GADT constructor constraints ignored

[wolfram_kahl]

Class constraints on GADT constructore appear to be
ignored.

I tried:

data Expr :: * -> * where
  Const :: Show a => a -> Expr a
  Apply :: Fct a b -> Expr a -> Expr b

and there is no way to define the expected constraint-less
Show instance --- for details, see Expr0.lhs.

[wolfram_kahl]

Logged In: YES 
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For a simpler example, see Expr1.lhs

[wolfram_kahl]

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The expected solution would be that constrained constructors are
translated into constructors with an additional dictionary
argument,
as sketched in Expr1a.lhs.

[simonpj]

Fixed at last (the implication-consraint patch)

Simon