[wyager]

> This includes optional-chaining,

Haskell has bind (>>=) and do-syntax, rust has `and_then`. This is pretty standard with any ADT-supporting language.

> if-let

Most languages with ADT support have good pattern matching that subsumes if-let syntax and allows you to do other things as well. Swift's pattern matching, such as it is, is a bit of a mess.

> guard-let unwrapping

Haskell has `guard` and `MonadFail`, which address the use cases for this in a more principled way. `guard` for boolean matching (like equality) and `MonadFail` for structural matching (like pattern matching on a constructor).

Rust has (?) and `try`, which are syntactic sugar around a simple match/return statement that addresses most use cases of guard-let.

Obviously there are going to be small differences between this implementations, but Swift doesn't really excel here.

> `5 == Just 5` fails.

As it probably should. Supporting such implicit casting could lead to some obvious confusion and buggy behavior. Ideally, the fact that "a == b" typechecks should indicate that "a" and "b" are of the same type.

In Haskell, you would just do `Just 5 == x` if you want to check that something is, in fact, `Just 5`. If that really wasted a lot of space somehow, you can define something like `x ==: y = Just x == y`.

[comex]

Rust copied 'if let' from Swift (~2 years ago) despite having decent pattern matching; community consensus today is that it's highly worthwhile as a feature. There have also been proposals to add 'guard let'. So, while I don't have enough Swift experience to judge its ADT support overall, I wouldn't cite those features as evidence that it's wanting. They might not be as natural in traditional FP languages like Haskell, but they seem to fit pretty well in the stylistic niche Rust and Swift (among other languages) have carved out.

edit: FWIW, there's some overlap between '?' and 'guard let', but not that much. Using hypothetical Rust syntax, they'd overlap in this case:

    guard let Some(x) = foo else {
        return Err(GettingFoo);
    }

better expressed as

    let x = foo.ok_or(GettingFoo)?;

…but if you want to return something other than a Result, or the ADT being matched on is something custom rather than Option/Result, there's no good way to make '?' do the job.

[tomjakubowski]

> Rust copied 'if let' from Swift (~2 years ago) despite having decent pattern matching; community consensus today is that it's highly worthwhile as a feature.

Rust copied 'if let' and made it far nicer and more capable by allowing arbitrary fallible pattern matches to be the condition. Then Swift 2 copied that improvement back with 'if case'.

[Ericson2314]

? is bind for Either monad if you squint.

`context(e?)` becomes `e >>= \x -> context(x)`

[majewsky]

You mean `e >>= context` ? ;)

[Ericson2314]

Hah!

I should have been clear about CPSing things first so I didn't need to eta-expand :)

[ridiculous_fish]

Optional chaining is syntactically much more lightweight than bind and do-syntax. >>= and `and_then` have the annoying property of reversing the order of function calls. `if let` is also more pleasant to read - note you can destructure multiple values naturally (no tuple required) and incorporate boolean tests.

It's totally fair to point out that Haskell is more principled - users can build Haskell's Maybe, but not Swift's Optional.

> Rust has (?) and `try`, which are syntactic sugar around a simple match/return statement that addresses most use cases of guard-let

They aren't really comparable. Rust's `try!` addresses one case: you have a Result and want to propagate any error to the caller. This is closest to Swift's `try`. But Swift's `guard` is much more flexible: it allows destructuring, boolean tests, multiple assignment, etc., and it also allows arbitrary actions on failure: return, exit(), throw, etc., with the compiler checking that all paths result in some sort of exit.

In practice this is used for all sorts of early-outs, not just error handling. It neatly avoids `if` towers-of-indenting-doom. I think the best analog is Haskell's do-notation.

There's the same tradeoff here. Rust's try! is a macro that anyone can build, while Swift's `guard` is necessarily built into the language. But any Swift programmer will tell you how much they appreciate this feature, unprincipled though it may be. Use it for a while and you become a believer!

> As it probably should. Supporting such implicit casting could lead to some obvious confusion and buggy behavior. Ideally, the fact that "a == b" typechecks should indicate that "a" and "b" are of the same type.

The principled stand! Both languages make the right decision for their use case. Swift's Optionals are much more commonly encountered than Haskell's Maybe (because of Cocoa), and so the language's syntax design optimizes for dealing with Optional. They're more central to Swift than its ADTs.