typeclass - Haskell Type Class Implied -

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i want able types b

class g   f :: g -> int  class b g   h :: g -> int  instance g => b g     h = f

i'm getting compile error:

illegal instance declaration `b g' …       (all instance types must of form (t a1 ... an)        a1 ... *distinct type variables*,        , each type variable appears @ once in instance head.

you should not this. however, it's reasonable write like

class b g => g   f :: g -> int

this produces useful entailment. defaultsignatures extension (which dislike various reasons), can write

class b g   h :: g -> int   default h :: g => g -> int   h = f

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