Example project using a constraint to denote deferred computations.

I introduce a type class Defer; used to mark arguments which should be left unevaluated until needed.

Similar to types like => T in Scala.

The example scenario is that of findMap:

findMap :: forall f a b. Foldable f => (a -> Maybe b) -> f a -> Maybe b

This function applies a testing function a -> Maybe b to each a in an f a. The result is the first value constructed with Just, else Nothing.

Ideally we would like the computation to stop when we hit the first Just. But with the way things currently are, it doesn't.

I demonstrate a change to Semigroup and First which allows this behaviour.