obey: StdIn nextLine.
         StdOut print: Robot location; cr].

On Tue, 9 Apr 2019 at 02:58, Roelof Wobben <[hidden email]> wrote:

yes,  this is a real  tests from the pharo track on exercism.io

I understand what you mean but maybe I overthinking things.
But if we have a robot facing north and the robot turns to the left  , im my oponion it faces now to the east.

like this test is saying :

test04_RotatesTheRobotsDirection90DegreesClockwiseChangesTheDirectionFromEastToSouth
    | result |
    result := robotSimulatorCalculator
        moveDirection: 'east'
        position:
            (Dictionary new
                add: 'x' -> 0;
                add: 'y' -> 0;
                yourself)
        instructions: 'R'.
    self
        assert: result
        equals:
            (Dictionary new
                add: 'direction' -> 'south';
                add:
                    'position'
                        ->
                            (Dictionary new
                                add: 'x' -> 0;
                                add: 'y' -> 0;
                                yourself);
                yourself)


but I cannot come to the same outcome with this code :

pointToName: aPoint

  ^aPoint x isZero

     ifTrue:  [aPoint y > 0 ifTrue: [#north] ifFalse: [#south]]

     ifFalse: [aPoint x > 0 ifTrue: [#west ] ifFalse: [#east ]]


maybe exercism.io is not a good way to practice and learn smalltalk but I found not a better one. or smalltalk is not for me.

Roelof

Op 8-4-2019 om 16:44 schreef Richard O'Keefe:

The basic issue here is abstraction.

An instance of "Robot" in your program is not a

physical object.  How could it possibly point North,

South, or Nor-nor-west?  It cannot.

Its location and direction are abstract values

*metaphorically* related to real world notions

like position vectors and velocity vectors.

"North" in this program is not a real thing,

it is an *idea* which could be represented by

'North', 'north', #North, #north, $N, $n,

'Raki',  'raki',  #Raki,  #raki,  $R, $r,

137, (0@ -1), a picture of the star Polaris,

the colour red (the conventional colour for

that end of a compass needle which points north),

a sound recording of a lecture by Alfred North

Whitehead, or anything you please, as long as,

inside the program, it *acts* the way *you* want

"north" to act (which is not necessarily the way

the physical direction North acts, and in fact in

this case it most certainly is not).

Locations and movements in a 2D space are, in Smalltalk,

commonly represented by Points.  "Represented by."

As for this method:

test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth
    | result |
    result := robotSimulatorCalculator
        moveDirection: 'north'
        position:
            (Dictionary new
                add: 'x' -> 0;
                add: 'y' -> 0;
                yourself)
        instructions: 'A'.
    self
        assert: result
        equals:
            (Dictionary new
                add: 'direction' -> 'north';
                add:
                    'position'
                        ->
                            (Dictionary new
                                add: 'x' -> 0;
                                add: 'y' -> 1;
                                yourself);
                yourself)

PLEASE tell me that is not what they are actually using.

Let's start with

  (Dictionary new)

     add: k1 -> v1;

     ...

     add: kn -> vn;

     yourself

Did you know that sending #add: to a dictionary is not

portable?  Storing actual Association objects inside

Dictionaries was originally an encapsulation error and

remains a performance error, so there are Smalltalks

that do not make that mistake.  The *portable* way to

make a Dictionary is

    (Dictionary new)

       at: k1 put: v1;

       ...

       at: kn put: vn;

       yourself.

And why in the name of sanity are the keys *strings*

instead of *symbols*?  This is not Smalltalk.  It is

Javascript in drag.

Now exercism.io has a habit of insisting on particular

implementations.  For example, I completed the SML track,

and found that the test code ONLY worked with Poly and

not with any of the three SML implementations I already

had on my machine.  Since you are doing this in Pharo,

I take it that exercism.io will insist on the Smalltalk

track being done in Pharo, and in that case it is

*nauseating* to use a Dictionary when you could use a

Point.  Old-fashioned Smalltalk style would have been

to return something like

   #(<direction> <x> <y>)

e.g. #(north 1 0), and I still prefer that.

In fact *good* Smalltalk style for something like this

would be

test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth

  robotSimulatorCalculator

    moveTo: 0@0;

    head: #north;

    obey: 'A'.

  self assert: robotSimulatorCalculator heading equals: #north.

  self assert: robotSimulatorCalculator location equals: 0@1.

-- We're starting to get the idea that identifiers like

robotSimulatorCalculator are not a very good idea when

simulatedRobot would do the job as well or better.

(This is also pointing us towards Betrand Meyer's

Command/Query Separation principle, but we shan't

go there today.)

This is important feedback to give to the exercism.io

people.  The test code should use a SMALLTALK interface,

not a warmed-over JAVASCRIPT interface.

Now, how do we map between direction *names* and

direction *points*?  Well, we have to start by

laying down clearly what we *mean* by the directions.

To move North one step is to add 1 to y and 0 to x.

(We know that from the appalling test case above.)

To move South one step is to add -1 to y and 0 to x.

(South is the opposite of North.)

To move East one step, oh we have a problem.

THIS NEEDS SPELLING OUT.  And one of the things the

exercism.io exercises are HORRIBLY BAD AT is specifying

the problem.  Nearly every single exercise I have tried,

I have been unable to tell what the problem is without

examining the test cases, and that is not the way

exercises are supposed to work.  (Yeah, that's why I'm

screaming about it.  I've taught a class using exercises

like this that were not of my writing and vague specifications

really upset the students.  People who had taken the class

under someone else several years before were still angry

about it.)

The geometric classes in Smalltalk were written to support

graphic user interfaces.  And in user interfaces, the y

coordinate increases DOWN.  So if we take the compass rose

and rotate it so that North is DOWN, it follows that

West is right and East is left.  So

To move East one step is to add -1 to x and 0 to y.

To move West one step is to add 1 to x and 0 to y.

The chances are excellent that the problem specification

is inconsistent with this.  Sigh.  Let's proceed, though.

North  0@1

South  0@ -1

East   -1@0

West   1@0

pointToName: aPoint

  ^aPoint x isZero

     ifTrue:  [aPoint y > 0 ifTrue: [#north] ifFalse: [#south]]

     ifFalse: [aPoint x > 0 ifTrue: [#west ] ifFalse: [#east ]]

nameToPoint: aSymbol

  aSymbol = #north ifTrue: [^0 @ 1].

  aSymbol = #south ifTrue: [^0 @ -1].

  aSymbol = #west  ifTrue: [^1 @ 0].

  aSymbol = #east  ifTrue: [^-1 @ 0].

  aSymbol error: 'not a compass direction in lower case'.

Another problem I had with exercism was a "Space-Age"

exercise where the README.md capitalised the planet names

but test_Space-Age.<whatever> insisted on lower case.

That might well happen here.

Just for grins,

Dictionary>>

asPoint

  ^(self at: 'x') @ (self at: 'y')

Point>>

asDictionary

  ^(Dictionary new)

     at: 'x' put: self x;

     at: 'y' put: self y;

     yourself

On Mon, 8 Apr 2019 at 22:15, Roelof Wobben <[hidden email]> wrote:

Richard thanks.

One thing I do not see direct.

you said :

A direction could be represented by a pair of integers

dx, dy such that |dx|+|dy| = 1.  It could also be

represented by a Point with integer components.

for me a direction is the direction the robot is facing so something like north or east.

the challenge also wants a output like this :

test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth
    | result |
    result := robotSimulatorCalculator
        moveDirection: 'north'
        position:
            (Dictionary new
                add: 'x' -> 0;
                add: 'y' -> 0;
                yourself)
        instructions: 'A'.
    self
        assert: result
        equals:
            (Dictionary new
                add: 'direction' -> 'north';
                add:
                    'position'
                        ->
                            (Dictionary new
                                add: 'x' -> 0;
                                add: 'y' -> 1;
                                yourself);
                yourself)

so how do I "convert" the point you are using to the text.

Or do I misunderstood you somewhere wrong.

Roelof

Op 8-4-2019 om 10:57 schreef Richard O'Keefe:

One thing I have often seen and lamented is students

writing excessively complicated code with way too many

classes.  There is a huge difference between

  "A Robot knows its position and direction."

and

  "A Robot has-a Position and has-a Direction."

The first is the important one.  The second is

an over-commitment to too many classses.  For a

problem like this, you really really do not want

a Direction class, and you certainly have no use

for double dispatch.

A position can be represented by a pair of integers

x, y.  It could also be represented by a Point with

integer components.

A direction could be represented by a pair of integers

dx, dy such that |dx|+|dy| = 1.  It could also be

represented by a Point with integer components.

For movement, you need to be able to add the direction

to the location, which could be simply

x := x + dx.  y := y + dy.

or it could be

position := position + direction.

For turning, you need to be able to rotate a direction

vector by ninety degrees.  Now it so happens that

Point has methods #leftRotated and #rightRotated.

So we can do the following:

   a Robot has position (a Point) and direction (aPoint)

   position := 0 @ 0.

   direction := 0 @ 1.

To move forward without turning:

   position := position + direction.

To turn left without moving:

   direction := direction leftRotated.

To turn right without moving:

   direction := direction rightRotated.

To obey a sequence of characters, commands:

   commands do: [:each |

      each caseOf: {

         [$A] -> [--move forward--].

         [$L] -> [--turn left--].

         [$R] -> [--turn right--]

      }].

One of the key ideas in extreme programming is

"You Ain't Gonna Need It", abbreviated to YAGNI!

The idea is *DON'T* generalise beyond your immediate

needs.  In this case, for example, the likelihood of

*this* program needing to deal with more general

kinds of movement is ZERO.  And the only reason for

using Point here instead of just using a few simple

assignment statements is that Point already exists,

so costs nothing to write, and as a familiar class,

code using it should be easy to read.

If someone challenges you to do something counter-productive,

refuse the challenge.

On Mon, 8 Apr 2019 at 17:21, Roelof Wobben <[hidden email]> wrote:

I can try to explain what I trying to solve.

I have a Robot which can turn left,  turn right or moveForward.

now I have a string like 'LAR'

that means the robot needs to turn left (l) , move forward one place (A) and turn left.
and I have to keep track to which direction the robot is facing and on which coordinate it stands.

so to summarize with the above string

lets say the robot is facing north on coordinate (0,0)
then it has to turn left , so its facing east and still on coordinate (0,0)
then it has to move forward, so its still  facing east but are on coordinate(0,1)
then it has to turn right, so its facing north and on coordinate (0,1)

and TimMacKinnon has challenged me to do this with double dispatch.

So I think now I need a object Direction, a sub object North and a sub - sub object TurnLeft, turnRight and moveForward.

So I can use double dispath first the direction North, East, South, West
and then use double dispatch to find the right move.

Roelof





Op 8-4-2019 om 06:50 schreef Richard O'Keefe:

It would really REALLY **REALLY** help if we knew what

the heck you were trying to do.  There is an excellent

chance that it is MUCH simpler than you think.  If you

cannot show us the Smalltalk version of the problem,

can you show us the version for some other language?

On Sun, 7 Apr 2019 at 20:15, Roelof Wobben <[hidden email]> wrote:

Op 6-4-2019 om 15:15 schreef K K Subbu:
> On 06/04/19 4:49 PM, Roelof Wobben wrote:
>> Hello,
>>
>> I just learned double dispatch.
>> And now for the Robot challenge of exercism Tim has pointed me to
>> this
>> article(https://blog.metaobject.com/2019/04/accessors-have-message-obsession.html)
>>
>> but I fail to see how the move method looks like in that article.
>> I had a conversation with Tim in the exercism channel and the way he
>> explains it, it looks like double dispatch for me.
>>
>> Am I on the right track or do I oversee something here.
> unary methods like moveRight perform specific ops and are not
> parametric, so only a single dispatch, depending on the receiver, is
> needed.
>
> If you change it to move: aDistanceOrAngle, then performing requests
> like "move: 3 cms" or "move: 30 degrees" will depend not only on the
> receiver but also on the class of the argument. This would need double
> dispatch (aka multiple polymorphism). The first dispatch would be
> based on the receiver and the receiver's method would then dispatch it
> based on the class of the argument (i.e. Distance>>move or Angle>>move )
>
> HTH .. Subbu
>
>


hmm, still stuck

I have now a class Direction with as instance variables north, south,
east, west
and made the accessors.

then I thought I need a initialize like this :

initialize
    north = Direction( 0, -1).
    east  = Direction( 1,  0).
    south = Direction( 0,  1).
    west  = Direction(-1,  0).

but the Direction (0,-1)  is a problem . the compiler does not like the
(0,-1) part

to give you the big picture. I have a Robot which can turnRight ,
turnLeft and moveForward and I try to understand how the page would work
in my case.

So I have a object Direction as described above and a Object MoveForward
which is a subobject of Direction.
MoveForward has only 1 method :

IsMove
    ^  'A'

Roelof

Hi,

Time to time I hear people like Richard saying “Dolphin is the dialect most beautiful Smalltalk he used” and others praising it in different levels. 

As Pharo “architect” (or whatever I am, but at least I’m sure I have to pay attention to the IDE :P), I’m interested to know what elements of Dolphin dialect you find “beautiful”, “enjoyable” and productive. 

What it is?

- the MVP?

- integration with Windows? The way this integration is done? (If so… how is it done?)

…

I am very interested on knowing this with some detail level. That doesn’t mean I will react and do something, but I want to have a better understanding and put it in my radar to take inspiration to enhance the Pharo experience :P

Esteban

On 10 Apr 2019, at 03:09, Richard O'Keefe <[hidden email]> wrote:

On this laptop I have

 - Squeak

 - Pharo

 - GNU Smalltalk

 - VisualAge Smalltalk

 - VisualWorks Smalltalk

 - Smalltalk/X

plus some oddballs like susie, amber, and CSOM.

On another laptop I have

 - Strongtalk

 - Dolphin

And of course I have my own 'astc' Smalltalk-via-C compiler.

I have to say that Dolphin is easily the most *beautiful* Smalltalk

environment I've used.  (Yes, I'm the kind of person who has four

different C compilers on the same machine and uses them all.  You

don't want to know how many Javascript implementations...)

The important thing here is that there are at least two aspects to

"Smalltalk".  There is Smalltalk-the-approach-to-OO and there is

Smalltalk-the-many-related-but-different-IDEs.  When it comes to

productivity, the IDE is important.  Really important.  But when

it comes to thinking about programming and solving tasks like

exercism ones, it's the approach that matters.  And that approach

pays off in languages like Javascript and Ruby and Python as well.

I used to be a University lecturer.  Now I'm a (sub)contractor.

I used to see a LOT of student code that

 - had way too many classes

 - did not use existing well-known classes when it should

 - failed to encapsulate private state

 - put responsibilities in the wrong places

and that was Java code.  What prepared me to see such issues in Java?

Lots and lots of practice in Smalltalk.

And lots of reading Smalltalk, and figuring out what made it easy or

hard to read.

I do not know how much time you have on your hands,

but you might find it profitable to look at

http://rosettacode.org/wiki/Rosetta_Code

specifically

http://rosettacode.org/wiki/Category:Smalltalk

Look at the bottom of that page for a list of 258

problems solved in Smalltalk.

On Tue, 9 Apr 2019 at 03:20, Roelof Wobben <[hidden email]> wrote:

Thanks,

for the discusson and lessons.

I will think about it and also think if smalltalk is for me.
I did the pharo Mooc and still have a lot of problems making the smalltalk way click in my head so I can solve little problems like this.

Out of coriousy what dialect do you use?


Op 8-4-2019 om 17:11 schreef Richard O'Keefe:

You are expected to use my code fragments for *ideas*,

not to incorporate them *literally* in your code.  As

I explained, *without seeing the specification*, I have

no way to tell whether the specification uses a left-handed

or right-handed coordinate system.

For what it's worth, here's a complete program in my

Smalltalk dialect.  It doesn't plug into the exercism

testing framework because I can do not know what it

looks like.  But if it makes the code more complicated

that this, it's doing it wrong.

require: 'geometry.st'  "Point"
require: 'print.st'     "OutputStream>>print:"
   
Object subclass: #Robot
  instanceVariableNames: 'position direction'
  poolDirectionaries:    'FileStream'

  methods for: 'initialising'
    pvtPostNew
      position  := 0@0.
      direction := 1@0.

  methods for: 'accessing'
    direction
      ^direction copy

    location
      ^location copy

    obey: commands
      commands do: [:each |
        each caseOf: {
          [$A] -> [position  := position  + direction].
          [$L] -> [direction := direction leftRotated].
          [$R] -> [direction := direction rightRotated]
        }].

  class methods for: 'main'
    start
      [StdIn atEnd] whileFalse: [
         |robot|
         robot := Robot new.
         Robot obey: StdIn nextLine.
         StdOut print: Robot location; cr].

On Tue, 9 Apr 2019 at 02:58, Roelof Wobben <[hidden email]> wrote:

yes,  this is a real  tests from the pharo track on exercism.io

I understand what you mean but maybe I overthinking things.
But if we have a robot facing north and the robot turns to the left  , im my oponion it faces now to the east.

like this test is saying :

test04_RotatesTheRobotsDirection90DegreesClockwiseChangesTheDirectionFromEastToSouth
    | result |
    result := robotSimulatorCalculator
        moveDirection: 'east'
        position:
            (Dictionary new
                add: 'x' -> 0;
                add: 'y' -> 0;
                yourself)
        instructions: 'R'.
    self
        assert: result
        equals:
            (Dictionary new
                add: 'direction' -> 'south';
                add:
                    'position'
                        ->
                            (Dictionary new
                                add: 'x' -> 0;
                                add: 'y' -> 0;
                                yourself);
                yourself)


but I cannot come to the same outcome with this code :

pointToName: aPoint

  ^aPoint x isZero

     ifTrue:  [aPoint y > 0 ifTrue: [#north] ifFalse: [#south]]

     ifFalse: [aPoint x > 0 ifTrue: [#west ] ifFalse: [#east ]]


maybe exercism.io is not a good way to practice and learn smalltalk but I found not a better one. or smalltalk is not for me.

Roelof

Op 8-4-2019 om 16:44 schreef Richard O'Keefe:

The basic issue here is abstraction.

An instance of "Robot" in your program is not a

physical object.  How could it possibly point North,

South, or Nor-nor-west?  It cannot.

Its location and direction are abstract values

*metaphorically* related to real world notions

like position vectors and velocity vectors.

"North" in this program is not a real thing,

it is an *idea* which could be represented by

'North', 'north', #North, #north, $N, $n,

'Raki',  'raki',  #Raki,  #raki,  $R, $r,

137, (0@ -1), a picture of the star Polaris,

the colour red (the conventional colour for

that end of a compass needle which points north),

a sound recording of a lecture by Alfred North

Whitehead, or anything you please, as long as,

inside the program, it *acts* the way *you* want

"north" to act (which is not necessarily the way

the physical direction North acts, and in fact in

this case it most certainly is not).

Locations and movements in a 2D space are, in Smalltalk,

commonly represented by Points.  "Represented by."

As for this method:

test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth
    | result |
    result := robotSimulatorCalculator
        moveDirection: 'north'
        position:
            (Dictionary new
                add: 'x' -> 0;
                add: 'y' -> 0;
                yourself)
        instructions: 'A'.
    self
        assert: result
        equals:
            (Dictionary new
                add: 'direction' -> 'north';
                add:
                    'position'
                        ->
                            (Dictionary new
                                add: 'x' -> 0;
                                add: 'y' -> 1;
                                yourself);
                yourself)

PLEASE tell me that is not what they are actually using.

Let's start with

  (Dictionary new)

     add: k1 -> v1;

     ...

     add: kn -> vn;

     yourself

Did you know that sending #add: to a dictionary is not

portable?  Storing actual Association objects inside

Dictionaries was originally an encapsulation error and

remains a performance error, so there are Smalltalks

that do not make that mistake.  The *portable* way to

make a Dictionary is

    (Dictionary new)

       at: k1 put: v1;

       ...

       at: kn put: vn;

       yourself.

And why in the name of sanity are the keys *strings*

instead of *symbols*?  This is not Smalltalk.  It is

Javascript in drag.

Now exercism.io has a habit of insisting on particular

implementations.  For example, I completed the SML track,

and found that the test code ONLY worked with Poly and

not with any of the three SML implementations I already

had on my machine.  Since you are doing this in Pharo,

I take it that exercism.io will insist on the Smalltalk

track being done in Pharo, and in that case it is

*nauseating* to use a Dictionary when you could use a

Point.  Old-fashioned Smalltalk style would have been

to return something like

   #(<direction> <x> <y>)

e.g. #(north 1 0), and I still prefer that.

In fact *good* Smalltalk style for something like this

would be

test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth

  robotSimulatorCalculator

    moveTo: 0@0;

    head: #north;

    obey: 'A'.

  self assert: robotSimulatorCalculator heading equals: #north.

  self assert: robotSimulatorCalculator location equals: 0@1.

-- We're starting to get the idea that identifiers like

robotSimulatorCalculator are not a very good idea when

simulatedRobot would do the job as well or better.

(This is also pointing us towards Betrand Meyer's

Command/Query Separation principle, but we shan't

go there today.)

This is important feedback to give to the exercism.io

people.  The test code should use a SMALLTALK interface,

not a warmed-over JAVASCRIPT interface.

Now, how do we map between direction *names* and

direction *points*?  Well, we have to start by

laying down clearly what we *mean* by the directions.

To move North one step is to add 1 to y and 0 to x.

(We know that from the appalling test case above.)

To move South one step is to add -1 to y and 0 to x.

(South is the opposite of North.)

To move East one step, oh we have a problem.

THIS NEEDS SPELLING OUT.  And one of the things the

exercism.io exercises are HORRIBLY BAD AT is specifying

the problem.  Nearly every single exercise I have tried,

I have been unable to tell what the problem is without

examining the test cases, and that is not the way

exercises are supposed to work.  (Yeah, that's why I'm

screaming about it.  I've taught a class using exercises

like this that were not of my writing and vague specifications

really upset the students.  People who had taken the class

under someone else several years before were still angry

about it.)

The geometric classes in Smalltalk were written to support

graphic user interfaces.  And in user interfaces, the y

coordinate increases DOWN.  So if we take the compass rose

and rotate it so that North is DOWN, it follows that

West is right and East is left.  So

To move East one step is to add -1 to x and 0 to y.

To move West one step is to add 1 to x and 0 to y.

The chances are excellent that the problem specification

is inconsistent with this.  Sigh.  Let's proceed, though.

North  0@1

South  0@ -1

East   -1@0

West   1@0

pointToName: aPoint

  ^aPoint x isZero

     ifTrue:  [aPoint y > 0 ifTrue: [#north] ifFalse: [#south]]

     ifFalse: [aPoint x > 0 ifTrue: [#west ] ifFalse: [#east ]]

nameToPoint: aSymbol

  aSymbol = #north ifTrue: [^0 @ 1].

  aSymbol = #south ifTrue: [^0 @ -1].

  aSymbol = #west  ifTrue: [^1 @ 0].

  aSymbol = #east  ifTrue: [^-1 @ 0].

  aSymbol error: 'not a compass direction in lower case'.

Another problem I had with exercism was a "Space-Age"

exercise where the README.md capitalised the planet names

but test_Space-Age.<whatever> insisted on lower case.

That might well happen here.

Just for grins,

Dictionary>>

asPoint

  ^(self at: 'x') @ (self at: 'y')

Point>>

asDictionary

  ^(Dictionary new)

     at: 'x' put: self x;

     at: 'y' put: self y;

     yourself

On Mon, 8 Apr 2019 at 22:15, Roelof Wobben <[hidden email]> wrote:

Richard thanks.

One thing I do not see direct.

you said :

A direction could be represented by a pair of integers

dx, dy such that |dx|+|dy| = 1.  It could also be

represented by a Point with integer components.

for me a direction is the direction the robot is facing so something like north or east.

the challenge also wants a output like this :

test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth
    | result |
    result := robotSimulatorCalculator
        moveDirection: 'north'
        position:
            (Dictionary new
                add: 'x' -> 0;
                add: 'y' -> 0;
                yourself)
        instructions: 'A'.
    self
        assert: result
        equals:
            (Dictionary new
                add: 'direction' -> 'north';
                add:
                    'position'
                        ->
                            (Dictionary new
                                add: 'x' -> 0;
                                add: 'y' -> 1;
                                yourself);
                yourself)

so how do I "convert" the point you are using to the text.

Or do I misunderstood you somewhere wrong.

Roelof

Op 8-4-2019 om 10:57 schreef Richard O'Keefe:

One thing I have often seen and lamented is students

writing excessively complicated code with way too many

classes.  There is a huge difference between

  "A Robot knows its position and direction."

and

  "A Robot has-a Position and has-a Direction."

The first is the important one.  The second is

an over-commitment to too many classses.  For a

problem like this, you really really do not want

a Direction class, and you certainly have no use

for double dispatch.

A position can be represented by a pair of integers

x, y.  It could also be represented by a Point with

integer components.

A direction could be represented by a pair of integers

dx, dy such that |dx|+|dy| = 1.  It could also be

represented by a Point with integer components.

For movement, you need to be able to add the direction

to the location, which could be simply

x := x + dx.  y := y + dy.

or it could be

position := position + direction.

For turning, you need to be able to rotate a direction

vector by ninety degrees.  Now it so happens that

Point has methods #leftRotated and #rightRotated.

So we can do the following:

   a Robot has position (a Point) and direction (aPoint)

   position := 0 @ 0.

   direction := 0 @ 1.

To move forward without turning:

   position := position + direction.

To turn left without moving:

   direction := direction leftRotated.

To turn right without moving:

   direction := direction rightRotated.

To obey a sequence of characters, commands:

   commands do: [:each |

      each caseOf: {

         [$A] -> [--move forward--].

         [$L] -> [--turn left--].

         [$R] -> [--turn right--]

      }].

One of the key ideas in extreme programming is

"You Ain't Gonna Need It", abbreviated to YAGNI!

The idea is *DON'T* generalise beyond your immediate

needs.  In this case, for example, the likelihood of

*this* program needing to deal with more general

kinds of movement is ZERO.  And the only reason for

using Point here instead of just using a few simple

assignment statements is that Point already exists,

so costs nothing to write, and as a familiar class,

code using it should be easy to read.

If someone challenges you to do something counter-productive,

refuse the challenge.

On Mon, 8 Apr 2019 at 17:21, Roelof Wobben <[hidden email]> wrote:

I can try to explain what I trying to solve.

I have a Robot which can turn left,  turn right or moveForward.

now I have a string like 'LAR'

that means the robot needs to turn left (l) , move forward one place (A) and turn left.
and I have to keep track to which direction the robot is facing and on which coordinate it stands.

so to summarize with the above string

lets say the robot is facing north on coordinate (0,0)
then it has to turn left , so its facing east and still on coordinate (0,0)
then it has to move forward, so its still  facing east but are on coordinate(0,1)
then it has to turn right, so its facing north and on coordinate (0,1)

and TimMacKinnon has challenged me to do this with double dispatch.

So I think now I need a object Direction, a sub object North and a sub - sub object TurnLeft, turnRight and moveForward.

So I can use double dispath first the direction North, East, South, West
and then use double dispatch to find the right move.

Roelof





Op 8-4-2019 om 06:50 schreef Richard O'Keefe:

It would really REALLY **REALLY** help if we knew what

the heck you were trying to do.  There is an excellent

chance that it is MUCH simpler than you think.  If you

cannot show us the Smalltalk version of the problem,

can you show us the version for some other language?

On Sun, 7 Apr 2019 at 20:15, Roelof Wobben <[hidden email]> wrote:

Op 6-4-2019 om 15:15 schreef K K Subbu:
> On 06/04/19 4:49 PM, Roelof Wobben wrote:
>> Hello,
>>
>> I just learned double dispatch.
>> And now for the Robot challenge of exercism Tim has pointed me to
>> this
>> article(https://blog.metaobject.com/2019/04/accessors-have-message-obsession.html)
>>
>> but I fail to see how the move method looks like in that article.
>> I had a conversation with Tim in the exercism channel and the way he
>> explains it, it looks like double dispatch for me.
>>
>> Am I on the right track or do I oversee something here.
> unary methods like moveRight perform specific ops and are not
> parametric, so only a single dispatch, depending on the receiver, is
> needed.
>
> If you change it to move: aDistanceOrAngle, then performing requests
> like "move: 3 cms" or "move: 30 degrees" will depend not only on the
> receiver but also on the class of the argument. This would need double
> dispatch (aka multiple polymorphism). The first dispatch would be
> based on the receiver and the receiver's method would then dispatch it
> based on the class of the argument (i.e. Distance>>move or Angle>>move )
>
> HTH .. Subbu
>
>


hmm, still stuck

I have now a class Direction with as instance variables north, south,
east, west
and made the accessors.

then I thought I need a initialize like this :

initialize
    north = Direction( 0, -1).
    east  = Direction( 1,  0).
    south = Direction( 0,  1).
    west  = Direction(-1,  0).

but the Direction (0,-1)  is a problem . the compiler does not like the
(0,-1) part

to give you the big picture. I have a Robot which can turnRight ,
turnLeft and moveForward and I try to understand how the page would work
in my case.

So I have a object Direction as described above and a Object MoveForward
which is a subobject of Direction.
MoveForward has only 1 method :

IsMove
    ^  'A'

Roelof

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