Re: Beginner's Language?

[brangdon@cix.co.uk (Dave Harris)]

dale@cs.rmit.edu.au (Dale Stanbrough) wrote (abridged):
> You can have values such as '#doSoemthing' (it's been a -long- time
> since i've done Smalltalk, so please forgive if i get the syntax
> wrong). You can convert this symbol into a method call to a routine
> called 'doSomething'

Actually that is a library call, called #perform:. Arguably it is no more 
part of the language than #ifTrue:. Eg:

    |aSymbol|
    aSymbol:= #message.
    anObject perform: aSymbol.

has the same effect as:

    anObject message.

The equivalent in C++ would be like:

    void (Object::*aPmf)();
    aPmf = &Object::message;
    (anObject->*aPmf)();

which has the same effect as:

    anObject->message();

I agree this stuff need not be taught on the first day. Still, it is a 
useful and powerful technique. I suspect a Smalltalk course could include 
it earlier than a C++ course. (The syntax alone could make C++ students 
run screaming, never mind the type-checking issues.)

The #message notation itself is part of the language; it produces a 
Symbol. A Symbol is what Java calls an "interned" String, meaning that 
two Symbols with the same value will have the same identity. This means 
that comparing Symbols for equality is fast, because we don't have to 
look at each of the characters in them like we do with Strings. This 
means they are quite commonly used in Smalltalk; often in situations 
where C++ would use an enum. They are not just used with #perform:.

Although this is fairly subtle stuff, I would hope a Smalltalk course 
would cover the difference between:

     string1 == string2.
     string1 = string2.

just as a C++ course would cover the difference between:

     string1 == string2;
     strcmp( string1, string2 );

because a programmer who doesn't grasp this does not yet grasp the object 
model.

Finally, in Smalltalk we can construct Symbols on the fly. Eg:

    |aPrefix aSuffix aSymbol|
    aPrefix := 'mess'.
    aSuffix := 'age'.
    aSymbol := (aPrefix, aSuffix) asSymbol.
    anObject perform: aSymbol.

I hope it is clear what that is doing. There is no C++ equivalent.

  Dave Harris, Nottingham, UK | "Weave a circle round him thrice,
      brangdon@cix.co.uk      |   And close your eyes with holy dread,
                              |  For he on honey dew hath fed
 http://www.bhresearch.co.uk/ |   And drunk the milk of Paradise."