[swift-evolution] Proposal: Keep var on Function Parameters and Pattern Matching

[Francisco Costa]

Hello all,

There is currently a proposal to remove `var` from Function Parameters and
Pattern Matching which seems to have been already accepted

https://github.com/apple/swift-evolution/blob/master/proposals/0003-remove-var-parameters-patterns.md

I cannot find the discussion on it, probably because it predates the open
sourcing of Swift. While some of its arguments may sound reasonable, I
think they have been greatly exaggerated.

The main point of the proposal seems to be that the `var` attribute can be
easily confused with `inout`.  It then provides examples where `var` would
be completely useless:

```
func foo(i: Int) {
  i += 1 // Illegal
}

func foo(var i: Int) {
  i += 1 // Legal but useless since we are not using this local value
anywhere
}
```

We today made a quick survey around the backend developers on the office
(not familiar with Swift) and not a single one suggested that the second
method would mutate the original parameter value. In fact, to all of them
it was clear that `var` creates a local copy of the value and that the
method is also missing a `return` if we want to use the incremented value.

The Swift Language Guide gives a good explanation on this as well:

"Variable parameters ... give a new modifiable copy of the parameter’s
value for your function to work with."

This is probably a concept most beginners to the language can either guess
intuitively or learn in a couple of seconds.

Especially taking into account that the way we use `inout` parameters is so
much advertised in Swift, to the point where calling a method with them
requires to explicitly identify them with `&`, it seems that by giving up
the `var` attribute we would lose a convenient elegance in exchange for no
real benefit to beginners.

In fact, in my experience the `var` attribute is most often used in
scenarios where immutability is intentionally preserved, we pass a
immutable value to a method and it returns another immutable (modified)
version of that value.

Another common scenario is when you need to further modify a value that was
unwrapped from an Optional. I would argue that this seems elegant and easy
to read:

```
if var x = getOptionalInt() {
  x += 1
  return x
}
```

While this (as has been proposed) seems like we are fighting a language
limitation:

```
if let x = getOptionalInt() {
  var x = x
  x += 1
  return x
}
```

I understand that discussing back and forth proposals that have been
already accepted is counter-productive, but since this discussion doesn't
seem to have been published, I would kindly ask if we can give it a second
thought :)

Thanks,
Francisco
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