[Haskell-cafe] typeclass constraints

23 Aug 2013

      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