Promise.cast and Promise.resolve

Quildreen Motta quildreen at
Wed Jan 29 06:39:55 PST 2014

On 29 January 2014 10:45, Paolo Amadini <paolo.02.prg at> wrote:

> On 29/01/2014 5.12, Kris Kowal wrote:
> > In this case, a half pursuit of type purity is a side quest at the
> > expense of users. Having two ways to resolve and two ways to observe a
> > promise is unnecessarily confusing. In my experience, one method like
> > "then", that unwraps recursively, and one function, like "Promise.cast",
> > that automatically lifts if necessary, and "then" handlers that return
> > into the waiting hands of "Promise.cast" are coherent and ergonomic.
> > Having a choice between "cast" and "resolve" and a choice between "then"
> > and "chain", will leave developers unnecessarily confused and worried
> > all the while they use or abandon Promises as too subtle.
> As an imperative programmer, I confirm I'm left worried and confused ;-)
> But I understand that functional programming might need more complexity.

It is, actually, more simplicity. The concept of `Promise.resolve` and
`Promise.chain` is simpler than `Promise.cast` and `Promise.then` (i.e.:
they represent orthogonal concepts, not "complected"). `Promise.cast` and
`Promise.then` may be, arguably, *easier* to work with, from a user POV,
since you don't need to make as many choices. I would argue that it would
make more sense to write `cast` and `then` in terms of `resolve` and
`chain`, however. But this seems to have already been decided.

Answering your previous question:

var p1 = Promise.resolve(Promise.cast(1));
var p2 = Promise.cast(Promise.cast(1));

`p1` will be a `Promise(Promise(1))`, whereas `p2` will be `Promise(1)` —
IOW, `Promise.resolve` will just put anything inside of a `Promise`,
regardless of that being a promise or a regular value, whereas
`Promise.cast` will put the innermost value that is not a promise inside of
a promise.

var p3 = new Promise(resolve => resolve(Promise.cast(1));
// is the same as:
var p3 = Promise.resolve(Promise.cast(1))

Quildreen "(Soreλ\a)" Motta
*— JavaScript Alchemist / Minimalist Designer / PLT hobbyist —*
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