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foldl/7

Module​

This predicate is provided by apply.pl.

Load this module before using the predicate:

:- consult('/v1/lib/apply.pl').

Description​

Left-folds four lists in lockstep using Goal. Goal is called as call(Goal, Elem1, Elem2, Elem3, Elem4, Acc0, Acc1).

Signature​

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

Examples​

Fold four lists in lockstep to compute a combined result​

This scenario demonstrates how to use foldl/7 to fold four lists simultaneously.

Here are the steps of the scenario:

  • Given the program:
quad_sum(W, X, Y, Z, Acc0, Acc) :- Acc is Acc0 + W + X + Y + Z.
  • Given the query:
consult('/v1/lib/apply.pl'),
foldl(quad_sum, [1,2], [3,4], [5,6], [7,8], 0, Result).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4365
answer:
  has_more: false
  variables: ["Result"]
  results:
  - substitutions:
    - variable: Result
      expression: 36

Fold four empty lists returns the initial accumulator​

Here are the steps of the scenario:

  • Given the program:
quad_sum(W, X, Y, Z, Acc0, Acc) :- Acc is Acc0 + W + X + Y + Z.
  • Given the query:
consult('/v1/lib/apply.pl'),
foldl(quad_sum, [], [], [], [], 100, Result).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4067
answer:
  has_more: false
  variables: ["Result"]
  results:
  - substitutions:
    - variable: Result
      expression: 100

Fold four lists to build a structured result​

Here are the steps of the scenario:

  • Given the program:
make_quad(W, X, Y, Z, Acc0, [[W,X,Y,Z]|Acc0]).
  • Given the query:
consult('/v1/lib/apply.pl'),
foldl(make_quad, [a], [1], [x], [true], [], Quads).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4193
answer:
  has_more: false
  variables: ["Quads"]
  results:
  - substitutions:
    - variable: Quads
      expression: "[[a,1,x,true]]"