must_be/2

Module

This predicate is provided by error.pl.

Load this module before using the predicate:

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

Description

Succeeds when Term satisfies Type. Throws:

• error(instantiation_error, must_be/2) when Term is insufficiently instantiated;
• error(type_error(Type, Term), must_be/2) when Term has the wrong type;
• error(existence_error(type, Type), must_be/2) when Type is unknown.

Signature

must_be(+Type, @Term) is det

Examples

Validate an atom with must_be/2

This scenario demonstrates how to load error.pl and validate a value type with must_be/2.

Here are the steps of the scenario:

• Given the program:

• Given the query:

consult('/v1/lib/error.pl'),
must_be(atom, hello),
Result = ok.

• When the query is run
• Then the answer we get is:

height: 42
gas_used: 4075
answer:
  has_more: false
  variables: ["Result"]
  results:
  - substitutions:
    - variable: Result
      expression: ok

must_be/2 throws instantiation_error for unbound values

This scenario demonstrates that must_be/2 raises an instantiation error when the checked value is a variable.

Here are the steps of the scenario:

• Given the program:

• Given the query:

consult('/v1/lib/error.pl'),
must_be(atom, X).

• When the query is run
• Then the answer we get is:

height: 42
gas_used: 4269
answer:
  has_more: false
  variables: ["X"]
  results:
  - error: "error(instantiation_error,must_be/2)"