[m-users.] Typeclasses

[Dirk Ziegemeyer]

Hi Mark,

> Am 13.10.2017 um 19:52 schrieb Mark Brown <mark at mercurylang.org>:
> 
> The typeclass method you have given is more general than map.search,
> so map.search can't implement it.

Thank you for your advice to use different names for the variables to receive a better error message.

Is it possible at all to define a typeclass key_value_store with the method search_value and then to declare map as an instance of this typeclass?

> 
>> 
>> :- import_module map.
>> 
>> :- typeclass key_value_store(Store) where [
>>    pred search_value(Store::in, K::in, V::out) is semidet
>> ].
>> 
>> :- instance key_value_store(map(K, V)) where [
>>    pred(search_value/3) is map.search
>> ].
> 
> The type signature for search_value in this instance is equivalent to
> pred(map(K2, V2), K1, V1), where K1 and V1 are from the typeclass
> declaration and K2 and V2 are from the instance declaration. They are
> renamed apart because each declaration is a separate scope. So this
> doesn't actually say that the types of the key and value arguments
> need to match those types that are in the store.
> 
>> The compiler throws this error message:
>> 
>> typeclass_test.m:029: In clause for type class method implementation:
>> typeclass_test.m:029:   in argument 2 of predicate `map.search'/3:
>> typeclass_test.m:029:   type error: variable `HeadVar__2' has type `(some [K]
>> typeclass_test.m:029:   K)',
>> typeclass_test.m:029:   expected type was `(some [K] K)'.
> 
> The quantifiers are a clue that these type variables are actually from
> different scopes. For error messages like this one, it can be helpful
> to change the variable names to be distinct for the purposes of
> debugging. E.g., with the above numbering the error message is:
> 
> typeclass_test.m:016: In clause for type class method implementation:
> typeclass_test.m:016:   in argument 2 of call to predicate `map.search'/3:
> typeclass_test.m:016:   type error: variable `K' has type `(some [K1] K1)',
> typeclass_test.m:016:   expected type was `(some [K2] K2)'.
> typeclass_test.m:016: In clause for type class method implementation:
> typeclass_test.m:016:   in argument 3 of call to predicate `map.search'/3:
> typeclass_test.m:016:   type error: variable `V' has type `(some [V1] V1)',
> typeclass_test.m:016:   expected type was `(some [V2] V2)’.

Dirk