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
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
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