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From: budd@orstcs.cs.ORST.EDU
Newsgroups: comp.lang.smalltalk
Subject: Translating Smalltalk
Message-ID: <245100008@orstcs>
Date: Mon, 8-Jun-87 14:09:00 EDT
Article-I.D.: orstcs.245100008
Posted: Mon Jun  8 14:09:00 1987
Date-Received: Fri, 12-Jun-87 04:08:51 EDT
Lines: 37
Nf-ID: #N:orstcs:245100008:000:1775
Nf-From: orstcs.cs.ORST.EDU!budd    Jun  8 11:09:00 1987


Somebody asked about translating Smalltalk into other code, and how message
passing is done.  Note that the dynamic message passing portion of
smalltalk is not the problem in translating Smalltalk to machine code;
I will use C as my target language, as it is more readable; We can easily
translate

	object send: arg1 message: arg2 with: arg3 arguments: arg4

into some C code that looks like:

*(FindMethod(object,'send:message:with:arguments))(arg1,arg2,arg3,arg4)

That is, we write a little routine that does our message lookup for us,
returns a method (function) and then executes it.  This is how object
oriented programming is done in Scheme, for example.  The need for memory
management complicates this only slightly.
If no method exists FindMethod returns a pointer to an error reporting
routine.

The real problem in translating Smalltalk is the notion of Blocks.  Blocks
must carry with them the context in which they were defined, and there is
no way to easily simulate this dynamic binding is a staticly scoped
language like C.  (This is a classic funarg problem).

Which leads one to wonder if a language similar to Smalltalk, but without
blocks, could be defined.  I think yes.  And if this were done, could you
build interpreters/compilers and generate efficient machine code without
sacrificing the nice features of Smalltalk; i.e., experimental programming,
rapid prototyping, reusability.  I think yes.  Finally, if you did this,
could you use optional declarations of object class to speed things up by 
eliminating the method lookup; (the idea being that you produce and debug 
your code, then insert declarations to speed it up); I think yes.  

Watch this space for future announcements.

--tim budd, oregon state university,  budd@oregon-state.csnet