purescript-infinite-lists

Getting back unsafe head's type, safely

This library defines a type for strictly infinite (but lazy) lists.

As such, some of the Data.List API is not implemented here -- anything that relies on finiteness, for example -- while other functions get simplified types.

Also note that equality of countable sets is undecidable in general, but inequality can be determined after a finite number of steps. As such using eq and compare may not terminate.

For example:

head :: forall a. List a -> a
tail :: forall a. List a -> List a

are now totally safe.

Data.List.Infinite is meant to be imported qualified, as almost all its exposed functions clash with other modules.

Note also that the Semigroup instance provided is not extended by the finite version.

bower i --save purescript-infinite-lists

It's amusing that, had they been provided, length could be given in O(1) time instead of O(n), and last could be given a simple definition, retaining the same type signature: last _ = Nothing.