And, if you like the division between `then` and `chain`:
class MonadicPromise extends Promise {
constructor(exec) {
class MP extends MonadicPromise {
constructor(exec) {
Promise.call(this, (f,r) => exec( v => f([v]), r));
}
}
return new MP(exec);
}
chain(f, r) {
return super.then(v => f(v[0]), r);
}
then(f, r) {
return super.then(v => Promise.resolve(v[0]).then(f, r), r);
}
// See https://github.com/domenic/promises-unwrapping/issues/95
resolve(v) {
return new MonadicPromise(function(r) { r(v); });
}
};
You can put MonadicPromises in your async maps if you wish to be able
to use chain on the values; otherwise they are indistinguishable from
standard promises. There is a bit of extra cost for
MonadicPromise.resolve, since it always creates a wrapper -- but the
common-case non-monadic Promise doesn't need to pay for this.
--scott
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