Question

I’ve written some Haskell code which needs -XUndecidableInstances to compile. I do understand why that happens, that there is a certain condition which is violated and therefore GHC yells.

However, I’ve never come across a situation where the type checker would actually hang or wind up in an endless loop.

What does a non-terminating instance definition look like – can you give an example?

Answer 1

For example:

{-# LANGUAGE UndecidableInstances #-}

class C a where
    f :: a -> a

instance C [[a]] => C [a] where
    f = id

g x = x + f [x]

What is happening: When typing f [x] the compiler needs to ensure that x :: C [a] for some a. The instance declaration says that x can be of type C [a] only if it is also of type C [[a]]. So the compiler needs to ensure that x :: C [[a]], etc. and gets caught in an infinite loop.

See also Can using UndecidableInstances pragma locally have global consequences on compilation termination?