5 ms·

This code doesn't look right to me: // list :: [Promise a] -> Promise [a] var list = function(promises) { var listPromise = new Promise();

by graue 14y ago

This code doesn't look right to me: // list :: [Promise a] -> Promise [a] var list = function(promises) { var listPromise = new Promise(); for (var k in listPromise) promises[k] = listPromise[k]; Perhaps the assignment is supposed to be the other way around? for (var k in promises) listPromise[k] = promises[k];

aethertap 14y ago
I agree, I was going to ask the same question. Unless there's some subtlety that I'm missing, the order you propose makes much more sense.
ams6110 14y ago
I'm glad someone else thought that looked odd.
pyrtsa 14y ago
I asked the same question in Twitter. Turns out James was actually augmenting (i.e. modifying) the array object `promises` to behave as a promise itself. I don't think this was a particularly beautiful way of doing it but it seems to work now that I think of it. Promise libraries, like RSVP.js [1] he referred to, typically implement a way to construct a promise with a depends-on-many relationship, as a function possibly called `all([p1, p2, ...])` (with the same type signature as for `list`), `and(p1, p2, ...)` or something similar. IMO, defining the `list` function that way would've been clearer to the reader and more FP'ish, treating the `promises` argument in as a value and not a mutable object. [1]: https://github.com/tildeio/rsvp.js/blob/master/lib/rsvp/all.js https://github.com/tildeio/rsvp.js/blob/master/lib/rsvp/all....
- graue 14y ago
  A day later I looked at this again and I'm a little closer to understanding. var listPromise = new Promise(); creates an object that, being a Promise object, has certain methods and internal state, derived from the prototype of Promise. for (var k in listPromise) promises[k] = listPromise[k]; This confused me because I thought "k" was a stand-in for a numeric index, e.g. that it was doing promises[0] = listPromise[0], promises[1] = listPromise[1], etc. That is not what's going on. Rather, "k" refers to attributes and/or methods that objects of the Promise class have by default. It's copying those onto `promises` — the array `promises` itself, not the individual items `promises[i]`, which keep their existing methods and attributes. Coming from a Python background, I think I would have found this more obvious if the variable "k" were instead called "method" or "attr". If it was `for (var method in listPromise)` it'd be much clearer what's going on, whereas single-letter variables like i, j, and k are, to me, stand-ins for integers. It was also confusing, as you said, that the function uses destructive update rather than treating the input as a value. James did mention this ("augmenting the list with promise methods"), but it's still unexpected, especially when the function is preceded by a Haskell type signature. The reason I only say I'm closer to understanding, and not quite there yet, is I'm not sure what it means to do `new Promise()` or what is being copied over in the above for-loop. I tried James's code with a Promises/A+ implementation, rsvp.js (https://github.com/tildeio/rsvp.js https://github.com/tildeio/rsvp.js), but it won't let me do `new Promise()` because it works differently: > var promise = new RSVP.Promise(); TypeError: You must pass a resolver function as the sole argument to the promise constructor Per an example in RSVP.js's readme, it's expecting this: var promise = new RSVP.Promise(function(resolve, reject){ // set up a callback that calls either resolve(...) // or reject(...) }); If James is using a specific promises implementation in his code, it appears to be the one he defined in a past blog post (http://blog.jcoglan.com/2011/03/11/promises-are-the-monad-of-asynchronous-programming/ http://blog.jcoglan.com/2011/03/11/promises-are-the-monad-of...), which in turn builds on a module from his JS.Class library (http://jsclass.jcoglan.com/deferrable.html http://jsclass.jcoglan.com/deferrable.html), which I hadn't heard of before. I still think this is a great article, but that code snippet has proven to be quite a puzzle.

by graue 14y ago

aethertap 14y ago

I agree, I was going to ask the same question. Unless there's some subtlety that I'm missing, the order you propose makes much more sense.

ams6110 14y ago

I'm glad someone else thought that looked odd.

pyrtsa 14y ago

I asked the same question in Twitter. Turns out James was actually augmenting (i.e. modifying) the array object `promises` to behave as a promise itself. I don't think this was a particularly beautiful way of doing it but it seems to work now that I think of it. Promise libraries, like RSVP.js [1] he referred to, typically implement a way to construct a promise with a depends-on-many relationship, as a function possibly called `all([p1, p2, ...])` (with the same type signature as for `list`), `and(p1, p2, ...)` or something similar. IMO, defining the `list` function that way would've been clearer to the reader and more FP'ish, treating the `promises` argument in as a value and not a mutable object. [1]: https://github.com/tildeio/rsvp.js/blob/master/lib/rsvp/all.js https://github.com/tildeio/rsvp.js/blob/master/lib/rsvp/all....

graue 14y ago
A day later I looked at this again and I'm a little closer to understanding. var listPromise = new Promise(); creates an object that, being a Promise object, has certain methods and internal state, derived from the prototype of Promise. for (var k in listPromise) promises[k] = listPromise[k]; This confused me because I thought "k" was a stand-in for a numeric index, e.g. that it was doing promises[0] = listPromise[0], promises[1] = listPromise[1], etc. That is not what's going on. Rather, "k" refers to attributes and/or methods that objects of the Promise class have by default. It's copying those onto `promises` — the array `promises` itself, not the individual items `promises[i]`, which keep their existing methods and attributes. Coming from a Python background, I think I would have found this more obvious if the variable "k" were instead called "method" or "attr". If it was `for (var method in listPromise)` it'd be much clearer what's going on, whereas single-letter variables like i, j, and k are, to me, stand-ins for integers. It was also confusing, as you said, that the function uses destructive update rather than treating the input as a value. James did mention this ("augmenting the list with promise methods"), but it's still unexpected, especially when the function is preceded by a Haskell type signature. The reason I only say I'm closer to understanding, and not quite there yet, is I'm not sure what it means to do `new Promise()` or what is being copied over in the above for-loop. I tried James's code with a Promises/A+ implementation, rsvp.js (https://github.com/tildeio/rsvp.js https://github.com/tildeio/rsvp.js), but it won't let me do `new Promise()` because it works differently: > var promise = new RSVP.Promise(); TypeError: You must pass a resolver function as the sole argument to the promise constructor Per an example in RSVP.js's readme, it's expecting this: var promise = new RSVP.Promise(function(resolve, reject){ // set up a callback that calls either resolve(...) // or reject(...) }); If James is using a specific promises implementation in his code, it appears to be the one he defined in a past blog post (http://blog.jcoglan.com/2011/03/11/promises-are-the-monad-of-asynchronous-programming/ http://blog.jcoglan.com/2011/03/11/promises-are-the-monad-of...), which in turn builds on a module from his JS.Class library (http://jsclass.jcoglan.com/deferrable.html http://jsclass.jcoglan.com/deferrable.html), which I hadn't heard of before. I still think this is a great article, but that code snippet has proven to be quite a puzzle.