10.4 Abstract instance declarations

Abstract instance declarations are instance declarations whose implementations are hidden. An abstract instance declaration has the same form as an instance declaration, but without the ‘where [...]’ part. An abstract instance declaration declares that a sequence of types is an instance of a particular type class without defining how the type class methods are implemented for those types. Like abstract type declarations, abstract instance declarations are only useful in the interface section of a module. Each abstract instance declaration must be accompanied by a corresponding non-abstract instance declaration that defines how the type class methods are implemented.

Here's an example:

     :- module hashable.
     :- interface.
     :- import_module int, string.
     
     :- typeclass hashable(T) where [func hash(T) = int].
     :- instance hashable(int).
     :- instance hashable(string).
     
     :- implementation.
     
     :- instance hashable(int) where [func(hash/1) is hash_int].
     :- instance hashable(string) where [func(hash/1) is hash_string].
     
     :- func hash_int(int) = int.
     hash_int(X) = X.
     
     :- func hash_string(string) = int.
     hash_string(S) = H :-
             % use the standard library predicate string.hash/2
             string.hash(S, H).
     
     :- end_module hashable.