[Haskell-cafe] Curious data family bug

[Simon Peyton-Jones]

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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