#10805: Could not deduce (a ~ b) implies (f a ~ f b), because a type function may not be injective -------------------------------------+------------------------------------- Reporter: htebalaka | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- I don't know if the summary I provided is what's really going on here, but I'm a little baffled why this doesn't typecheck. I can't see what injectivity has to do with it, since we're not trying to deduce that the arguments are the same from the result of an application being the same; rather we're doing the opposite. I've encountered plenty of obvious arithmetic laws that I've needed to unsafeCoerceConstraint away with doing type level arithmetic, but this seems like it should be trivial regardless. {{{#!hs {-# LANGUAGE GADTs, ExplicitNamespaces, TypeOperators, DataKinds #-} {-# LANGUAGE ConstraintKinds, KindSignatures, PatternGuards #-} import Data.Type.Equality ((:~:)(..)) import GHC.TypeLits (Nat, type (<=), type (-), type (+), type (<=?)) data SNat :: Nat -> * where SSucc :: SNat a -> SNat (a + 1) SZero :: SNat 0 heqHet :: SNat a -> SNat b -> Maybe (a :~: b) heqHet SZero SZero = Just Refl heqHet (SSucc a) (SSucc b) | Just Refl <- heqHet a b = Just Refl heqHet _ _ = Nothing data Slice :: Nat -> Nat -> * where Slice :: ((start + 1) <= end, end <= numElements) => SNat numElements -> SNat start -> SNat end -> Slice numElements (end - start) heqHet' :: Slice a b -> Slice a b' -> Bool heqHet' (Slice _ start end) (Slice _ start' end') | Just _ <- heqHet start start' , Just _ <- heqHet end end' = True | otherwise = False }}} {{{#!hs Slice.hs:24:10: Could not deduce (((start + 1) <=? a) ~ ((start + 1) <=? a)) from the context (b ~ (end - start), (start + 1) <= end, end <= a) bound by a pattern with constructor Slice :: forall (numElements :: Nat) (start :: Nat) (end :: Nat). ((start + 1) <= end, end <= numElements) => SNat numElements -> SNat start -> SNat end -> Slice numElements (end - start), in an equation for ‘heqHet'’ at Slice.hs:24:10-26 or from (b' ~ (end1 - start1), (start1 + 1) <= end1, end1 <= a) bound by a pattern with constructor Slice :: forall (numElements :: Nat) (start :: Nat) (end :: Nat). ((start + 1) <= end, end <= numElements) => SNat numElements -> SNat start -> SNat end -> Slice numElements (end - start), in an equation for ‘heqHet'’ at Slice.hs:24:30-48 NB: ‘GHC.TypeLits.<=?’ is a type function, and may not be injective Relevant bindings include heqHet' :: Slice a b -> Slice a b' -> Bool (bound at Slice.hs:24:1) In the pattern: Slice _ start end In an equation for ‘heqHet'’: heqHet' (Slice _ start end) (Slice _ start' end') | Just _ <- heqHet start start', Just _ <- heqHet end end' = True | otherwise = False Failed, modules loaded: none. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10805 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler