Do the unthinkable: catch errors in pure code

Undefined.hs

-- Not for the weak at heart.
-- If you use this, you should feel bad about yourself!

import Control.Exception
import System.IO.Unsafe

-- Given a value, check if it has anything undefined in WNHF. If so, return the exception message, else the value
fromError :: a -> Either String a
fromError x = unsafePerformIO $ catch (Right x <$ evaluate x) getMessage
  where
    getMessage :: SomeException -> IO (Either String a)
    getMessage = pure . Left . show