Opened 4 years ago

Last modified 4 years ago

#10808 new bug

Odd interaction between record update and type families

Reported by: diatchki Owned by:
Priority: normal Milestone:
Component: Compiler (Type checker) Version: 7.10.2
Keywords: TypeFamilies Cc: glguy
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:


Consider the following example

{-# 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.

Change History (2)

comment:1 Changed 4 years ago by glguy

Cc: glguy added
Type of failure: None/UnknownGHC rejects valid program

comment:2 Changed 4 years ago by thomie

Keywords: TypeFamilies added
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