{-# LANGUAGE CPP #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
import Data.Kind
import Data.Type.Equality
import GHC.TypeLits
#define N 2
type family V (e :: Nat) :: Constraint where
V (e :: Nat) = (e `CmpNat` N ~ 'EQ, e `CmpNat` N ~ 'LT)
fails to generate a correctly expanded output, thus causing compilation failure down the line.
The relevant part of the expanded output is:
type family V (e :: Nat) :: Constraint where
V (e :: Nat) = (e `CmpNat` 2 ~ 'EQ, e `CmpNat` N ~ 'LT)
Notice that CPPHS substituted for the first occurrence of N, but not the second. Most likely it's getting confused by the occurrence of two ' characters, and leaving whatever is inside untouched. But that's not a valid Haskell character, so I would've expected this to work correctly.
There's a simple work-around by putting the second part in a line by itself:
V (e :: Nat) = ( e `CmpNat` N ~ 'EQ
, e `CmpNat` N ~ 'LT)
in which case the expansion correctly replaces N in both lines.
So, there's a work around; but would be nice if it wasn't needed.
This program:
fails to generate a correctly expanded output, thus causing compilation failure down the line.
The relevant part of the expanded output is:
Notice that CPPHS substituted for the first occurrence of
N, but not the second. Most likely it's getting confused by the occurrence of two'characters, and leaving whatever is inside untouched. But that's not a valid Haskell character, so I would've expected this to work correctly.There's a simple work-around by putting the second part in a line by itself:
in which case the expansion correctly replaces
Nin both lines.So, there's a work around; but would be nice if it wasn't needed.