Smalltalk › Pharo › Pharo Smalltalk Users

How does Boolean ifTrue work?

_‹ Previous Topic Next Topic _›

Classic

List

Threaded

54 messages Options

123

CodeDmitry

Oct 31, 2016; 1:17pm

How does Boolean ifTrue work?

This post was updated on Oct 31, 2016; 1:24pm.

CONTENTS DELETED

The author has deleted this message.

Sven Van Caekenberghe-2

Oct 31, 2016; 1:22pm

Re: How does Boolean ifTrue work?

5697 posts

> On 31 Oct 2016, at 14:17, CodeDmitry <[hidden email]> wrote:
>
> I am looking at
>
> ifTrue: alternativeBlock
> "If the receiver is false (i.e., the condition is false), then the value is
> the
> false alternative, which is nil. Otherwise answer the result of evaluating
> the argument, alternativeBlock. Create an error notification if the
> receiver is nonBoolean. Execution does not actually reach here because
> the expression is compiled in-line."
>
> self subclassResponsibility
>
> In order to perform the block, ifTrue must somehow end up evaluating the
> block, but this code only sends a subclassResponsibility message to itself
> and implicitly returns itself.
>
> Where does the block actually get evaluated?

... [show rest of quote]

In the sub classes of Boolean, True and False. Have a look at ALL implementors of #ifTrue: or #ifFalse:

There is only one instance of True, the constant true, same for False and false.

HTH,

Sven

>
>
>
> --
> View this message in context: http://forum.world.st/How-does-Boolean-ifTrue-work-tp4920873.html
> Sent from the Pharo Smalltalk Users mailing list archive at Nabble.com.
>

CyrilFerlicot

Oct 31, 2016; 1:23pm

Re: How does Boolean ifTrue work?

1067 posts

In reply to this post by CodeDmitry

On 31/10/2016 14:17, CodeDmitry wrote:

> I am looking at
>
> ifTrue: alternativeBlock
> "If the receiver is false (i.e., the condition is false), then the value is
> the
> false alternative, which is nil. Otherwise answer the result of evaluating
> the argument, alternativeBlock. Create an error notification if the
> receiver is nonBoolean. Execution does not actually reach here because
> the expression is compiled in-line."
>
> self subclassResponsibility
>
> In order to perform the block, ifTrue must somehow end up evaluating the
> block, but this code only sends a subclassResponsibility message to itself
> and implicitly returns itself.
>
> Where does the block actually get evaluated?
>
>

... [show rest of quote]

Hi!

Boolean is only an abstract class. You never manipulate a Boolean, you
manipulate True or False.

The #ifTrue: method works with a dispatch between True and False.

Try to check: `True browse` or `False browse` :)

>
>
> --
> View this message in context: http://forum.world.st/How-does-Boolean-ifTrue-work-tp4920873.html
> Sent from the Pharo Smalltalk Users mailing list archive at Nabble.com.
>

--
Cyril Ferlicot

http://www.synectique.eu

2 rue Jacques Prévert 01,
59650 Villeneuve d'ascq France

signature.asc (817 bytes) Download Attachment

Nicolas Passerini

Oct 31, 2016; 1:24pm

Re: How does Boolean ifTrue work?

104 posts

In reply to this post by CodeDmitry

Well, in fact Boolean class has no instances, the boolean value true is an instance of True class, you should look at the ifTrue: methods in the subclasses of Boolean True and False.

But also, ifTrue: expressions are inlined, as the comment says in the code you posted. So I think that all this ifTrue: methods are never actually executed.

2016-10-31 14:17 GMT+01:00 CodeDmitry <[hidden email]>:

I am looking at

ifTrue: alternativeBlock
"If the receiver is false (i.e., the condition is false), then the value is
the
false alternative, which is nil. Otherwise answer the result of evaluating
the argument, alternativeBlock. Create an error notification if the
receiver is nonBoolean. Execution does not actually reach here because
the expression is compiled in-line."

self subclassResponsibility

In order to perform the block, ifTrue must somehow end up evaluating the
block, but this code only sends a subclassResponsibility message to itself
and implicitly returns itself.

Where does the block actually get evaluated?

--
View this message in context: http://forum.world.st/How-does-Boolean-ifTrue-work-tp4920873.html
Sent from the Pharo Smalltalk Users mailing list archive at Nabble.com.

... [show rest of quote]

CodeDmitry

Oct 31, 2016; 1:25pm

Re: How does Boolean ifTrue work?

This post was updated on Oct 31, 2016; 1:56pm.

In reply to this post by CodeDmitry

CONTENTS DELETED

The author has deleted this message.

Thierry Goubier

Oct 31, 2016; 1:37pm

Re: How does Boolean ifTrue work?

1618 posts

Hi Dmitry,

you found it yourself, sort of!

Le 31/10/2016 à 14:25, CodeDmitry a écrit :
> Wait, so unlike JavaScript; True and False are actual singletons and
> ifTrue/ifFalse/ifTrue:ifFalse: uses visitor pattern to have the actual True
> or False handle the block rather than doing it themselves?

Yes, true and false are singletons (resp single instance of the True
class, False class).

No, this is not a visitor pattern, just polymorphism (True and False
have their own implementation of
ifTrue:/ifFalse:/ifTrue:ifFalse:/ifFalse:ifTrue:).

Regards,

Thierry

jtuchel

Oct 31, 2016; 1:58pm

Re: How does Boolean ifTrue work?

1922 posts

In reply to this post by CodeDmitry

When I teach Smalltalk, this is one of the most beautiful exercises for
students.
Those who get it will love Smalltalk, those who don't often leave the
project soon ;-)

BTW: This is not a visitor or any other pattern, it really is just
Polymorphism at work and the reason why back in the 90ies, people said
if statements should be replaced by Polymorphism

Joachim

Am 31.10.16 um 14:17 schrieb CodeDmitry:

... [show rest of quote]

--
-----------------------------------------------------------------------
Objektfabrik Joachim Tuchel mailto:[hidden email]
Fliederweg 1 http://www.objektfabrik.de
D-71640 Ludwigsburg http://joachimtuchel.wordpress.com
Telefon: +49 7141 56 10 86 0 Fax: +49 7141 56 10 86 1

CodeDmitry

Oct 31, 2016; 1:58pm

Re: How does Boolean ifTrue work?

This post was updated on Oct 31, 2016; 2:26pm.

In reply to this post by Thierry Goubier

CONTENTS DELETED

The author has deleted this message.

Thierry Goubier

Oct 31, 2016; 2:17pm

Re: How does Boolean ifTrue work?

1618 posts

Le 31/10/2016 à 14:58, CodeDmitry a écrit :
> But still, how is the actual argument "alternativeBlock" passed to the
> True/False from a Boolean?
>
> The message does not cache the message inside itself before passing the
> message, and it does not pass the alternative block along with the message.

If I get correctly what you are saying, this is because the
ifTrue:ifFalse: message is never sent to an instance of Boolean.

It is sent either to true, the single instance of class True, or to
false, the single instance of class False, where the implementation is
what you expect:

False>>#ifTrue:ifFalse:

ifTrue: trueAlternativeBlock ifFalse: falseAlternativeBlock
"Answer the value of falseAlternativeBlock. Execution does not
actually reach here because the expression is compiled in-line."

^falseAlternativeBlock value

If you're interested in the details, do also read the comment.

Regards,

Thierry

Sven Van Caekenberghe-2

Oct 31, 2016; 2:17pm

Re: How does Boolean ifTrue work?

5697 posts

In reply to this post by CodeDmitry

myCollection isEmpty ifTrue: [ self inform: 'The collection is empty' ].

First #isEmpty is sent to myCollection which results in either true or false as value.

Then #ifTrue: is sent to this value.

Now, if the value was true (the sole instance of class True), the code in True>>#ifTrue: is executed, which will evaluate the block by sending it #value. If the value was false, the code in False>>#ifTrue: is executed, and nil is returned and the block is not executed.

Like others said, it is just polymorphism at work.

(And yes, the compiler optimises some of this logic a bit, but the effect is the same).

> On 31 Oct 2016, at 14:58, CodeDmitry <[hidden email]> wrote:
>
> But still, how is the actual argument "alternativeBlock" passed to the
> True/False from a Boolean?
>
> The message does not cache the message inside itself before passing the
> message, and it does not pass the alternative block along with the message.
>
>
>
> --
> View this message in context: http://forum.world.st/How-does-Boolean-ifTrue-work-tp4920873p4920886.html
> Sent from the Pharo Smalltalk Users mailing list archive at Nabble.com.
>

CodeDmitry

Oct 31, 2016; 2:32pm

Re: How does Boolean ifTrue work?

This post was updated on Oct 31, 2016; 2:41pm.

In reply to this post by Thierry Goubier

CONTENTS DELETED

The author has deleted this message.

philippeback

Oct 31, 2016; 2:51pm

Re: How does Boolean ifTrue work?

3147 posts

In reply to this post by Sven Van Caekenberghe-2

but you should use myCollection ifEmpty: [ ... ]

Phil

On Mon, Oct 31, 2016 at 3:25 PM, Sven Van Caekenberghe <[hidden email]> wrote:

myCollection isEmpty ifTrue: [ self inform: 'The collection is empty' ].

First #isEmpty is sent to myCollection which results in either true or false as value.

Then #ifTrue: is sent to this value.

Now, if the value was true (the sole instance of class True), the code in True>>#ifTrue: is executed, which will evaluate the block by sending it #value. If the value was false, the code in False>>#ifTrue: is executed, and nil is returned and the block is not executed.

Like others said, it is just polymorphism at work.

(And yes, the compiler optimises some of this logic a bit, but the effect is the same).

> On 31 Oct 2016, at 14:58, CodeDmitry <[hidden email]> wrote:
>
> But still, how is the actual argument "alternativeBlock" passed to the
> True/False from a Boolean?
>
> The message does not cache the message inside itself before passing the
> message, and it does not pass the alternative block along with the message.
>
>
>
> --
> View this message in context: http://forum.world.st/How-does-Boolean-ifTrue-work-tp4920873p4920886.html
> Sent from the Pharo Smalltalk Users mailing list archive at Nabble.com.
>

... [show rest of quote]

philippeback

Oct 31, 2016; 2:52pm

Re: How does Boolean ifTrue work?

3147 posts

In reply to this post by CodeDmitry

For the full story, these objects and ifTrue: etc are special cases in the VM code.

For speed.

Phil

On Mon, Oct 31, 2016 at 3:32 PM, CodeDmitry <[hidden email]> wrote:

I realized it a bit before you posted but thanks for confirming.

Boolean seems to indeed just be a "Dynamic Interface"; since Smalltalk does
not have "Java Interfaces"(nor want them for the same reason JavaScript
doesn't), the developers wanted to still have True and False be subclasses
of Booleans.

In all honesty Boolean seems to be there purely for the "common sense" of
it, rather than need. Smalltalk is duck typed and has no need for
interfaces. If it's a Boolean, you know it has those methods so Boolean is
not actually useful for anything; True and False may as well just be
subclasses of Object.

--
View this message in context: http://forum.world.st/How-does-Boolean-ifTrue-work-tp4920873p4920891.html
Sent from the Pharo Smalltalk Users mailing list archive at Nabble.com.

... [show rest of quote]

Thierry Goubier

Oct 31, 2016; 2:55pm

Re: How does Boolean ifTrue work?

1618 posts

In reply to this post by CodeDmitry

Le 31/10/2016 à 15:32, CodeDmitry a écrit :
> I realized it a bit before you posted but thanks for confirming.
>
> Boolean seems to indeed just be a "Dynamic Interface"; since Smalltalk does
> not have "Java Interfaces"(nor want them for the same reason JavaScript
> doesn't), the developers wanted to still have True and False be subclasses
> of Booleans.

Boolean is a nice header for classification. Everybody is familiar with
Boolean algebra, including hardware guys :)

> In all honesty Boolean seems to be there purely for the "common sense" of
> it, rather than need. Smalltalk is duck typed and has no need for
> interfaces. If it's a Boolean, you know it has those methods so Boolean is
> not actually useful for anything; True and False may as well just be
> subclasses of Object.

Not exactly. Boolean carry 15 methods that are not overridden in either
False nor True (and 11 methods which are), so Boolean is also used as a
way to share code between the classes True and False and avoid code
duplication. A nice case for having a common superclass below Object.

Thierry

CodeDmitry

Oct 31, 2016; 2:58pm

Re: How does Boolean ifTrue work?

This post was updated on Oct 31, 2016; 3:04pm.

CONTENTS DELETED

The author has deleted this message.

Esteban A. Maringolo

Oct 31, 2016; 3:00pm

Re: How does Boolean ifTrue work?

2343 posts

In reply to this post by CodeDmitry

2016-10-31 11:32 GMT-03:00 CodeDmitry <[hidden email]>:

> I realized it a bit before you posted but thanks for confirming.
>
> Boolean seems to indeed just be a "Dynamic Interface"; since Smalltalk does
> not have "Java Interfaces"(nor want them for the same reason JavaScript
> doesn't), the developers wanted to still have True and False be subclasses
> of Booleans.
>
> In all honesty Boolean seems to be there purely for the "common sense" of
> it, rather than need. Smalltalk is duck typed and has no need for
> interfaces. If it's a Boolean, you know it has those methods so Boolean is
> not actually useful for anything; True and False may as well just be
> subclasses of Object.

Not exactly, look at the methods in Boolean and you'll see many
methods are common to all.

So Boolean has a reason to be the superclass of both True and False.

Esteban A. Maringolo

NorbertHartl

Oct 31, 2016; 3:03pm

Re: How does Boolean ifTrue work?

3479 posts

In reply to this post by philippeback

Am 31.10.2016 um 15:59 schrieb [hidden email]:

but you should use myCollection ifEmpty: [ ... ]

Why?

Norbert

Phil

On Mon, Oct 31, 2016 at 3:25 PM, Sven Van Caekenberghe <[hidden email]> wrote:
myCollection isEmpty ifTrue: [ self inform: 'The collection is empty' ].

First #isEmpty is sent to myCollection which results in either true or false as value.

Then #ifTrue: is sent to this value.

Now, if the value was true (the sole instance of class True), the code in True>>#ifTrue: is executed, which will evaluate the block by sending it #value. If the value was false, the code in False>>#ifTrue: is executed, and nil is returned and the block is not executed.

Like others said, it is just polymorphism at work.

(And yes, the compiler optimises some of this logic a bit, but the effect is the same).

> On 31 Oct 2016, at 14:58, CodeDmitry <[hidden email]> wrote:
>
> But still, how is the actual argument "alternativeBlock" passed to the
> True/False from a Boolean?
>
> The message does not cache the message inside itself before passing the
> message, and it does not pass the alternative block along with the message.
>
>
>
> --
> View this message in context: http://forum.world.st/How-does-Boolean-ifTrue-work-tp4920873p4920886.html
> Sent from the Pharo Smalltalk Users mailing list archive at Nabble.com.
>

... [show rest of quote]

... [show rest of quote]

Esteban A. Maringolo

Oct 31, 2016; 3:06pm

Re: How does Boolean ifTrue work?

2343 posts

In reply to this post by CodeDmitry

2016-10-31 11:58 GMT-03:00 CodeDmitry <[hidden email]>:
> Ah so it's more of a "dynamic abstract class" than a "dynamic interface", so
> they do get visited during the traversal of the inheritence chain(rather
> than skipped immediately to Object).

I don't know what you mean by "dynamic". The whole object system of
Smalltalk is dynamic by design.

Esteban A. Maringolo

Esteban A. Maringolo

Oct 31, 2016; 3:08pm

Re: How does Boolean ifTrue work?

2343 posts

In reply to this post by NorbertHartl

2016-10-31 12:11 GMT-03:00 Norbert Hartl <[hidden email]>:
>
> Am 31.10.2016 um 15:59 schrieb [hidden email]:
>
> but you should use myCollection ifEmpty: [ ... ]
>
> Why?

It enables you to implement polymorphism with an object that might not
be aCollection, but behaves like one.
Of course you could also implement #isEmpty, but in my experience the
less you test (#isEmpty) and the more you delegate (ifEmpty:) the
better. :)

Regards,

Esteban A. Maringolo

Denis Kudriashov

Oct 31, 2016; 3:10pm

Re: How does Boolean ifTrue work?

2224 posts

In reply to this post by NorbertHartl

2016-10-31 16:11 GMT+01:00 Norbert Hartl <[hidden email]>:

Am 31.10.2016 um 15:59 schrieb [hidden email]:

but you should use myCollection ifEmpty: [ ... ]

Why?

because it shorter :)

123