how does 'undefined' work in Haskell

Question

I\'m curious about the \'undefined\' value in Haskell. Its interesting because you can put it just about anywhere, and Haskell will be happy. The following are all a-ok

Answer 1

There are two separated things to note about undefined:

• You can put undefined almost everywhere and typechecker will be happy. It's because undefined have type (forall a. a).
• You can put undefined almost everywhere and it will have some well defined representation at run time.

For the second, GHC commentary clearly says:

The representation of ⊥ must be a pointer: it is an object that when evaluated throws an exception or enters an infinite loop.

For more details you might want to read the paper Spinless Tagless G-Machine.