I would like help understanding a type error I'm getting with GHC 6.10.4. GHC reports a type mismatch for the types in a satisfiable equality constraint. The function "idLazy" below demonstrates the error. I would appreciate if someone can explain what's going on. Thanks, -heatsink {-# LANGUAGE TypeFamilies, EmptyDataDecls, ScopedTypeVariables, FlexibleContexts #-} module Test where import Control.Monad -- These types are used as type indices data Pure data Lazy -- Delayed evaluation in the IO monad data Delayed t a = Delayed (IO (t a)) {- This function definition gives me the error Test.hs:24:0: Couldn't match expected type `Delayed (t Pure)' against inferred type `t Lazy' But that's exactly what my constraint says! What's wrong? -} idLazy :: forall (t :: * -> * -> *). t Lazy ~ Delayed (t Pure) => Delayed (t Pure) Lazy -> Delayed (t Pure) Lazy idLazy x = x -- Below, I provide an instance of 't' that makes idLazy well-typed. -- A type-indexed data structure data Tree a = Leaf Int | Branch (Subtree a a) (Subtree a a) -- The first parameter to 'Subtree' determines what type the outermost -- constructor will have. The second parameter determines what type -- the inner constructors will have. type family Subtree a :: * -> * -- Two instances of the data structure type instance Subtree Pure = Tree type instance Subtree Lazy = Delayed Tree {- If I use this type signature, there is no error idLazy :: Subtree Lazy ~ Delayed (Subtree Pure) => Delayed (Subtree Pure) Lazy -> Delayed (Subtree Pure) Lazy -} _________________________________________________________________ Hotmail: Trusted email with powerful SPAM protection. http://clk.atdmt.com/GBL/go/201469227/direct/01/