[Haskell-cafe] some questions about Template Haskell

TP paratribulations at free.fr
Fri Jun 28 22:55:40 CEST 2013

Hi everybody,

I am trying to learn Template Haskell, and I have two independent questions.

1/ First, the following code (which is not in its final version, but it is a 
test) does not compile:

{-# LANGUAGE TemplateHaskell #-}
module Pr where
import Language.Haskell.TH

pr :: Name -> ExpQ
pr n = [| putStrLn $ (nameBase n) ++ " = " ++ show $(varE n) |]

I obtain:

No instance for (Lift Name) arising from a use of `n'
    Possible fix: add an instance declaration for (Lift Name)
    In the first argument of `nameBase', namely `n'

Why? Indeed, there is no typeclass constraint on n in the definition of 

ghci> :t nameBase
nameBase :: Name -> String

Contrary to lift for example:
ghci> :t lift
lift :: Lift t => t -> Q Exp

2/ If I define in a module:

j = 3

and then define in another module:

h x = $([|j|])
main = do
print $ h undefined

I obtain "3" as expected.

However, I do not achieve to make this system work with an infix 

infix $([| j |]) +

I obtain:

parse error on input `$('

What is the problem?

Thanks in advance,


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