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From: nr@elan.Princeton.EDU (Norman Ramsey)
Newsgroups: comp.lang.modula3
Subject: Closures in m3
Message-ID: <7006@rossignol.Princeton.EDU>
Date: 18 Feb 91 19:56:07 GMT
Sender: news@cs.Princeton.EDU
Organization: Dept. of Computer Science, Princeton University
Lines: 29


Many of the table, list, et cetera data structure packages make it
possible to enumerate elements of the table.  Typically the procedures
implementing the enumeration accept a REFANY as arguments, and this
REFANY is used as a closure argument.  An example from List:

TYPE
    WalkProc = PROCEDURE (arg, element: REFANY) RAISES {};
        (* The type of procedures for walking over lists. *)

PROCEDURE Walk( l: T; p: WalkProc; arg: REFANY := NIL );
    (*
    Applies p to each of the elements of l in turn.  The only exceptions raised
    are those possibly raised by p. *)

I suggest that it would be more elegant for enumeration to use
closures (as in Thread).

TYPE
  WalkClosure = OBJECT METHODS apply(element:REFANY) END;

PROCEDURE Walk( l: T; p: WalkClosure );


I would like to hear people's comments on this proposal.  If you won't
buy ``more elegant,'' perhaps you'll buy ``typechecked at compile time.''
--
Norman Ramsey
nr@princeton.edu