[m-rev.] for post-commit review: clarify some parts of the reference manual

[Julien Fischer]

On Wed, 7 Feb 2024, Zoltan Somogyi wrote:

> diff --git a/doc/reference_manual.texi b/doc/reference_manual.texi
> index 0aa356ce5..5f2edc148 100644
> --- a/doc/reference_manual.texi
> +++ b/doc/reference_manual.texi
> @@ -7223,12 +7223,16 @@ The @code{typeclass} declaration
>  gives the name of the type class that it is defining,
>  the names of the type variables which are parameters to the type class,
>  and the operations (i.e.@: methods) which form the interface of the type class.
> -For each method, all parameters of the typeclass must be determined by
> -the type variables appearing in the type signature of the method.
> -A variable is determined by a type signature
> -if it appears in the type signature,
> -but if functional dependencies are present,
> -then it may also be determined from the other variables
> +For each method, all parameters of the typeclass must be determined
> +by the type declaration of the method.
> +The values of @emph{most} parameter type variables
> +are determined by having them occur
> +in the declared type of an argument of the method.
> +However, in the presence of functional dependencies,
> + at c where exactly? The possibilities are the typeclass declaration and
> + at c the method itself; are both allowed?

Functional dependencies can only occur on typeclass declarations. They are not
allowed on methods. Any FDs that are used to determine type parameters need to
be from the typeclass declaration or inherited from any superclasses.

The reference manual doesn't mention if other (i.e non-FD) typeclass
constraints are allowed on individual methods (i.e. what Haskell, or at least
GHC, calls constrained class methods), although doing so appears to work. (If
this support is intentional we should probably mention it somewhere in the
reference manual.)

The diff looks fine.

Julien.