Add functions to convert to / from a Promise to a LazyPromise

[thomashoneyman]

Description of the change

It can be useful to convert a Promise into a LazyPromise or vice versa -- for example, several other libraries like web-fetch can create Promise values, but to be inserted in a chain of binds with LazyPromise values you'll need to convert it first. This PR adds two helper functions for this purpose.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[thomashoneyman]

cc: @robertdp as you helped with this module -- let me know if you have any thoughts!

[thomashoneyman]

These two functions could also be renamed toLazyPromise :: Effect (Promise a) -> LazyPromise a and fromLazyPromise :: LazyPromise a -> Effect (Promise a) if desired. I'm open to other names.

[garyb]

I think the current names are good since they're in the Lazy module already - I assume the common usage would be something like: LazyPromise.toPromise, whereas I'd call them Promise.toLazyPromise etc. if they lived in the non-lazy module 🙂

[robertdp]

An unintuitive issue here is promise flattening. LazyPromise uses Box to add structure and prevent flattening. So removing it will reintroduce flattening.

[robertdp]

Does inferring the type for toPromise add the Flatten constraint?

[thomashoneyman]

It doesn't infer the constraint. Are you envisioning something like this implementation?

toPromise :: forall a. Flatten a b => LazyPromise a -> Effect (P.Promise b)
toPromise (LazyPromise p) = runEffectFn2 P.then_ (mkEffectFn1 \(Box b) -> pure (P.resolve b)) =<< p

or

toPromise :: forall a. Flatten a a => LazyPromise a -> Effect (P.Promise a)

I'm unfortunately not familiar with promise flattening 😬.

[robertdp]

Ah, I assumed the module P referred to Web.Promise. This is where the type-safe functions that model flattening are.

I would recommend using Web.Promise.then_ and Web.Promise.resolve for these new functions, instead of the internal unsafe functions.

[thomashoneyman]

Does this inferred type signature look correct to you?

toPromise :: forall a b c. Flatten a c => Flatten c b => LazyPromise a -> Effect (P.Promise b)
toPromise (LazyPromise p) = Promise.then_ (\(Box b) -> pure (Promise.resolve b)) =<< p

[robertdp]

The double flatten constraint looks redundant to me. It looks like the unnecessary one is coming from the Promise.then_ constraint. How about:

toPromise :: forall a b. Flatten a b => LazyPromise a -> Effect (P.Promise b)
toPromise (LazyPromise p) = runEffectFn2 P.then_ (mkEffectFn1 \(Box b) -> pure (Promise.resolve b)) =<< p

[robertdp]

I think this is now basically identical to your version here: https://github.com/purescript-web/purescript-web-promise/pull/12/files#r655780671

I reckon go with your implementation in the link, you had it right.

[thomashoneyman]

Mind taking another look at the latest?

[robertdp]

LGTM 👍