#9018: GHC suppresses too much kind information ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- GHC tends to hide important kind polymorphism in error messages and in GHCi. (I came across this when looking at #9017.) For example, try this: {{{ bash$ cat Test.hs {-# LANGUAGE PolyKinds #-} module Test where foo :: m a foo = foo bash$ ghci Test.hs ghci> :t foo foo :: m a ghci> :i foo foo :: m a bash$ ghci Test.hs -XPolyKinds ghci> :t foo foo :: m a ghci> :i foo goo :: m a bash$ ghci Test.hs -fprint-explicit-foralls -fprint-explicit-kinds ghci> :t foo foo :: foo :: forall (m :: * -> *) d. m d ghci> :i foo foo :: forall (k :: BOX) (m :: k -> *) (d :: k). m d bash$ ghci Test.hs -XPolyKinds -fprint-explicit-foralls -fprint-explicit- kinds ghci> :t foo foo :: forall (k :: BOX) (m :: k -> *) (d :: k). m d ghci> :i foo foo :: forall (k :: BOX) (m :: k -> *) (d :: k). m d }}} Pretty confusing eh? * Without `-fprint-explicit-foralls -fprint-explicit-kinds` you don't see the kinds on the types at all. * Without `-XPolyKinds` in GHCi, when you say `:t foo` you are asking for the type of the expression `foo` (it could be an arbitrary expression). So `foo` is instantiated and then re-generalised; but without `-XPolyKinds` we don't get a poly-kinded type. Hence the difference in what is printed by `:type` and `:info`. Here's a proposal: even without `-fprint-explicit-foralls`, we should print any foralls that bind a type variable whose kind includes a kind variable. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9018 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler