Mu in the same file

src/Main.hs

{-# LANGUAGE DeriveFunctor #-}

module Main where

newtype Mu f = Mu (f (Mu f))
newtype K a b = K a
newtype F a = F (Mu (K a)) deriving Functor

main :: IO ()
main = return ()

$ cabal build
Building panic-0.1.0.0...
Preprocessing executable 'panic' for panic-0.1.0.0...
[1 of 1] Compiling Main             ( src/Main.hs, dist/build/panic/panic-tmp/Main.o )

src/Main.hs:7:37:
    No instance for (Functor Mu)
      arising from the first field of ‘F’ (type ‘Mu (K a)’)
    Possible fix:
      use a standalone 'deriving instance' declaration,
        so you can specify the instance context yourself
    When deriving the instance for (Functor F)

Mu in a separate file

src/Mu.hs

module Mu where

newtype Mu f = Mu (f (Mu f))

src/Main.hs

{-# LANGUAGE DeriveFunctor #-}

module Main where

import Mu

newtype K a b = K a
newtype F a = F (Mu (K a)) deriving Functor

main :: IO ()
main = return ()

$ cabal build
Building panic-0.1.0.0...
Preprocessing executable 'panic' for panic-0.1.0.0...
[1 of 2] Compiling Mu               ( src/Mu.hs, dist/build/panic/panic-tmp/Mu.o )
[2 of 2] Compiling Main             ( src/Main.hs, dist/build/panic/panic-tmp/Main.o )

src/Main.hs:8:37:ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.2 for x86_64-unknown-linux):
	Prelude.(!!): index too large


Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug