In the following code, GHC is unable to match Obj Hask with *, despite the declaration that type instance Obj Hask = * just above.
What's preventing this?
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
module Minimal where
type family Obj t :: *
type family Arr t :: Obj t -> Obj t -> *
data Hask
type instance Obj Hask = *
type instance Arr Hask = (->)
-- Minimal.hs:15:26: error:
-- • Couldn't match kind ‘Obj Hask’ with ‘*’
-- Expected kind ‘Obj Hask -> Obj Hask -> *’,
-- but ‘(->)’ has kind ‘* -> * -> *’
-- • In the type ‘(->)’
-- In the type instance declaration for ‘Arr’
-- |
-- 15 | type instance Arr Hask = (->)
-- | ^^^^
It's able to note the equivalence if I give it a nudge
{-# LANGUAGE StandaloneKindSignatures #-}
-- …
type Function :: Obj Hask -> Obj Hask -> *
type Function = (->)
type instance Arr Hask = Function
-- compiles just fine
This is the one spot where type families surprisingly don't "just work".
GHC internally splits declarations into groups and can't use a type instance to typecheck things in its own group. Adding intermediate declarations affects this grouping, which is why things may or may not typecheck depending on that.
Another more reliable way to break groups is to add empty Template Haskell splices ($(return []) or newDeclarationGroup).