[Haskell-cafe] typeclass constraints

2013-08-23 Thread TP 
Hi everybody,


There is something I do not understand in the way typeclass constraints are 
inferred.


1/ Take the following function definition:

sum' [] = []
sum' (x:xs) = x + sum' xs

GHCI correctly gives:

 :t sum'
sum' :: Num [a] = [[a]] - [a]

So it has inferred that the type list has to be an instance of Num for sum' 
to be able to work. It will give an error if we try to use sum' without 
implementing the instance.


2/ Now, take the following definition:


{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH
import Language.Haskell.TH.Syntax

p :: a - ExpQ
p n = [| show n |]


We obtain an error if we try to load it in GHCI:

No instance for (Lift a) arising from a use of `n'
Possible fix:
  add (Lift a) to the context of
the type signature for p :: a - ExpQ
In the first argument of `show', namely `n'
In the Template Haskell quotation [| show n |]
In the expression: [| show n |]

And indeed, if we use instead:


{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH
import Language.Haskell.TH.Syntax

p :: Lift a = a - ExpQ
p n = [| show n |]


it works correctly.


Why GHC is able to infer the typeclass constraint (Num a) in 1/, but not 
(Lift a) in 2/?


Thanks in advance,

TP


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Re: [Haskell-cafe] typeclass constraints

2013-08-23 Thread Adam Gundry 
Hi TP,

The difference is that in your second example, you have specified the
type signature

p :: a - ExpQ

so GHC checks whether p has this type, and correctly objects that it
doesn't. If you leave off the type signature, as you did for sum', the
right thing will be inferred.

Hope this helps,

Adam


On 23/08/13 10:23, TP wrote:
 Hi everybody,
 
 
 There is something I do not understand in the way typeclass constraints are 
 inferred.
 
 
 1/ Take the following function definition:
 
 sum' [] = []
 sum' (x:xs) = x + sum' xs
 
 GHCI correctly gives:
 
 :t sum'
 sum' :: Num [a] = [[a]] - [a]
 
 So it has inferred that the type list has to be an instance of Num for sum' 
 to be able to work. It will give an error if we try to use sum' without 
 implementing the instance.
 
 
 2/ Now, take the following definition:
 
 
 {-# LANGUAGE TemplateHaskell #-}
 
 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax
 
 p :: a - ExpQ
 p n = [| show n |]
 
 
 We obtain an error if we try to load it in GHCI:
 
 No instance for (Lift a) arising from a use of `n'
 Possible fix:
   add (Lift a) to the context of
 the type signature for p :: a - ExpQ
 In the first argument of `show', namely `n'
 In the Template Haskell quotation [| show n |]
 In the expression: [| show n |]
 
 And indeed, if we use instead:
 
 
 {-# LANGUAGE TemplateHaskell #-}
 
 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax
 
 p :: Lift a = a - ExpQ
 p n = [| show n |]
 
 
 it works correctly.
 
 
 Why GHC is able to infer the typeclass constraint (Num a) in 1/, but not 
 (Lift a) in 2/?
 
 
 Thanks in advance,
 
 TP
 
 
 ___
 Haskell-Cafe mailing list
 Haskell-Cafe@haskell.org
 http://www.haskell.org/mailman/listinfo/haskell-cafe
 


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Re: [Haskell-cafe] typeclass constraints

2013-08-23 Thread Ivan Lazar Miljenovic 
On 23 August 2013 19:23, TP paratribulati...@free.fr wrote:
 Hi everybody,


 There is something I do not understand in the way typeclass constraints are
 inferred.


 1/ Take the following function definition:

 sum' [] = []
 sum' (x:xs) = x + sum' xs

You haven't specified a type signature here, so GHC will derive the
most generic one possible.


 GHCI correctly gives:

 :t sum'
 sum' :: Num [a] = [[a]] - [a]

 So it has inferred that the type list has to be an instance of Num for sum'
 to be able to work. It will give an error if we try to use sum' without
 implementing the instance.


 2/ Now, take the following definition:

 
 {-# LANGUAGE TemplateHaskell #-}

 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax

 p :: a - ExpQ
 p n = [| show n |]

You have defined a type signature here, so GHC will try to use it...
except (as you've noted) GHC will then complain that it's wrong.

If GHC auto-magically fixed incorrect type signatures, then one of the
major advantages of the type system (i.e. specify a type for a
function and then use that to guarantee that the function matches the
specification of what we wanted) will no longer be valid.

 

 We obtain an error if we try to load it in GHCI:

 No instance for (Lift a) arising from a use of `n'
 Possible fix:
   add (Lift a) to the context of
 the type signature for p :: a - ExpQ
 In the first argument of `show', namely `n'
 In the Template Haskell quotation [| show n |]
 In the expression: [| show n |]

 And indeed, if we use instead:

 
 {-# LANGUAGE TemplateHaskell #-}

 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax

 p :: Lift a = a - ExpQ
 p n = [| show n |]
 

 it works correctly.


 Why GHC is able to infer the typeclass constraint (Num a) in 1/, but not
 (Lift a) in 2/?


 Thanks in advance,

 TP


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 Haskell-Cafe@haskell.org
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-- 
Ivan Lazar Miljenovic
ivan.miljeno...@gmail.com
http://IvanMiljenovic.wordpress.com

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Re: [Haskell-cafe] typeclass constraints

2013-08-23 Thread TP 
Adam Gundry wrote:

 If you leave off the type signature, as you did for sum', the
 right thing will be inferred.

Thanks Adam and Ivan. Very stupid question...

TP


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