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 wrote:

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/

On Friday 18 March 2011 13:35:22, JP Moresmau wrote:

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.

In (IdSig i), i has type Id, hence search i :: Result Id but you want something of type `Result id'. Fortunately it's easy to transform, since search i is a ResultId, so instance (Search id id) => Search id (Sig id) where search (SigGen g) = search g search (IdSig i) = case search i of ResultId y -> ResultId y _ -> error "foo" compiles (and probably does what you want).

On Friday 18 March 2011 14:40:40, JP Moresmau wrote:

Thanks to you all, I think I understand better. instance Search Id Id where search _ _ i = only (FoundId i)

Is too restrictive on the first type, so declaring instead: instance Search id Id where search _ _ i = only (FoundId i)

Not sure what GHC does with that, but at least in spirit that violates the FunDep of class Search id a | a -> id where ... Even if it works now, it may well not work in the future. I'd go for a more stable solution respecting the intent of FunDeps (i.e. there should only be one type t with an instance Search t Id).

Fixed the issue!! Now the initial "id" is not Id and everybody is happy (and the code still seems to work as intended)

thanks again

JP