4 ms·

So, just out of curiosity: I know in ECMAScript this is probably mainly for the purpose of giving you a convenient way to lift either (α → β) or (α → Promise β)

So, just out of curiosity: I know in ECMAScript this is probably mainly for the purpose of giving you a convenient way to lift either (α → β) or (α → Promise β) into (Promise α → Promise β) without static typing or having two "then" functions, by forcing β and Promise β to be disjoint. It also allows looser composition underneath, since you can choose which kind of return to provide at runtime without having to re-wrap the bare case manually. But are there any other reasons for Promise of Promise to be squashed away like that?

Arnavion 5y ago
Apart from simplicity, it was designed to be used with "thenables". If the value that a Promise resolved with had a `then()` member, the value would be treated like a first-party Promise. So even if there was a separate fmap-then and bind-then, the fmap-then would have to deal with thenables and act like bind-then anyway. There's a bunch of history in [1], particularly [2] and [3], if you care to read them. [1]: https://github.com/promises-aplus/promises-spec/issues https://github.com/promises-aplus/promises-spec/issues [2]: https://github.com/promises-aplus/promises-spec/issues/75 https://github.com/promises-aplus/promises-spec/issues/75 [3]: https://github.com/promises-aplus/promises-spec/issues/94 https://github.com/promises-aplus/promises-spec/issues/94
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