Curry flavored promises

http://lhorie.github.io/mithril-blog/curry-flavored-promises.html

by lhorie • 12 years ago

28 17 12 years ago

lhorie.github.io

gberger• 12 years ago

    //here's an AJAX request and a chain of promises
    m.request({method: "GET", url: "/api/projects"})
        .then(console.log) // log all projects
        .then(pastItems) // filter past projects
        .then(console.log) // log past projects only

The calls to console.log will not work.

You need to do this:

    .then(function() { console.log(arguments); })

Or this:

    .then(console.log.bind(console))

gonzoisme• 12 years ago

Yeah, the example couldn't work. Also, isn't this an example of partial application, not currying?

ephemeralgomi• 12 years ago

Yup.

_ZeD_• 12 years ago

well, it depends on the console.log implementation :)

On firefox I get

    >>> console.log.call(undefined, "Hi")
    Hi

bendrucker3• 12 years ago

console.log is bound in Firefox and in Node. In all other browsers calling console.log.call(null) is a TypeError.

cwmma• 12 years ago

this

    console.log.bind(console)

doesn't work in old IE you need to do

    Function.prototype.bind.call(console.log, console)

anaphor2• 12 years ago

Currying is a functional programming technique that lets us pre-define arguments in a function

No it isn't, that's partial application. Currying is (lambda (a b c) ...) => (lambda (a) (lambda (b) (lambda (c) ...))) generalized to any number of parameters.

reddyonrails• 12 years ago

total_sum = total.curry

sum_of_cubes = total_sum.(lambda { |x| x^3} )

sum_of_cubes.(1,2)

In ruby way

bilalq• 12 years ago

Do these promises work differently from most other implementations? Just sticking a `.then(console.log)` seems like it would yield `undefined` to the next function in the promise chain, no?

esailija2• 12 years ago

Maybe the code was untested or only tested in firefox or node, where `console.log` is bound (which is sane default for singletons if you ask me)

bendrucker3• 12 years ago

It's true that console.log is bound in FF and Node but that s not the issue here. Passing console.log to then will cause the next handler to receive the return value of the console.log call, i.e. undefined.

phpnode• 12 years ago

agree, this sounds like a bug, console.log() returns undefined so that's what the next promise in the chain should receive.

lhorie• 12 years ago

Hi author here.

Thanks for the feedback! Just wanted to clarify some points:

I should probably have noted that the usage of console.log is illustrative and is typically only used for debugging (the idea being that console.log could be used in Firefox, or you could wrap it in a log helper function, or use alert or whatever).

Typically, in a Mithril app, you'd either assign the promise to a variable or bind to an m.prop at the end of the chain, as per the docs on web services ( http://lhorie.github.io/mithril/web-services.html )

Regarding console.log returning undefined: Mithril does deviate from A+ promises (again, as per the docs - http://lhorie.github.io/mithril/mithril.deferred.html ). The reason for the spec deviation is precisely to allow the use case of mixing procedural impurity into a chain (whereas the undefined case covered by the spec isn't really useful, given that undefined is the lack of a value, as opposed to null, which is a value with no meaning).

@anaphor: huh, reading about currying again, you're right. Little misunderstanding on my part. Sorry about that.

anaphor2• 12 years ago

That's okay, I've seen people with PhDs confuse them before, it's a common mistake, and currying gets you partial application "for free". Good article nevertheless!

Curry flavored promises

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