Implement map combinator.

thun/thun.pl

@@ -242,6 +242,34 @@ combo(genrec, [R1, R0, Then, If|S],
     Quoted = [If, Then, R0, R1, genrec],
     append(R0, [Quoted|R1], Else).
 
+/*
+This is a crude but servicable implementation of map combinator.
+Obviously it would be nice to take advantage of the implied parallelism.
+Instead the quoted program, stack, and each term in the arg list are
+transformed to a simple Joy expression that runs the program on a prepared
+stack.  These expressions are collected in a list and the whole thing is
+evaluated with infra on an empty list, so the result is the mapped list.
+
+The chief advantage of doing it this way (as opposed to using Prolog's map)
+is that the whole state remains in the continuation expression.
+*/
+
+combo(map, [_,   []|S],         [[]|S], E,        E ) :- !.
+combo(map, [P, List|S], [Mapped, []|S], E, [infra|E]) :-
+    prepare_mapping(P, S, List, Mapped).
+
+% Set up a program for each term in ListIn
+%
+%     [term S] [P] infrst
+%
+% prepare_mapping(P, S, ListIn, ListOut).
+
+prepare_mapping(P, S, In, Out) :- prepare_mapping(P, S, In, [], Out).
+
+prepare_mapping(    _, _,     [],                   Out,  Out) :- !.
+prepare_mapping(    P, S, [T|In],                   Acc,  Out) :-
+    prepare_mapping(P, S,    In,  [[T|S], P, infrst|Acc], Out).
+
 
 /*
 Compiler