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)