These are GHC types, but here is a self-contained example:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances #-}
data Id=Id String
data Result id =ResultId Id
| ResultGen id
data Sig id=IdSig Id
| SigGen id
class Search id a | a -> id where
search :: a -> Result id
instance Search Id Id where
search i = ResultId i
instance (Search id id) => Search id (Sig id) where
search (SigGen g) = search g
search (IdSig i) = search i
The last line fails. I don't understand why this doesn't compile.
Thanks,
JP
On Fri, Mar 18, 2011 at 12:56 PM, Dimitrios Vytiniotis
Hi, can you elaborate a bit? What is Id and what is (Sig id) and IdSig in your example? Can you reproduce an example that you believe should compile but doesn't?
thanks d-
-----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of JP Moresmau Sent: 16 March 2011 21:46 To: Haskell Cafe Subject: [Haskell-cafe] Could not deduce ... using functional dependencies with GHC7
Hello, when moving to GHC7 a body of code that I'm not immensely familiar with, I got an error compiling with GHC 7.0.2. Can somebody kindly explain to me what it means and how to get around it? This is in the scion code base so is using some GHC types.
I have a class with a functional dependency (I think :-p): class Search id a | a -> id where search :: (SrcSpan -> Bool) -> SrcSpan -> a -> SearchResults id
And some instances, notably: instance Search Id Id where search _ _ i = only (FoundId i)
And the error occurs on another instance: instance (Search id id) => Search id (Sig id) where search p s (IdSig i) = search p s i ... other cases here, that work
The error is:
Could not deduce (id ~ Id) from the context (Search id id) bound by the instance declaration at lib\Scion\Inspect\Find.hs:477:10-45 `id' is a rigid type variable bound by the instance declaration at lib\Scion\Inspect\Find.hs:477:18 When using functional dependencies to combine Search Id Id, arising from the dependency `a -> id' in the instance declaration at lib\Scion\Inspect\Find.hs:183:10 Search id Id, arising from a use of `search' at lib\Scion\Inspect\Find.hs:479:32-37 In the expression: search p s i In an equation for `search': search p s (IdSig i) = search p s i
If I replace the call to search with the actual implementation of search for Id, it compiles, so at some level the code "makes sense". I much prefer when error messages end with "Possible fix:..."!
Thanks!
-- JP Moresmau http://jpmoresmau.blogspot.com/
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-- JP Moresmau http://jpmoresmau.blogspot.com/