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apply.pl -- Apply predicates on a list |
This module defines meta-predicates that apply a predicate on all members of a list.
All predicates support partial application in the Goal argument. This means that these calls are identical:
?- maplist(=, [foo, foo], [X, Y]). ?- maplist(=(foo), [X, Y]).
call(Goal, Xi)
succeeds.
call(Goal, Xi)
fails.
call(Pred, X)
succeeds and
Excluded contains the remaining elements.
call(Pred, Xi, Place)
,
where Place must be unified to one of <
, =
or >
.
Pred must be deterministic.
maplist(G, [X_11, ..., X_1n], [X_21, ..., X_2n], ..., [X_m1, ..., X_mn]) :- call(G, X_11, ..., X_m1), call(G, X_12, ..., X_m2), ... call(G, X_1n, ..., X_mn).
This family of predicates is deterministic iff Goal is deterministic
and List1 is a proper list, i.e., a list that ends in []
.
call(Goal, ElemIn, _)
fails are omitted from ListOut. For example (using library(yall)):
?- convlist([X,Y]>>(integer(X), Y is X^2), [3, 5, foo, 2], L). L = [9, 25, 4].
foldl
family of predicates is defined as
follows, with V0 an initial value and V the final value of the
folding operation:
foldl(G, [X_11, ..., X_1n], [X_21, ..., X_2n], ..., [X_m1, ..., X_mn], V0, V) :- call(G, X_11, ..., X_m1, V0, V1), call(G, X_12, ..., X_m2, V1, V2), ... call(G, X_1n, ..., X_mn, V<n-1>, V).
No implementation for a corresponding foldr
is given. A foldr
implementation would consist in first calling reverse/2 on each of
the m input lists, then applying the appropriate foldl
. This is
actually more efficient than using a properly programmed-out
recursive algorithm that cannot be tail-call optimized.
scanl
family of predicates is defined as
follows, with V0 an initial value and V the final value of the
scanning operation:
scanl(G, [X_11, ..., X_1n], [X_21, ..., X_2n], ..., [X_m1, ..., X_mn], V0, [V0, V1, ..., Vn] ) :- call(G, X_11, ..., X_m1, V0, V1), call(G, X_12, ..., X_m2, V1, V2), ... call(G, X_1n, ..., X_mn, V<n-1>, Vn).
scanl
behaves like a foldl
that collects the sequence of
values taken on by the Vx accumulator into a list.