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foldl/5

Module​

This predicate is provided by apply.pl.

Load this module before using the predicate:

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

Description​

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

Signature​

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

Examples​

Fold two lists in lockstep to compute dot product​

This scenario demonstrates how to use foldl/5 to fold two lists simultaneously.

Here are the steps of the scenario:

  • Given the program:
add_product(X, Y, Acc0, Acc) :- Acc is Acc0 + (X * Y).
  • Given the query:
consult('/v1/lib/apply.pl'),
foldl(add_product, [1,2,3], [4,5,6], 0, DotProduct).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4337
answer:
  has_more: false
  variables: ["DotProduct"]
  results:
  - substitutions:
    - variable: DotProduct
      expression: 32

Fold two empty lists returns the initial accumulator​

Here are the steps of the scenario:

  • Given the program:
add_product(X, Y, Acc0, Acc) :- Acc is Acc0 + (X * Y).
  • Given the query:
consult('/v1/lib/apply.pl'),
foldl(add_product, [], [], 99, Result).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4046
answer:
  has_more: false
  variables: ["Result"]
  results:
  - substitutions:
    - variable: Result
      expression: 99

Fold two lists to build a pair list​

Here are the steps of the scenario:

  • Given the program:
make_pair(X, Y, Acc0, [[X,Y]|Acc0]).
  • Given the query:
consult('/v1/lib/apply.pl'),
foldl(make_pair, [a,b], [1,2], [], Pairs).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4214
answer:
  has_more: false
  variables: ["Pairs"]
  results:
  - substitutions:
    - variable: Pairs
      expression: "[[b,2],[a,1]]"