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Comment by chowells | Hacker News Reader

I can't imagine using a haskell-like language without laziness. It's what makes it possible to actually write small reusable functions.

Tell me, have you ever used foldr in Purescript? It just doesn't lead to reusable logic there, so I have no idea why you would.

But in Haskell, foldr is used everywhere. Laziness means that logic built with it is actually reusable.

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Here is foldl implemented using foldr in PureScript, along with an example of using laziness to gain modularity:

http://try.purescript.org/?gist=63d81adf9f5257ffa4f576df2658...

> I can't imagine using a haskell-like language without laziness. It's what makes it possible to actually write small reusable functions.

I don't understand your point. Why is laziness a requirement to write small reusable functions? Are you thinking about currying?

OCaml is (relatively) similar to Haskell and is not lazy. Function currying does not require laziness.

It's unrelated to currying.

It's also hard to explain, but if you're used to Haskell, working with an eager-by-default language such as OCaml or ML is mildly annoying. You can adapt, of course, but it does seem as if gluing stuff together is trickier with eager evaluation.

There are downsides to lazy-by-default, of course.

There's a classic paper that answers this well: "Why Functional Programming Matters."

(Not a quick read, but not too huge, and it is a classic that is well worth reading sometime).

Folds are used in purescript all the time! Sure a foldr is less useful because it doesn't have the nice lazy preserving properties, but you can have strict left folds, which are tail recursive, and thus run in constant space.

Elm, another haskell-like, _strict_ language, models entire applications around the strict left fold over events https://guide.elm-lang.org/architecture/

We actually do the same in Jobmachine. The application is driven by a strict left fold over incoming events and current state.

The hard thing to swallow with purescript is whether row types are really worth the complexity they add.

I think it's a much easier pill to swallow than laziness.

There's a reason I said foldr. In Haskell, even foldl is implemented with foldr. It's that powerful of a tool. In contrast, foldl is far weaker, and necessarily strict in its input. It loses nothing moving to mandatory strictness. But foldr loses everything.

I don't really use foldr even in Haskell. It's almost always a performance problem.

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