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foldl/4

Module​

This predicate is provided by apply.pl.

Load this module before using the predicate:

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

Description​

Left-folds List using Goal. Goal is called as call(Goal, Elem, Acc0, Acc1).

Signature​

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

Examples​

Fold a list of integers into a sum​

This scenario demonstrates how to load apply.pl and use foldl/4 to aggregate a list with an accumulator.

Here are the steps of the scenario:

  • Given the program:
sum(Elem, Acc0, Acc) :- Acc is Acc0 + Elem.
  • Given the query:
consult('/v1/lib/apply.pl'),
foldl(sum, [1,2,3,4], 0, Total).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4294
answer:
  has_more: false
  variables: ["Total"]
  results:
  - substitutions:
    - variable: Total
      expression: 10

Fold an empty list returns the initial accumulator​

Here are the steps of the scenario:

  • Given the program:
sum(Elem, Acc0, Acc) :- Acc is Acc0 + Elem.
  • Given the query:
consult('/v1/lib/apply.pl'),
foldl(sum, [], 42, Total).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4019
answer:
  has_more: false
  variables: ["Total"]
  results:
  - substitutions:
    - variable: Total
      expression: 42

Fold with non-numeric accumulator (list building)​

Here are the steps of the scenario:

  • Given the program:
cons(Elem, Acc0, [Elem|Acc0]).
  • Given the query:
consult('/v1/lib/apply.pl'),
foldl(cons, [a,b,c], [], Result).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4208
answer:
  has_more: false
  variables: ["Result"]
  results:
  - substitutions:
    - variable: Result
      expression: "[c,b,a]"