# `parameter/2`

## Description

    parameter(Number, Term)

Used in parametric objects (and parametric categories), this private method provides runtime access to the parameter values of the entity that contains the predicate clause whose body is being executed by using the argument number in the entity identifier. This predicate is implemented as a unification between its second argument and the corresponding implicit execution-context argument in the predicate clause making the call. This unification occurs at the clause head when the second argument is not instantiated (the most common case). When the second argument is instantiated, the unification must be delayed to runtime and thus occurs at the clause body.

Entity parameters can also be accessed using parameter variables, which use the syntax `_VariableName_`. The compiler recognizes occurrences of these variables in directives and clauses. Parameter variables allows us to abstract parameter positions thus simplifying code maintenance.

## Modes and number of proofs

    parameter(+integer, ?term) - zero_or_one

## Errors

`Number` is a variable:
`instantiation_error`
`Number` is neither a variable nor an integer value:
`type_error(integer, Number)`
`Number` is smaller than one or greater than the parametric entity identifier arity:
`domain_error(out_of_range, Number)`
`Entity` identifier is not a compound term:
`type_error(compound, Entity)`

## Examples

    :- object(box(_Color, _Weight)).

        ...

        % this clause is translated into
        % a fact upon compilation
        color(Color) :-
            parameter(1, Color).

        % upon compilation, the >/2 call will be
        % the single goal in the clause body
        heavy :-
            parameter(2, Weight),
            Weight > 10.
        ...

The same example using parameter variables:

    :- object(box(_Color_, _Weight_)).

        ...

        color(_Color_).

        heavy :-
            _Weight_ > 10.

        ...

See also

context/1, self/1, sender/1, this/1