[Haskell-cafe] (Why) must this function remain nameless?

[Clinton Mead]

The following code (somewhat contrived) code compiles on GHC without issue,
and towards the end repeatedly uses the lambda function "(\fn (D x) -> f fn
x)". But when I simply try to give it a name, like so:

h = (\fn (D x) -> f fn x)

I then get a compile error.

How can I name this function, or must it remain forever nameless?

( Ideone link: http://ideone.com/mtuYnK )

---

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE NoMonomorphismRestriction #-}

import GHC.Exts (Constraint)

type family Parent a

class C a b where
  f :: b -> a -> String

instance (C (Parent a) b) => C a b where
  f _ _ = f (undefined :: b) (undefined :: Parent a)

data A1 = A1
data A2 = A2
data A3 = A3
data A4 = A4

type instance Parent A2 = A1
type instance Parent A3 = A2
type instance Parent A4 = A3

data F1 = F1
data F3 = F3

instance C A1 F1 where
  f _ _ = "F1"

instance C A3 F3 where
  f _ _ = "F3"

type family Constraints t a :: Constraint

data D t = forall a. (Constraints t a) => D a

type instance Constraints A1 a = (C a F1)
type instance Constraints A2 a = (C a F1, C a F3)
type instance Constraints A3 a = (C a F1, C a F3)
type instance Constraints A4 a = (C a F1, C a F3)

main =
  do
    putStrLn (f F1 A1)
    putStrLn (f F1 A2)
    putStrLn (f F1 A3)
    putStrLn (f F1 A4)
    putStrLn (f F3 A3)
    putStrLn (f F3 A4)
    putStrLn $ (\fn (D x) -> f fn x) F1 ((D A1) :: D A1)
    putStrLn $ (\fn (D x) -> f fn x) F1 ((D A2) :: D A1)
    putStrLn $ (\fn (D x) -> f fn x) F3 ((D A3) :: D A2)

-- h = (\fn (D x) -> f fn x)
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