Hidden features of Haskell -

what lesser-known useful features of haskell programming language. (i understand language lesser-known, work me. explanations of simple things in haskell, defining fibonacci sequence 1 line of code, upvoted me.)

• try limit answers haskell core
• one feature per answer
• give example , short description of feature, not link documentation
• label feature using bold title first line

my brain exploded

if try compile code:

{-# language existentialquantification #-} data foo = forall a. foo ignorefoo f = 1 foo = f 

you error message:

$ ghc foo.hs  foo.hs:3:22:     brain exploded.     can't handle pattern bindings existentially-quantified constructors.     instead, use case-expression, or do-notation, unpack constructor.     in binding group         foo     in pattern binding: foo = f     in definition of `ignorefoo':         ignorefoo f = 1                                             foo = f