Posted by
Volker Zink on
Feb 21, 2005; 2:48pm
URL: https://forum.world.st/Trouble-with-symbols-tp3373204p3373206.html
There are some problems with your code. I try to point these problems
later on. First i want to know, why you don't use code like following:
sortAspect := #firstname.
coll := SortedCollection sortBlock: [:a :b | (a perform: sortAspect) <
(b perform: sortAspect)].
Should be much simpler. Now to your code:
Fernando wrote:
> Hi,
>
> Assume I have a class Person with methods #firstname and #lastname that
> I want to put into a SortedBlock.
>
> I want to create a generic sortBlock so I can sort either by #firstname
> or by #lastname.
>
> This code (see me previous 'Currying blocks' post) works:
>
> curry := [:query| [:a :b| (a perform: query ) < (b perform: query )]].
>
> curry2 := [:query| [:a :b| (a query asSymbol) < (b query asSymbol)].]
curry2 makes no sense because 'query' is a parameter to the outer block,
'a' and 'b' parameters to the inner block. Nevertheless you have code
like '(a query asSymbol)' which should produce an error because you send
the message 'query' to the variable 'a'. You should use perform: in
this case (like your first example '(a perform: query)').
> coll := SortedCollection sortBlock: (curry value: #lastname).
> or
> coll := SortedCollecton sortBlock: (curry2 value: #firstname).
>
> However, if I change te definition of the curryed block to:
> curry3 := [:query| [:a :b| (a query ) < (b query )]]
>
> It won't work and when I try to add something to the collection, I get
> a DNU error, complaning that Person doesnt understand 'query'.
Its the same as above. '(a query)' means you send the message 'query' to
'a' which isn't defined. But 'query' is not a method to invoke, its a
variable which holds a Symbol (#firstname or #lastname). So use '(a
perform: query)' which tell the object stored in the variable 'a' (an
instance of the class Person) to invoke a method which name (selector)
is the symbol stored in the variable 'query'.
> Why do I have to send the message #asSymbol (see curry2)? After all,
> I'm already sending a symbol when I evaluate curry2 value: #firstname.
>
> I think I'm missing something about symbols, but I'm not sure what.
You cannot send symbols. You can send only messages (maybe with
parameters) to objects not objects to objects. Messages normally invoke
methods defined in the class (or superclass) of the receiving object
which have the same selector (which is a Symbol).
> PS I'm using Dolphin 5.1
>
This basic stuff should be the same for all Smalltalks. Hope this helps
to clarify things a bit. Take a look at the class named 'Class'. There
is functionality about the methods. In VW you can send
getMethodDictionary to a class (i.e. to Class or Person) and you get the
methods defined in that class.
Volker