Prolog map procedure that applies predicate to list elements

Question

How do you write a Prolog procedure map(List, PredName, Result) that applies the predicate PredName(Arg, Res) to the elements of List, and returns the result in the list Result?

For example:

test(N,R) :- R is N*N.

?- map([3,5,-2], test, L).
L = [9,25,4] ;
no

Starve

Answer 1

This is usually called maplist/3 and is part of the Prolog prologue. Note the different argument order!

:- meta_predicate(maplist(2, ?, ?)).

maplist(_C_2, [], []).
maplist( C_2, [X|Xs], [Y|Ys]) :-
   call(C_2, X, Y),
   maplist( C_2, Xs, Ys).

The different argument order permits you to easily nest several maplist-goals.

?- maplist(maplist(test),[[1,2],[3,4]],Rss).
   Rss = [[1,4],[9,16]].

maplist comes in different arities and corresponds to the following constructs in functional languages, but requires that all lists are of same length. Note that Prolog does not have the asymmetry between zip/zipWith and unzip. A goal maplist(C_3, Xs, Ys, Zs) subsumes both and even offers more general uses.

• maplist/2 corresponds to all
• maplist/3 corresponds to map
• maplist/4 corresponds to zipWith but also unzip
• maplist/5 corresponds to zipWith3 and unzip3
• ...

Agnomen

+1. In modern Prologs, you'll find maplist somewhere in the library. – Defeatism

@DavidTonhofer: Please refer to meta-predicate for more! – Agnomen

I do not believe that the SWI-Prolog version of maplist/3 corresponds to map anymore. It seems to be all but delivers the input in pairs from both lists. Is there a predicate that has the same functionality as map? – Tactical

SWI has maplist/2/3/4/5. And, no SWI does not "deliver input in pairs", except you use such a predicate in the 1st argument. Please give a concrete example, where you think map cannot be expressed with maplist. Except for cases where the lists are of different length. – Agnomen

Perhaps I am confused by the documentation. maplist/3, "As maplist/2, operating on pairs of elements from two lists" implies to me (a novice prolog programmer) that maplist/3 has the same functionality as maplist/2 which is all but with different inputs to the first argument. This resulted in me ignoring maplist/3 and finding/writing my own map predicate. – Tactical

@bennyty: That documentation is infact a bit misleading. – Agnomen

@Agnomen I came to the same conclusion. could you explain how maplist/3 is not all? I get: ?- maplist(=, [1, 2, 3], [1, 2, 3]). true. not [true, true, true], whereas convlist/3 seems to be exactly map. – Better

@ErikKaplun: Consider As = [1,2,3], maplist(succ, As, Bs). Where do you see an all in here? That's a typical map. – Agnomen

@ErikKaplun: Now you talk about maplist/2 instead of maplist/3. See above... – Agnomen

Oh yeah, that's right, how embarrassing! Apologies, my Prolog-eye is still young! But then I should add that maplist/3 is also all, but can be used like map, or even better, map and all are the same thing in Prolog, due to predicates/relations being more general than functions. – Better