Re: [Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

2011-03-18 Thread 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-boun...@haskell.org 
[mailto:haskell-cafe-boun...@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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Re: [Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

2011-03-18 Thread JP Moresmau 
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
dimit...@microsoft.com 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-boun...@haskell.org 
 [mailto:haskell-cafe-boun...@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/

 ___
 Haskell-Cafe mailing list
 Haskell-Cafe@haskell.org
 http://www.haskell.org/mailman/listinfo/haskell-cafe





-- 
JP Moresmau
http://jpmoresmau.blogspot.com/

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Re: [Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

2011-03-18 Thread Sean Leather 
On Fri, Mar 18, 2011 at 13:35, 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.


This doesn't even work in GHC 6.12.3. The search call in the second case,
IdSig, restricts the result to Result Id, which is less polymorphic than
Result id. Either of the following are valid:

instance Search Id (Sig Id) where
  search (SigGen g)   = search g
  search (IdSig i)= search i

instance (Search id id) = Search id (Sig id) where
  search (SigGen g)   = search g

Sean
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Re: [Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

2011-03-18 Thread Daniel Fischer 
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).

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Re: [Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

2011-03-18 Thread JP Moresmau 
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)

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

On Fri, Mar 18, 2011 at 2:17 PM, Daniel Fischer
daniel.is.fisc...@googlemail.com wrote:
 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).




-- 
JP Moresmau
http://jpmoresmau.blogspot.com/

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Re: [Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

2011-03-18 Thread Daniel Fischer 
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

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Re: [Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

2011-03-18 Thread Iavor Diatchki 
Hi,
that's a bug in GHC---it erroneously accepts polymorphic instances which
violate the FD of a class.
-Iavor

On Fri, Mar 18, 2011 at 7:08 AM, Daniel Fischer 
daniel.is.fisc...@googlemail.com wrote:

 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

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 Haskell-Cafe@haskell.org
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[Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

2011-03-16 Thread JP Moresmau 
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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