Re: [Haskell-cafe] Undecidable instances with functional dependencies

2010-02-16 Thread Yves Parès 

However, TypeFamilies seems too be non portable as according to this
http://www.haskell.org/haskellwiki/GHC/Type_families, it works only as from
GHC 6.10.1.


Henning Thielemann-4 wrote:
 
 Miguel Mitrofanov schrieb:
 -- {-# LANGUAGE FunctionalDependencies#-}
 -- {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeFamilies #-}
 module Register where
 -- class Register a r | a - r
 class Register a where
 type R a
 -- instance Register Int Int
 instance Register Int where
 type R Int = Int
 -- instance Register Float Float
 instance Register Float where
 type R Float = Float
 -- instance (Register a1 r1, Register a2 r2) = Register (a1, a2) (r1, 
 r2)
 instance (Register a, Register b) = Register (a, b) where
 type R (a, b) = (R a, R b)

 So type functions are undecidable by default?
 
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-
Yves Parès

Live long and prosper
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Re: [Haskell-cafe] Undecidable instances with functional dependencies

2010-02-15 Thread Henning Thielemann 

Miguel Mitrofanov schrieb:

-- {-# LANGUAGE FunctionalDependencies#-}
-- {-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
module Register where
-- class Register a r | a - r
class Register a where
type R a
-- instance Register Int Int
instance Register Int where
type R Int = Int
-- instance Register Float Float
instance Register Float where
type R Float = Float
-- instance (Register a1 r1, Register a2 r2) = Register (a1, a2) (r1, 
r2)

instance (Register a, Register b) = Register (a, b) where
type R (a, b) = (R a, R b)


So type functions are undecidable by default?

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[Haskell-cafe] Undecidable instances with functional dependencies

2010-02-11 Thread Henning Thielemann 


I have the following class and instance

  class Register a r | a - r where

  instance (Register a ra, Register b rb) =
 Register (a,b) (ra,rb) where

and GHC refuses the instance because of violated Coverage Condition.
I have more instances like

  instance Register Int8  (Reg Int8)  where
  instance Register Word8 (Reg Word8) where

and for the set of instances I plan, the instance resolution will always 
terminate. I remember that the term 'undecidable instance' is not fixed 
and may be relaxed if a more liberal condition can be found. Is there a 
place, say a Wiki page, where we can collect examples where we think that 
the current check of GHC is too restrictive?

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Re: [Haskell-cafe] Undecidable instances with functional dependencies

2010-02-11 Thread Miguel Mitrofanov 

-- {-# LANGUAGE FunctionalDependencies#-}
-- {-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
module Register where
-- class Register a r | a - r
class Register a where
type R a
-- instance Register Int Int
instance Register Int where
type R Int = Int
-- instance Register Float Float
instance Register Float where
type R Float = Float
-- instance (Register a1 r1, Register a2 r2) = Register (a1, a2) (r1,  
r2)

instance (Register a, Register b) = Register (a, b) where
type R (a, b) = (R a, R b)

On 12 Feb 2010, at 00:32, Henning Thielemann wrote:



I have the following class and instance

 class Register a r | a - r where

 instance (Register a ra, Register b rb) =
Register (a,b) (ra,rb) where

and GHC refuses the instance because of violated Coverage Condition.
I have more instances like

 instance Register Int8  (Reg Int8)  where
 instance Register Word8 (Reg Word8) where

and for the set of instances I plan, the instance resolution will  
always terminate. I remember that the term 'undecidable instance' is  
not fixed and may be relaxed if a more liberal condition can be  
found. Is there a place, say a Wiki page, where we can collect  
examples where we think that the current check of GHC is too  
restrictive?

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