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Not sure promises are equivalent to the IO monad in particular. They could form a Monad instance, although that is not the interface they have chosen to expose.

Not sure promises are equivalent to the IO monad in particular. They could form a Monad instance, although that is not the interface they have chosen to expose. It does seem it would be more similar to the `Either` monad, however.

inglor 11y ago
The type signature, with `catch` is something like Either mixed with `IO`, it's just a specific form of continuation[1]. Promises are not _exactly_ like an IO monad because it would not be pragmatic, but it serves the same purpose (controlled side effects and boxing temporal state) in the same way (a bind operator). If you look at all the examples online for `IO` [2] they all translate 1-1 to promises just fine :) [1] : http://blog.sigfpe.com/2008/12/mother-of-all-monads.html http://blog.sigfpe.com/2008/12/mother-of-all-monads.html [2] : for example http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html http://adit.io/posts/2013-04-17-functors,_applicatives,_and_...
- asQuirreL 11y ago
  Oh yeah, I totally agree, the examples for the IO monad match up very well to what can be done with Promises. My point was more that this is the case because the IO monad is the canonical black box monad: All you can do with it stems from its implementation of the monad interface, and so anything that behaves like a Monad will look very similar. This is tantamount to saying that Promises have a potentially useful Monad instance (which I am completely in agreement with you on). The point about continuations is also a very interesting one, but I think again, stems from the same idea: Almost any monadic interface can be simulated with continuations.
- tel 11y ago
  Promises also have a notion of parallel tasking which isn't, at least in my mental model of IO, really IO. But of course we're talking about IO... so my mental model is just that: mine and mine alone, really.

inglor 11y ago

The type signature, with `catch` is something like Either mixed with `IO`, it's just a specific form of continuation[1]. Promises are not _exactly_ like an IO monad because it would not be pragmatic, but it serves the same purpose (controlled side effects and boxing temporal state) in the same way (a bind operator). If you look at all the examples online for `IO` [2] they all translate 1-1 to promises just fine :) [1] : http://blog.sigfpe.com/2008/12/mother-of-all-monads.html http://blog.sigfpe.com/2008/12/mother-of-all-monads.html [2] : for example http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html http://adit.io/posts/2013-04-17-functors,_applicatives,_and_...

asQuirreL 11y ago
Oh yeah, I totally agree, the examples for the IO monad match up very well to what can be done with Promises. My point was more that this is the case because the IO monad is the canonical black box monad: All you can do with it stems from its implementation of the monad interface, and so anything that behaves like a Monad will look very similar. This is tantamount to saying that Promises have a potentially useful Monad instance (which I am completely in agreement with you on). The point about continuations is also a very interesting one, but I think again, stems from the same idea: Almost any monadic interface can be simulated with continuations.
tel 11y ago
Promises also have a notion of parallel tasking which isn't, at least in my mental model of IO, really IO. But of course we're talking about IO... so my mental model is just that: mine and mine alone, really.