[Haskell-cafe] Fw: [darcs-devel] "Inferred type is less polymorphic than expected" and type witnesses

[Ryan Ingram]

Some questions first:
What's the type of this function supposed to be?
What's the type of unsafeCompare?
How is the data type with NilFL and :>: defined?

  -- ryan

On Mon, Jan 12, 2009 at 5:43 AM, Rob Hoelz <rob at hoelzro.net> wrote:
> Forwarding to Haskell Cafe per Eric's suggestion.
>
> Begin forwarded message:
>
> Date: Sun, 11 Jan 2009 23:01:31 -0600
> From: Rob Hoelz <rob at hoelzro.net>
> To: darcs-devel at darcs.net
> Subject: [darcs-devel] "Inferred type is less polymorphic than
> expected" and type witnesses
>
>
> Hello again, Darcs users and developers,
>
> As I mentioned in my last e-mail, I'm working on
> http://bugs.darcs.net/issue291.  It's actually gone pretty well, and I
> feel I'm just about finished (I've done all but sorting out the
> changes after leaving the editor), only I've encountered the compiler
> error you see in the subject of this message. This error only appears
> when compiling with witnesses. Here's the source for the function
> that it's complaining about:
>
> compare_changes_with_old (x :>: xs) (y :>: ys) =
>  case compare_changes_with_old xs ys of
>    nx :> ny -> if unsafeCompare x y
>      then ((x :>: nx) :> ny)
>      else (NilFL :> (y :>: ys))
> compare_changes_with_old NilFL NilFL = (NilFL :> NilFL)
> compare_changes_with_old NilFL ys@(_ :>: _) = (NilFL :> ys)
> compare_changes_with_old x@(_ :>: _) NilFL = (NilFL :> NilFL)
>
> Now, I have two questions:
>
> 1) What exactly does this error mean, and how do I get around it?
> 2) What are witness types, and what are they used for?
>
> I will gladly accept links to fine manuals as answers to either
> question, but simple explanations would be nice. =D  I thought I
> understood Haskell pretty well, but existentially qualified types have
> thrown me for a loop.
>
> Thanks much,
> Rob Hoelz
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