Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!shelby!neon!craig
From: craig@Neon.Stanford.EDU (Craig D. Chambers)
Newsgroups: comp.lang.modula3
Subject: Re: Closures in m3
Message-ID: <1991Feb22.232418.29567@Neon.Stanford.EDU>
Date: 22 Feb 91 23:24:18 GMT
References: <91Feb21.152416pst.16910@alpha.xerox.com>
Organization: Stanford University
Lines: 34

In article <91Feb21.152416pst.16910@alpha.xerox.com> Mike_Spreitzer.PARC@xerox.com writes:
>You are right about the first point: local procedures can't escape (given M3 as
>it is).  So the closure scheme is more convenient in some cases.  It's less
>convenient in others (where the local procedure would work, making the local
>procedure involves less overhead than making a whole closure object).  Wouldn't
>the local procedure scheme suffice in the vast majority of actual uses?  Note
>that if there
>were a syntax for objection creation that allowed method literals, the closure
>scheme can also approach the elegance of CLU iterators.

In fact, full closures (or even downwards-only closures) are *more*
powerful than CLU iterators (if this is what you meant by "elegant").
CLU iterators effectively take a single closure argument (the body of
the for loop).  If closures are first-class, then iterators can take
multiple closure arguments.  In our Self system we have many methods
that take multiple closure arguments.  For example, one iterator
method called "doFirst:Middle:Last:" takes a closure to execute for
the first element of the receiver collection, a different closure for
the interior elements, and a third closure for the last element of the
collection.  This is useful for instance when constructing a string
representation of the collection and needing to insert commas in
between elements; the code can insert a comma before all the elements
except the first by using a different closure for the first element
and the other elements.

One thing that makes closures usable in practice is a concise closure
literal syntax.  Since Self and Smalltalk both rely on closures and
messages for all control structures, even ifs and whiles, this concise
syntax had to be developed just to allow the language to be used at
all.  Without it, I wouldn't expect closures to receive much use.
This lack of concise syntax in Pascal-like languages has probably been
a major hurdle to widespread use of closures.

-- Craig Chambers