[m-dev.] for review: impure functions

[Tyson Dowd]

A diff of the changes to the top of purity.m:


--- purity.m	2000/04/05 08:23:42	1.1
+++ purity.m	2000/04/05 14:27:35
@@ -75,13 +75,29 @@
 %  semipure or impure predicates.  This promise cannot be checked, so we must
 %  trust the programmer.
 %
-%  XXX The current implementation doesn't handle impure functions.  The main
-%      reason is that handling of nested functions is likely to get pretty 
-%      confusing.  Because impure functions can't be reordered, the execution
-%      order would have to be strictly innermost-first, left-to-right, and 
-%      predicate arguments would always have to be evaluated before the
-%      predicate call.  Implied modes are right out.  All in all, they just
-%      won't be as natural as one might think at first.
+%  See the language reference manual for more information on syntax and
+%  semantics.
+%
+%  The current implementation now handles impure functions. 
+%  They are limited to being used as part of an explicit unification
+%  with a purity indicator before the goal.
+%  	impure X = some_impure_func(....)
+%  This eliminates any ordering problems with nested impure functions.
+%  Of course it also eliminates the benefits of using functions to
+%  cut down on the number of variables introduced.  The main use of
+%  impure functions is to interface nicely with foreign language
+%  functions.  
+%
+%  Wishlist:
+%  	It would be nice to use impure functions in DCG goals as well as
+%  	normal unifications.  
+%
+%  	We could give better error messages for impure calls inside
+%  	closures.  It's possible to give the context of these calls,
+%  	although we should be careful to pinpoint these as the source of
+%  	the error (no impurity allowed in closures) rather than as
+%  	errors to be corrected.
+%
 
 
 :- module purity.

-- 
       Tyson Dowd           # 
                            #  Surreal humour isn't everyone's cup of fur.
     trd at cs.mu.oz.au        # 
http://www.cs.mu.oz.au/~trd #
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