## #10805 closed bug (fixed)

# Could not deduce (a ~ b) implies (f a ~ f b), because a type function may not be injective

Reported by: | htebalaka | Owned by: | |
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Priority: | normal | Milestone: | 8.0.1 |

Component: | Compiler (Type checker) | Version: | 7.10.2 |

Keywords: | Cc: | ||

Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |

Type of failure: | GHC rejects valid program | Test Case: | |

Blocked By: | Blocking: | ||

Related Tickets: | Differential Rev(s): | ||

Wiki Page: |

### Description

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.

{-# 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

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.

### Change History (3)

### comment:1 Changed 4 years ago by

Resolution: | → fixed |
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Status: | new → closed |

### comment:3 Changed 4 years ago by

Milestone: | → 8.0.1 |
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I found https://ghc.haskell.org/trac/ghc/ticket/10742, which appears to be the same thing and is fixed in HEAD, so I'm closing