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Type-changing record update catch-all in sum type doesn't typecheck

Apologies for the confusing title/summary, but that is about as succinct as I could be.

Rather than try to describe this in english, here is the smallest example I could repro with.

I want to write this:

import Data.Char

data R a
  = Foo { x :: a, y :: a }
  | Bar { x :: a }
  | Baz { x :: a }

ordify :: R Char -> R Int
ordify r@(Foo {}) = r { x = ord (x r), y = ord (y r) }
ordify r          = r { x = ord (x r) }

But that fails to typecheck:

$ ghci Test.hs
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( Test.hs, interpreted )

Test.hs:10:21:
    Couldn't match type ‘Char’ with ‘Int’
    Expected type: R Int
      Actual type: R Char
    In the expression: r
    In the expression: r {x = ord (x r)}
Failed, modules loaded: none.

Instead, I have to resort to enumerating all the remaining constructors:

import Data.Char

data R a
  = Foo { x :: a, y :: a }
  | Bar { x :: a }
  | Baz { x :: a }

ordify :: R Char -> R Int
ordify r@(Foo {}) = r { x = ord (x r), y = ord (y r) }
ordify (Bar x) = Bar (ord x)
ordify (Baz x) = Baz (ord x)

The sum type has the property that every constructor has the field x, and some constructors have fields which are also parameterized over x's type 'a'. I would expect my first example, with the catch-all record-update to typecheck, but it doesn't. I suspect GHC thinks the catch-all case of ordify might be passed a Foo constructor, in which case the record update would be ill-typed, even though that is impossible due to the first case matching Foo explicitly.

Trac metadata
Trac field Value
Version 7.10.2
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture
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