[Haskell-cafe] Code that doesn't compile - but should :)

2010-03-16 Thread Giuseppe Maggiore 
Hi! Can anyone tell me why this code does not work? I cannot seem to
figure why it is broken...



{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
  UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables,
  TypeOperators, TypeSynonymInstances #-}

data Data k = Pair Integer (() - k)
data RecData = RecData (Data RecData)
mk_data x = RecData(Pair x (\() - mk_data (x+1)))

class Converter a f where convert :: a - f a

instance Converter RecData Data where
  convert (RecData r) = r

class Selector s a where select :: s - a

f :: (Selector s (a-f a), Converter a f) = s - (a-a)
f s =
  let method = select s
  in (\x -
let res = method x
in convert res)

--
Giuseppe Maggiore
Ph.D. Student (Languages and Games)
Microsoft Student Partner
Mobile: +393319040031
Office: +390412348444
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Re: [Haskell-cafe] Code that doesn't compile - but should :)

2010-03-16 Thread Ivan Lazar Miljenovic 
Giuseppe Maggiore giuseppe...@gmail.com writes:

 Hi! Can anyone tell me why this code does not work? I cannot seem to
 figure why it is broken...

The error message (and how you got it) would help...

 {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
   UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables,
   TypeOperators, TypeSynonymInstances #-}

You sure you have enough language extensions there? ;-)

-- 
Ivan Lazar Miljenovic
ivan.miljeno...@gmail.com
IvanMiljenovic.wordpress.com
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Re: [Haskell-cafe] Code that doesn't compile - but should :)

2010-03-16 Thread Giuseppe Maggiore 
The error message (obtained by loading the file with ghci) is:

GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main (
C:\Users\pulcy\Desktop\Papers\Monads\Objec
tiveMonad\HObject\Experiments\FunctorsProblems.hs, interpreted )

C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
Problems.hs:18:15:
Could not deduce (Selector s (f a - a))
  from the context (Selector s (a1 - f1 a1), Converter a1 f1)
  arising from a use of `select'
   at
C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObjec
t\Experiments\FunctorsProblems.hs:18:15-22
Possible fix:
  add (Selector s (f a - a)) to the context of
the type signature for `f'
  or add an instance declaration for (Selector s (f a - a))
In the expression: select s
In the definition of `method': method = select s
In the expression:
let method = select s in (\ x - let res = ... in convert res)

C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
Problems.hs:21:11:
Couldn't match expected type `a1' against inferred type `f a'
  `a1' is a rigid type variable bound by
   the type signature for `f'
 at
C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Expe
riments\FunctorsProblems.hs:16:18
In the expression: convert res
In the expression: let res = method x in convert res
In the expression: (\ x - let res = method x in convert res)
Failed, modules loaded: none.
Prelude

On Tue, Mar 16, 2010 at 2:31 AM, Ivan Lazar Miljenovic 
ivan.miljeno...@gmail.com wrote:

 Giuseppe Maggiore giuseppe...@gmail.com writes:

  Hi! Can anyone tell me why this code does not work? I cannot seem to
  figure why it is broken...

 The error message (and how you got it) would help...

  {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
 FlexibleInstances,
UndecidableInstances, FlexibleContexts, EmptyDataDecls,
 ScopedTypeVariables,
TypeOperators, TypeSynonymInstances #-}

 You sure you have enough language extensions there? ;-)


Barely :)



 --
 Ivan Lazar Miljenovic
 ivan.miljeno...@gmail.com
 IvanMiljenovic.wordpress.com http://ivanmiljenovic.wordpress.com/




-- 
Giuseppe Maggiore
Ph.D. Student (Languages and Games)
Microsoft Student Partner
Mobile: +393319040031
Office: +390412348444
___
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Re: [Haskell-cafe] Code that doesn't compile - but should :)

2010-03-16 Thread Chris Eidhof 
What about this?

 {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
  UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables,
  TypeOperators, TypeSynonymInstances #-}
 
 data Data k = Pair Integer (() - k)
 data RecData = RecData (Data RecData)
 mk_data x = RecData(Pair x (\() - mk_data (x+1)))
 

The I had to change the type of the Converter typeclass

 class Converter a f where convert :: f a - a
 
 -- instance Converter RecData Data where
 --  convert (RecData r) = r
 
 class Selector s a where select :: s - a
 

And explicitly quantify the type variables:

 f :: forall f s a . (Selector s (a-f a), Converter a f) = s - (a-a)
 f s =
  let method = select s :: a - f a
  in (\x -
let res = method x
in convert res)

-chris

On 16 mrt 2010, at 10:36, Giuseppe Maggiore wrote:

 The error message (obtained by loading the file with ghci) is:
 GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Main ( 
 C:\Users\pulcy\Desktop\Papers\Monads\Objec
 tiveMonad\HObject\Experiments\FunctorsProblems.hs, interpreted )
 
 C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
 Problems.hs:18:15:
 Could not deduce (Selector s (f a - a))
   from the context (Selector s (a1 - f1 a1), Converter a1 f1)
   arising from a use of `select'
at 
 C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObjec
 t\Experiments\FunctorsProblems.hs:18:15-22
 Possible fix:
   add (Selector s (f a - a)) to the context of
 the type signature for `f'
   or add an instance declaration for (Selector s (f a - a))
 In the expression: select s
 In the definition of `method': method = select s
 In the expression:
 let method = select s in (\ x - let res = ... in convert res)
 
 C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
 Problems.hs:21:11:
 Couldn't match expected type `a1' against inferred type `f a'
   `a1' is a rigid type variable bound by
the type signature for `f'
  at 
 C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Expe
 riments\FunctorsProblems.hs:16:18
 In the expression: convert res
 In the expression: let res = method x in convert res
 In the expression: (\ x - let res = method x in convert res)
 Failed, modules loaded: none.
 Prelude
 
  
 On Tue, Mar 16, 2010 at 2:31 AM, Ivan Lazar Miljenovic 
 ivan.miljeno...@gmail.com wrote:
 Giuseppe Maggiore giuseppe...@gmail.com writes:
 
  Hi! Can anyone tell me why this code does not work? I cannot seem to
  figure why it is broken...
 
 The error message (and how you got it) would help...
 
  {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, 
  FlexibleInstances,
UndecidableInstances, FlexibleContexts, EmptyDataDecls, 
  ScopedTypeVariables,
TypeOperators, TypeSynonymInstances #-}
 
 You sure you have enough language extensions there? ;-)
  
 Barely :)
  
 
 --
 Ivan Lazar Miljenovic
 ivan.miljeno...@gmail.com
 IvanMiljenovic.wordpress.com
 
 
 
 -- 
 Giuseppe Maggiore
 Ph.D. Student (Languages and Games)
 Microsoft Student Partner
 Mobile: +393319040031
 Office: +390412348444
 
 
 ___
 Haskell-Cafe mailing list
 Haskell-Cafe@haskell.org
 http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Code that doesn't compile - but should :)

2010-03-16 Thread Giuseppe Maggiore 
Well, first of all thanks!

Second, why the need for explicit quantification?
On Tue, Mar 16, 2010 at 2:39 AM, Chris Eidhof ch...@eidhof.nl wrote:

 What about this?

  {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
 FlexibleInstances,
   UndecidableInstances, FlexibleContexts, EmptyDataDecls,
 ScopedTypeVariables,
   TypeOperators, TypeSynonymInstances #-}
 
  data Data k = Pair Integer (() - k)
  data RecData = RecData (Data RecData)
  mk_data x = RecData(Pair x (\() - mk_data (x+1)))
 

 The I had to change the type of the Converter typeclass

  class Converter a f where convert :: f a - a
 
  -- instance Converter RecData Data where
  --  convert (RecData r) = r
 
  class Selector s a where select :: s - a
 

 And explicitly quantify the type variables:

  f :: forall f s a . (Selector s (a-f a), Converter a f) = s - (a-a)
  f s =
   let method = select s :: a - f a
   in (\x -
 let res = method x
 in convert res)

 -chris

 On 16 mrt 2010, at 10:36, Giuseppe Maggiore wrote:

  The error message (obtained by loading the file with ghci) is:
  GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
  Loading package ghc-prim ... linking ... done.
  Loading package integer ... linking ... done.
  Loading package base ... linking ... done.
  [1 of 1] Compiling Main (
 C:\Users\pulcy\Desktop\Papers\Monads\Objec
  tiveMonad\HObject\Experiments\FunctorsProblems.hs, interpreted )
 
 
 C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
  Problems.hs:18:15:
  Could not deduce (Selector s (f a - a))
from the context (Selector s (a1 - f1 a1), Converter a1 f1)
arising from a use of `select'
 at
 C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObjec
  t\Experiments\FunctorsProblems.hs:18:15-22
  Possible fix:
add (Selector s (f a - a)) to the context of
  the type signature for `f'
or add an instance declaration for (Selector s (f a - a))
  In the expression: select s
  In the definition of `method': method = select s
  In the expression:
  let method = select s in (\ x - let res = ... in convert res)
 
 
 C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
  Problems.hs:21:11:
  Couldn't match expected type `a1' against inferred type `f a'
`a1' is a rigid type variable bound by
 the type signature for `f'
   at
 C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Expe
  riments\FunctorsProblems.hs:16:18
  In the expression: convert res
  In the expression: let res = method x in convert res
  In the expression: (\ x - let res = method x in convert res)
  Failed, modules loaded: none.
  Prelude
 
 
  On Tue, Mar 16, 2010 at 2:31 AM, Ivan Lazar Miljenovic 
 ivan.miljeno...@gmail.com wrote:
  Giuseppe Maggiore giuseppe...@gmail.com writes:
 
   Hi! Can anyone tell me why this code does not work? I cannot seem to
   figure why it is broken...
 
  The error message (and how you got it) would help...
 
   {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
 FlexibleInstances,
 UndecidableInstances, FlexibleContexts, EmptyDataDecls,
 ScopedTypeVariables,
 TypeOperators, TypeSynonymInstances #-}
 
  You sure you have enough language extensions there? ;-)
 
  Barely :)
 
 
  --
  Ivan Lazar Miljenovic
  ivan.miljeno...@gmail.com
  IvanMiljenovic.wordpress.com http://ivanmiljenovic.wordpress.com/
 
 
 
  --
  Giuseppe Maggiore
  Ph.D. Student (Languages and Games)
  Microsoft Student Partner
  Mobile: +393319040031
  Office: +390412348444
 
 
  ___
  Haskell-Cafe mailing list
  Haskell-Cafe@haskell.org
  http://www.haskell.org/mailman/listinfo/haskell-cafe




-- 
Giuseppe Maggiore
Ph.D. Student (Languages and Games)
Microsoft Student Partner
Mobile: +393319040031
Office: +390412348444
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
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Re: [Haskell-cafe] Code that doesn't compile - but should :)

2010-03-16 Thread Chris Eidhof 
On 16 mrt 2010, at 10:58, Giuseppe Maggiore wrote:

 Well, first of all thanks!
 Second, why the need for explicit quantification?

I'm not sure, but I think it has to do with ambiguity. I think it's similar to 
the problem:

 readShow :: (Read a, Show a) - String - String
 readShow = show . read

We need to explicitly quantify over the type variables so that we can give an 
explicit type signature on the following line:

 let method = select s :: a - f a

There might be an easier way to do this, but I'm not sure how exactly.

-chris


 
 On Tue, Mar 16, 2010 at 2:39 AM, Chris Eidhof ch...@eidhof.nl wrote:
 What about this?
 
  {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, 
  FlexibleInstances,
   UndecidableInstances, FlexibleContexts, EmptyDataDecls, 
  ScopedTypeVariables,
   TypeOperators, TypeSynonymInstances #-}
 
  data Data k = Pair Integer (() - k)
  data RecData = RecData (Data RecData)
  mk_data x = RecData(Pair x (\() - mk_data (x+1)))
 
 
 The I had to change the type of the Converter typeclass
 
  class Converter a f where convert :: f a - a
 
  -- instance Converter RecData Data where
  --  convert (RecData r) = r
 
  class Selector s a where select :: s - a
 
 
 And explicitly quantify the type variables:
 
  f :: forall f s a . (Selector s (a-f a), Converter a f) = s - (a-a)
  f s =
   let method = select s :: a - f a
   in (\x -
 let res = method x
 in convert res)
 
 -chris
 
 On 16 mrt 2010, at 10:36, Giuseppe Maggiore wrote:
 
  The error message (obtained by loading the file with ghci) is:
  GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
  Loading package ghc-prim ... linking ... done.
  Loading package integer ... linking ... done.
  Loading package base ... linking ... done.
  [1 of 1] Compiling Main ( 
  C:\Users\pulcy\Desktop\Papers\Monads\Objec
  tiveMonad\HObject\Experiments\FunctorsProblems.hs, interpreted )
 
  C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
  Problems.hs:18:15:
  Could not deduce (Selector s (f a - a))
from the context (Selector s (a1 - f1 a1), Converter a1 f1)
arising from a use of `select'
 at 
  C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObjec
  t\Experiments\FunctorsProblems.hs:18:15-22
  Possible fix:
add (Selector s (f a - a)) to the context of
  the type signature for `f'
or add an instance declaration for (Selector s (f a - a))
  In the expression: select s
  In the definition of `method': method = select s
  In the expression:
  let method = select s in (\ x - let res = ... in convert res)
 
  C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
  Problems.hs:21:11:
  Couldn't match expected type `a1' against inferred type `f a'
`a1' is a rigid type variable bound by
 the type signature for `f'
   at 
  C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Expe
  riments\FunctorsProblems.hs:16:18
  In the expression: convert res
  In the expression: let res = method x in convert res
  In the expression: (\ x - let res = method x in convert res)
  Failed, modules loaded: none.
  Prelude
 
 
  On Tue, Mar 16, 2010 at 2:31 AM, Ivan Lazar Miljenovic 
  ivan.miljeno...@gmail.com wrote:
  Giuseppe Maggiore giuseppe...@gmail.com writes:
 
   Hi! Can anyone tell me why this code does not work? I cannot seem to
   figure why it is broken...
 
  The error message (and how you got it) would help...
 
   {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, 
   FlexibleInstances,
 UndecidableInstances, FlexibleContexts, EmptyDataDecls, 
   ScopedTypeVariables,
 TypeOperators, TypeSynonymInstances #-}
 
  You sure you have enough language extensions there? ;-)
 
  Barely :)
 
 
  --
  Ivan Lazar Miljenovic
  ivan.miljeno...@gmail.com
  IvanMiljenovic.wordpress.com
 
 
 
  --
  Giuseppe Maggiore
  Ph.D. Student (Languages and Games)
  Microsoft Student Partner
  Mobile: +393319040031
  Office: +390412348444
 
 
  ___
  Haskell-Cafe mailing list
  Haskell-Cafe@haskell.org
  http://www.haskell.org/mailman/listinfo/haskell-cafe
 
 
 
 -- 
 Giuseppe Maggiore
 Ph.D. Student (Languages and Games)
 Microsoft Student Partner
 Mobile: +393319040031
 Office: +390412348444
 
 

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