I think this is #12088. The problem is that open type family instances aren't used in kind checking in the same region of a module. The workaround is to put a top-level Template Haskell splice
> $(return []) between the two type instances. This is far from optimal, but fixing it is a Major Project. See https://ghc.haskell.org/trac/ghc/ticket/12088#comment:15 <https://ghc.haskell.org/trac/ghc/ticket/12088#comment:15> Richard > On Apr 12, 2018, at 7:47 PM, Iavor Diatchki <iavor.diatc...@gmail.com> wrote: > > Hello, > > I was experimenting with TypeInType and run into a problem, that can be > reduced to the following example. Does anyone have any insight on what > causes the error, in particular why is `IxKind` not being reduced? > > -Iavor > > > {-# Language TypeInType, TypeFamilies #-} > > module Help where > > import Data.Kind > > type family IxKind (m :: Type) :: Type > type family Value (m :: Type) :: IxKind m -> Type > > data T (k :: Type) (f :: k -> Type) > > type instance IxKind (T k f) = k > type instance Value (T k f) = f > > {- > [1 of 1] Compiling Help ( Desktop/Help.hs, interpreted ) > Desktop/Help.hs:13:31: error: > • Expected kind ‘IxKind (T k f) -> *’, but ‘f’ has kind ‘k -> *’ > • In the type ‘f’ > In the type instance declaration for ‘Value’ > | > 13 | type instance Value (T k f) = f > | ^ > Failed, no modules loaded. > -} > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
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