[Haskell-cafe] Curious data family bug

[Daniel Peebles]

Hmm, strange. I have a project that uses data families with dozens of
constructors per "clause"/instantiation of the type function. I use GADT
syntax to define them though as they also refine one of the parameter type
variables. Never had any issues with it, although I haven't tried building
that project on GHC 7 yet (as some of its dependencies didn't work last time
I tried), so maybe it's a new issue?

On Sun, Nov 14, 2010 at 2:15 PM, Michael Snoyman <michael at snoyman.com>wrote:

> 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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