Undefined at the type level

Question

Often when I\'m playing with Haskell code, I stub things out with a type annotation and undefined.

foo :: String -> Int
foo = undefined


        

Answer 1

This question was asked and answered a long time ago; best practices have evolved since.

These days, instead of undefined, for stubbing out code you should be using typed holes, and their type-level analogue, partial type signatures.

Answer 2

You can use EmptyDataDecls to stub out a type, and with KindSignatures you can give it a kind:

{-# LANGUAGE EmptyDataDecls, KindSignatures #-}

data Foo :: * -> *

You can also stub out the Monad instance without warnings with this option to GHC.

{-# OPTIONS_GHC -fno-warn-missing-methods #-}

instance Monad Foo

And then you don't need to leave any implementation for return and >>=.