[Haskell-cafe] Curious data family bug

[Michael Snoyman]

Sorry, I forgot to mention which compiler I was working with: 6.12.3.
I'm glad to hear it's working with 7.

Michael

On Mon, Nov 15, 2010 at 4:58 PM, Simon Peyton-Jones
<simonpj at microsoft.com> wrote:
> Hmm.  It works with HEAD (and hence I believe with the 7.0.1 RC2).  It looks similar to http://hackage.haskell.org/trac/ghc/ticket/4174, which is fixed.
>
> Anyway I've added it as a regression test, so it should never go wrong again.  Thanks for mentioning it.
>
> Simon
>
> | -----Original Message-----
> | From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-bounces at haskell.org] On
> | Behalf Of Michael Snoyman
> | Sent: 14 November 2010 19:16
> | To: Haskell Cafe
> | Subject: [Haskell-cafe] Curious data family bug
> |
> | Hey all,
> |
> | While trying to get a commit pushed for Yesod[1], Alexander Dunlap
> | pointed out one of his programs didn't work with the new code. After
> | some investigation, I was able to reproduce the bug with the following
> | code snippet:
> |
> | {-# LANGUAGE TypeFamilies #-}
> | data family Foo a
> | data Bar = Bar
> | data instance Foo Bar
> |     = Bar1 | Bar2 | Bar3 | Bar4 | Bar5 | Bar6 | Bar7 | Bar8 | Bar9
> |     deriving Eq
> |
> | This produces:
> |
> |     Couldn't match expected type `Main.R:FooBar'
> |            against inferred type `Foo Bar'
> |       NB: `Foo' is a type function
> |     In the first argument of `Main.$con2tag_R:FooBar', namely `a'
> |     In the expression: (Main.$con2tag_R:FooBar a)
> |     In the expression:
> |         case (Main.$con2tag_R:FooBar a) of {
> |           a#
> |             -> case (Main.$con2tag_R:FooBar b) of {
> |                  b# -> (a# GHC.Prim.==# b#) } }
> |
> | The especially strange thing about this bug is that it only occurs
> | when there are more than 8 constructors; if I remove Bar9, everything
> | seems to work. Does anyone have experience with this occuring?
> |
> | Michael
> |
> | [1] http://docs.yesodweb.com/blog/please-break-yesod/
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>