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  • Glasgow Haskell CompilerGlasgow Haskell Compiler
  • GHCGHC
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  • #15310
Closed
Open
Issue created Jun 25, 2018 by cedricshock@trac-cedricshock

Derive Generic1 instances for types of kind (k -> *) -> * that include applications of the parameter

The Generic1 class is polykinded. The DeriveGeneric extension can derive Generic1 instances for types of kind k -> * parameterized over a parameter with a kind other than k ~ *, but only if they don't apply the parameter to other types.

It currently cannot derive an instance for

newtype Fix f = In (f (Fix f))
  deriving (Generic1)

or for

data Child f = Child {
    ordinal :: Int,
    nickname :: f String
}
  deriving (Generic1)

It's possible to represent these types generically, either with composition that can include occurrences of the parameter or with new types that represent applications of the parameter.

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Version
Type FeatureRequest
TypeOfFailure OtherFailure
Priority low
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
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