What is the role of 'bottom' (⊥) in Haskell function definitions?

Question

I don\'t understand the role played by bottom (⊥ or _|_) in Haskell function definitions.

The definition of zip for example describes it as \"r

Answer 1

Riffing on AJFarmar's answer, I think this critical point was not made explicit:

• _|_ is not a valid literal or identifier in Haskell code!
• And therefore, zip [] _|_ = [] isn't valid code either!

That is implicit in what AJFarmar means by this quote:

[T]he line zip [] _|_ = [] does not act as a pattern match but an equation, stating the equality of zip [] _|_ and [].

To make it very crystal clear, zip [] _|_ = [] appears in the documentation comment for the definition of zip. It's not Haskell code—it's an English-language comment written in an informal technical notation that looks a little bit like Haskell code. Or, in other words, pseudo-code.