#10808: Odd interaction between record update and type families -------------------------------------+------------------------------------- Reporter: diatchki | 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: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Consider the following example {{{#!hs {-# LANGUAGE TypeFamilies #-} type family F a type family G a data T1 type instance F T1 = Char type instance G T1 = Int data T2 type instance F T2 = Bool type instance G T2 = Int data R a = R { x :: F a, y :: G a } r1 :: R T1 r1 = R { x = 'a', y = 2 } r2 :: R T2 r2 = r1 { x = True } -- error: Cannot match T1 with T2 r3 :: R T2 r3 = r1 { x = True, y = y r1 } -- OK }}} The error for `r2` is odd because types `R T1` and `R T2` differ only in the type of `x`, so if we start with a value of type `R T1`, and update the field where the types differ, we should end up with a valid value of type `R T2`. The fact that `r2` is OK is demonstrated by `r3` where we update the other field with its own value, and now the type checker is happy. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10808 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler