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Today's Topics:
1. fonction in a typeclass that does not mention the type
variable (TP)
2. Re: cabal package haskore-vintage fails to build (Brent Yorgey)
3. Re: fonction in a typeclass that does not mention the type
variable (Peter Hall)
4. Re: fonction in a typeclass that does not mention the type
variable (Peter Jones)
5. Re: fonction in a typeclass that does not mention the type
variable (Peter Jones)
6. Re: fonction in a typeclass that does not mention the type
variable (TP)
7. Re: fonction in a typeclass that does not mention the type
variable (TP)
----------------------------------------------------------------------
Message: 1
Date: Wed, 21 Aug 2013 18:39:33 +0200
From: TP <paratribulati...@free.fr>
To: beginners@haskell.org
Subject: [Haskell-beginners] fonction in a typeclass that does not
mention the type variable
Message-ID: <537fea-mm5.ln1@rama.universe>
Content-Type: text/plain; charset="ISO-8859-1"
Hi,
I have a simple test case containing a typeclass Foo with one type variable
a. The goal is to write once and for all a function symbolToInfixLevel
function that combines two other functions defined in the typeclass:
-----------------------
class Foo a where
symbolToConstructor :: String -> ( a -> a -> a )
infixLevel :: a -> Int
symbolToInfixLevel :: String -> Int
symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined
undefined
-----------------------
This yields an error because there is no "a" in the type signature for
symbolToInfixLevel:
$ runghc test_typeclass_without_typevariable.hs
test_typeclass_without_typevariable.hs:1:1:
The class method `symbolToInfixLevel'
mentions none of the type variables of the class Foo a
When checking the class method: symbolToInfixLevel :: String -> Int
In the class declaration for `Foo'
Now, if I define symbolToInfixLevel out of the typeclass:
-----------------------
class Foo a where
symbolToConstructor :: String -> ( a -> a -> a )
infixLevel :: a -> Int
symbolToInfixLevel :: String -> Int
symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined
undefined
-----------------------
Now, I obtain:
$ runghc test_typeclass_without_typevariable.hs
test_typeclass_without_typevariable.hs:7:24:
No instance for (Foo a0) arising from a use of `infixLevel'
In the expression: infixLevel
In the expression:
infixLevel $ (symbolToConstructor s) undefined undefined
In an equation for `symbolToInfixLevel':
symbolToInfixLevel s
= infixLevel $ (symbolToConstructor s) undefined undefined
How to get rid from this situation?
Thanks in advance,
TP
------------------------------
Message: 2
Date: Wed, 21 Aug 2013 13:01:01 -0400
From: Brent Yorgey <byor...@seas.upenn.edu>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] cabal package haskore-vintage fails
to build
Message-ID: <20130821170101.ga8...@seas.upenn.edu>
Content-Type: text/plain; charset=iso-8859-1
It also uses the cached version of the tar file which is already on
your disk, if you have previously installed the package, so it does
not even need to download anything.
-Brent
On Wed, Aug 21, 2013 at 01:00:12PM +0200, Sylvain HENRY wrote:
> It may be a bit quicker as it downloads and unpacks the package
> automatically.
>
> > cabal unpack haskore-vintage
> > cd haskore-vintage-0.1
> > vim src/Haskore/Utils.hs
> > cabal install
>
> Le 21/08/2013 09:58, Dimitri Hendriks a ?crit :
> >@Sylvain: why is cabal unpack better than tar zxvf ?
> >
> >
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 3
Date: Wed, 21 Aug 2013 18:05:42 +0100
From: Peter Hall <peter.h...@memorphic.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] fonction in a typeclass that does not
mention the type variable
Message-ID:
<CAA6hAk495VcVq2oSMfs6LhN4ARoSMAF=nokcnpnmkp28kgs...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Maybe I don't fully understand what you are trying to achieve, but I don't
think what you are trying to do makes sense. infixLevel and
symbolToConstructor will have different implementations for each instance
of Foo, and the compiler cannot possibly know which implementations you
mean. If the implementation of symbolToInfixLevel is independent of the
implementations of infixLevel and symbolToConstructor then it shouldn't
need them in its definition.
Peter
On 21 August 2013 17:39, TP <paratribulati...@free.fr> wrote:
> Hi,
>
> I have a simple test case containing a typeclass Foo with one type variable
> a. The goal is to write once and for all a function symbolToInfixLevel
> function that combines two other functions defined in the typeclass:
>
> -----------------------
> class Foo a where
>
> symbolToConstructor :: String -> ( a -> a -> a )
> infixLevel :: a -> Int
>
> symbolToInfixLevel :: String -> Int
> symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined
> undefined
> -----------------------
>
> This yields an error because there is no "a" in the type signature for
> symbolToInfixLevel:
>
> $ runghc test_typeclass_without_typevariable.hs
> test_typeclass_without_typevariable.hs:1:1:
> The class method `symbolToInfixLevel'
> mentions none of the type variables of the class Foo a
> When checking the class method: symbolToInfixLevel :: String -> Int
> In the class declaration for `Foo'
>
> Now, if I define symbolToInfixLevel out of the typeclass:
>
> -----------------------
> class Foo a where
>
> symbolToConstructor :: String -> ( a -> a -> a )
> infixLevel :: a -> Int
>
> symbolToInfixLevel :: String -> Int
> symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined
> undefined
> -----------------------
>
> Now, I obtain:
>
> $ runghc test_typeclass_without_typevariable.hs
> test_typeclass_without_typevariable.hs:7:24:
> No instance for (Foo a0) arising from a use of `infixLevel'
> In the expression: infixLevel
> In the expression:
> infixLevel $ (symbolToConstructor s) undefined undefined
> In an equation for `symbolToInfixLevel':
> symbolToInfixLevel s
> = infixLevel $ (symbolToConstructor s) undefined undefined
>
> How to get rid from this situation?
>
> Thanks in advance,
>
> TP
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
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------------------------------
Message: 4
Date: Wed, 21 Aug 2013 11:20:30 -0600
From: Peter Jones <mli...@pmade.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] fonction in a typeclass that does not
mention the type variable
Message-ID: <87y57vdkpt....@pmade.com>
Content-Type: text/plain
TP <paratribulati...@free.fr> writes:
> symbolToInfixLevel :: String -> Int
> symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined
>
> Now, I obtain:
>
> $ runghc test_typeclass_without_typevariable.hs
> test_typeclass_without_typevariable.hs:7:24:
> No instance for (Foo a0) arising from a use of `infixLevel'
> In the expression: infixLevel
> In the expression:
> infixLevel $ (symbolToConstructor s) undefined undefined
> In an equation for `symbolToInfixLevel':
> symbolToInfixLevel s
> = infixLevel $ (symbolToConstructor s) undefined undefined
>
> How to get rid from this situation?
Your `symbolToConstructor' function is too specific for both cases you
describe. You'd need to change the `String' argument to `a' and put it
back in the type class or write it like this:
symbolToInfixLevel :: (Foo a) => a -> Int
symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined
--
Peter Jones, Founder, Devalot.com
Defending the honor of good code
------------------------------
Message: 5
Date: Wed, 21 Aug 2013 11:37:23 -0600
From: Peter Jones <mli...@pmade.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] fonction in a typeclass that does not
mention the type variable
Message-ID: <87siy3djxo....@pmade.com>
Content-Type: text/plain
Peter Jones <mli...@pmade.com> writes:
> Your `symbolToConstructor' function is too specific for both cases you
> describe. You'd need to change the `String' argument to `a' and put it
> back in the type class or write it like this:
>
> symbolToInfixLevel :: (Foo a) => a -> Int
> symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined
Actually, the problem is with your `symbolToConstructor' function. It
needs to look like this:
symbolToConstructor :: (Foo a) => String -> a
Then symbolToInfixLevel can be:
symbolToInfixLevel :: String -> Int
--
Peter Jones, Founder, Devalot.com
Defending the honor of good code
------------------------------
Message: 6
Date: Wed, 21 Aug 2013 21:44:43 +0200
From: TP <paratribulati...@free.fr>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] fonction in a typeclass that does not
mention the type variable
Message-ID: <buhfea-vh9.ln1@rama.universe>
Content-Type: text/plain; charset="ISO-8859-1"
Peter Hall wrote:
> Maybe I don't fully understand what you are trying to achieve, but I don't
> think what you are trying to do makes sense. infixLevel and
> symbolToConstructor will have different implementations for each instance
> of Foo, and the compiler cannot possibly know which implementations you
> mean.
Indeed, my question was stupid. This is obvious in the second implementation
(which instance to choose?), and it is also true in the first one because
even if symbolToInfixLevel of one considered instance uses infixLevel and
symbolToConstructor of this same instance (what I am not sure of), at the
location where I use this function the compiler cannot known which instance
I mean.
> If the implementation of symbolToInfixLevel is independent of the
> implementations of infixLevel and symbolToConstructor then it shouldn't
> need them in its definition.
No, they are not independent: it is really a function that "composes"
functions infixLevel and symbolToConstructor. See my proposition in my
answer to Peter Jones.
------------------------------
Message: 7
Date: Wed, 21 Aug 2013 21:43:33 +0200
From: TP <paratribulati...@free.fr>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] fonction in a typeclass that does not
mention the type variable
Message-ID: <5shfea-vh9.ln1@rama.universe>
Content-Type: text/plain; charset="ISO-8859-1"
Peter Jones wrote:
> Actually, the problem is with your `symbolToConstructor' function. It
> needs to look like this:
>
> symbolToConstructor :: (Foo a) => String -> a
>
> Then symbolToInfixLevel can be:
>
> symbolToInfixLevel :: String -> Int
>
Sorry, I have not been able to write an example with your proposition. Might
you give more informations?
In the meantime, I have been able to write a working example proposing two
variants:
------------------------------
{-# LANGUAGE ScopedTypeVariables #-}
data Exp = Plus Exp Exp | Minus Exp Exp | Atom String
deriving Show
class Foo a where
symbolToConstructor :: String -> ( a -> a -> a )
infixLevel :: a -> Int
-- The second argument is only used to indicate the type of the
-- intermediary result (i.e. the type of the result of
-- symbolToConstructor).
symbolToInfixLevel :: String -> a -> Int
symbolToInfixLevel s u =
infixLevel $ (((symbolToConstructor s) undefined undefined)::a)
instance Foo Exp where
symbolToConstructor e = case e of
"+" -> Plus
"-" -> Minus
infixLevel e = case e of
Plus _ _ -> 6
Minus _ _ -> 7
main = do
-- First variant
print $ infixLevel $ (((symbolToConstructor "+") undefined undefined)::Exp)
print $ infixLevel $ (((symbolToConstructor "-") undefined undefined)::Exp)
-- Second variant
print $ symbolToInfixLevel "+" (undefined::Exp)
print $ symbolToInfixLevel "-" (undefined::Exp)
------------------------------
------------------------------
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