call_default_value(?Value, :Goal_1) is det

If `Value' is not bound, call `Goal_1' to determine its default value.

call_default_value(?Value, :Goal_1, +DefaultValue) is det

If `Value' cannot be determined by calling `Goal_1', the `DefaultValue' is used instead.

default_value(?Value, +DefaultValue) is det

Returns either the given value or the default value, in case there is no value given.

Example

Ordering is a meta-argument that allows a list of elements to be arbitrarily ordered. The use of default_value/2 here allows the original ordering of elements to be retained in case the Ordering argument is not instantiated.

default_value(=, Ordering),
once(call(Ordering, L1, L2))

default_value(?FromValue, -ToValue, +DefaultValue) is det

Returns the given value, unless the given value is a variable. In the latter case, the default value is returned instead.

@note We are sometimes using this predicate instead of default_value/2 from `library(default)' because we do not want variable occurrences of `FromValue' to get instantiated.