[Haskell-cafe] typeclass constraints

[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
> 
> 
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