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

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 wrote:

Forwarding to Haskell Cafe per Eric's suggestion.

Begin forwarded message:

Date: Sun, 11 Jan 2009 23:01:31 -0600 From: Rob Hoelz To: darcs-devel@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 _______________________________________________ darcs-devel mailing list (AUTOMATIC POSTINGS ONLY PLEASE!) darcs-devel@darcs.net http://lists.osuosl.org/mailman/listinfo/darcs-devel _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe