[swift-evolution] Optional Argument Chaining

Stephen Celis stephen.celis at gmail.com
Wed Dec 13 11:20:32 CST 2017

> On Dec 11, 2017, at 1:08 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> 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.


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