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
nameBase:
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
declaration:
infix $([| j |]) +
I obtain:
parse error on input `$('
What is the problem?
Thanks in advance,
TP
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