SWI-Prolog -- foldl/7

Availability::- use_module(library(apply)).(can be autoloaded)

foldl(:Goal, +List, +V0, -V)

foldl(:Goal, +List1, +List2, +V0, -V)

foldl(:Goal, +List1, +List2, +List3, +V0, -V)

foldl(:Goal, +List1, +List2, +List3, +List4, +V0, -V)

Fold an ensemble of m (0 <= m <= 4) lists of length n head-to-tail ("fold-left"), using columns of m list elements as arguments for Goal. The 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.

Examples

Accumulation using foldl/4-7

?- foldl(plus, [1,2,3], 0, N).
N = 6.

Convert a list of goals to a conjunction

Note that the last two arguments [of foldl/4-7] form a typical difference pair, so at least when the predicate is just building data structure, it is common for the last argument to be the "input" (the trivial case of the data structure) and the second-to-last argument to be the "output" (the complete data structure).

(From "An Elementary Prolog Library" by Richard O'Keefe)

flip_conj(Goal, (Goal, Conj), Conj).

?- foldl(flip_conj, [p, q, r], Conj, true).
Conj =  (p, q, r, true).

See also: - comma_list/2 for translating between a conjunction and a list.