[Haskell-cafe] MPTC inheritance

Paul Johnson paul at cogito.org.uk
Tue Feb 24 14:41:08 EST 2009

Derek Gladding wrote:
> Please forgive me if I'm still mentally contaminated by the OO way of 
> seeing (and discussing) the universe, but I'm trying to figure out how 
> to "inherit an interface" from a multi-parameter type class.
> [...]
> but this isn't allowed (kind mismatch).
Kinds are a meta-type system for types.  Because Haskell has such a rich 
type system, the types themselves need a type-like system.  These are 
"kinds".  You never declare kinds (apart from certain language 
extensions not in use here): the compiler infers them.

This suggests that your problem is in the types lower down.  Probably 
you are using "a" or "b" in a way that implies they take a type argument 
in one place (kind * -> *) and not in another place (kind *).

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