error:type_error/2

Availability::- use_module(library(error)).(can be autoloaded)

type_error(+ValidType, +Culprit)

Tell the user that Culprit is not of the expected ValidType. This error is closely related to domain_error/2 because the notion of types is not really set in stone in Prolog. We introduce the difference using a simple example.

Suppose an argument must be a non-negative integer. If the actual argument is not an integer, this is a type_error. If it is a negative integer, it is a domain_error.

Typical borderline cases are predicates accepting a compound term, e.g., point(X,Y). One could argue that the basic type is a compound-term and any other compound term is a domain error. Most Prolog programmers consider each compound as a type and would consider a compound that is not point(_,_) a type_error.

Examples

Convert a string of 0s and 1s to an integer

This shows how to convert almost any "string" of 0s and 1s to an integer.

It takes advantage of the built-in term reading, thus avoiding an explicit loop and arithmetic.

binary_string_to_int(Str, Int) :-
    (   ( string(Str) ; atom(Str) )
    ->  atomics_to_string(["0b", Str], Literal)
    ;   is_list(Str)
    ->  atomics_to_string(["0b"|Str], Literal)
    ;   type_error(string_type, Str)
    ),
    catch(term_string(Int, Literal, []),
          error(syntax_error(_), _),
          type_error(string_of_0s_and_1s, Str)).

?- binary_string_to_int('101', X).
X = 5.

?- binary_string_to_int([0, 0, 1, 0, 1], X).
X = 5.

?- binary_string_to_int(["1", "1", "0"], X).
X = 6.