Hidden features of Haskell [closed]

Answer 1

My brain just exploded

If you try to compile this code:

{-# LANGUAGE ExistentialQuantification #-}
data Foo = forall a. Foo a
ignorefoo f = 1 where Foo a = f

You will get this error message:

$ ghc Foo.hs

Foo.hs:3:22:
    My brain just exploded.
    I can't handle pattern bindings for existentially-quantified constructors.
    Instead, use a case-expression, or do-notation, to unpack the constructor.
    In the binding group for
        Foo a
    In a pattern binding: Foo a = f
    In the definition of `ignorefoo':
        ignorefoo f = 1
                    where
                        Foo a = f