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has_type/2

Module​

This predicate is provided by type.pl.

Load this module before using the predicate:

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

Description​

Succeeds when Term satisfies Type without throwing. Fails when Type is known but Term does not match it.

Signature​

has_type(+Type, @Term) is semidet

Examples​

Validate a byte list with has_type/2​

This scenario demonstrates how to load type.pl and check a structured type using has_type/2.

Here are the steps of the scenario:

  • Given the program:
  • Given the query:
consult('/v1/lib/type.pl'),
has_type(list(byte), [0,1,255]),
Result = ok.
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4488
answer:
  has_more: false
  variables: ["Result"]
  results:
  - substitutions:
    - variable: Result
      expression: ok

has_type/2 fails when the type does not match​

This scenario demonstrates that has_type/2 fails quietly when the value does not satisfy the requested type.

Here are the steps of the scenario:

  • Given the program:
  • Given the query:
consult('/v1/lib/type.pl'),
has_type(integer, hello).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4019
answer:
  has_more: false
  results: