> On Dec 11, 2017, at 1:08 PM, Matthew Johnson via swift-evolution
> <[email protected]> wrote:
>
> It’s worth mentioning that the problem this thread is discussing can be
> generalized to idioms / applicative. The specific case here is for Optional
> but many other types could benefit from an elegant syntactic solution to this
> problem. It might be worth exploring a more general solution. Here’s a link
> to how this is handled in Idris:
> http://docs.idris-lang.org/en/latest/tutorial/interfaces.html#idiom-brackets.
>
> Matthew
Just want to +1 a more general, less "Optional"-specific solution. You can do a
ton of interesting things with applicative structure.
Failures from "Optional" and "throws" operate sequentially and halt
immediately: the first "nil" or "throw" encountered prevents all work chained
from it[1]. Another upcoming sequential operation that's worth thinking about
in this discussion: "async"/"await".
Applicative structures throw away that sequential constraint. Let's consider
some fun things that happen when we take it away in a couple of the examples
above.
If "throws" were applicative, you could accumulate a bunch of errors at once.
do {
// made-up syntax
let user = User(|name: try validate(name: name), email: try
validate(email: email)|)
} catch {
print(error) // .manyErrors(["name is too short", "email is invalid"])
}
Currently, the above would halt on the first error.
If "async"/"await" were applicative, you could fire off a bunch of asynchronous
requests in parallel.
let homepageData = HomepageData(|await fetchCurrentUser(), await
fetchProducts()|)
In the proposed version of "async"/"await", the above would block on
"fetchCurrentUser()" and only call "fetchProducts()" after the response.
"Optional" would get to use that same sugar! An example from the original email:
getPostageEstimate(|source: john.address, destination: alice.address,
weight 2.0|)
--
[1]: "Optional" and "throws" are monadic structures and "flatMap" is the
abstraction of this sequential operation.
Stephen
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