There might be something obviously wrong with this, but how about:
match foo {
let foo = Foo { field: x } => ...
}
Gareth
On 3 May 2013 02:12, Patrick Walton <[email protected]> wrote:
> Hi everyone,
>
> There's consensus that `@` (imported from Haskell) is a bad binding
> operator for patterns, because it leads to the confusing-looking `@@` in,
> for example:
>
> struct Foo {
> field: int
> }
>
> ...
>
> match foo {
> foo@@Foo { field: x } => ...
> }
>
> However, there is not consensus as to what to change it to. Suggestions
> are `=` and `as`.
>
> The problem with `=` is that, if implemented naively, it makes our grammar
> ambiguous:
>
> let x = y = 3; // is x the result of evaluating `y = 3` (i.e. unit)
> // or are x and y bound to 3?
>
> The easiest way to fix this problem is to forbid `=` in irrefutable
> patterns, such as those introduced by `let`. However, this bifurcates the
> pattern grammar into the irrefutable-pattern grammar and the
> refutable-pattern grammar, with some conceptually-ugly overlap.
>
> The alternative is `as`, like OCaml. However, this conflicts with `as` in
> the expression grammar. A subset of the expression grammar is part of the
> pattern grammar in order to permit matching against constants. Removing
> `as` expressions from the subset of expression productions permitted in
> patterns would mean that this would no longer do what you expect:
>
> match 22.0f32 / 7.0f32 {
> math::PI as f32 => println("Good grief!"),
> _ => {}
> }
>
> So both `=` and `as` have drawbacks.
>
> I don't really have any preference at all; I just need to know what to
> implement. Opinions?
>
> Patrick
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