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's nothing really special about undefined. It is just an exceptional value -- you could represent it with an infinite loop, or a crash, or a segfault. One way to write it is as a crash:

undefined :: a
undefined | False = undefined

Or a loop:

undefined = undefined

It is an exceptional value that can be an element of any type.

Since Haskell is lazy you can still use such values in computations. E.g.

 > length [undefined, undefined]
 2

But otherwise, it is just a natural consequence of polymorphism and non-strictness.