Partial application

[yvt]

A new macro that acts like a curried function (e.g., matcher!($pat)($expr) == try_match!($expr, $pat)) could be useful in some situations.

Issues

• What should it be called? What about syntax? A few candidates:
  • matcher!($pat)
  • try_matcher!($pat)
  • try_match!(_, $pat), try_match!(, $pat), try_match!($pat)
• Should it return Option<MatchedVars> or Result<MatchedVars, Original>?
  • Related: Add a macro that evaluates to Option<_> #8
• Should the if guard and the success expression (=> $expr) capture outer variables by move?

Examples

Iterator::filter_map

Assumes the closure returns Option<MatchedVars>

let array = [Var1(42), Var2, Var1(10)];
let filtered: Vec<_> = array
    .iter()
//  .filter_map(|x| try_match!(x, &Var1(_0) if _0 > 20).ok())
    .filter_map(matcher!(&Var1(_0) if _0 > 20))
    .collect();
assert_eq!(filtered, [42]);

Assumes the closure returns Result<MatchedVars, Original>

let array = [Var1(42), Var2, Var1(10)];
let filtered: Vec<_> = array
    .iter()
//  .filter_map(|x| try_match!(x, &Var1(_0) if _0 > 20).ok())
    .map(matcher!(&Var1(_0) if _0 > 20))
    .filter_map(Result::ok)
    .collect();
assert_eq!(filtered, [42]);

Applicable use cases in the wild: Tamschi/quote-miner, yvt/Stella2

Iterator::map + Fallible Iterator::collect

Assumes the closure returns Result<MatchedVars, Original>

let array = [Var1(42), Var2, Var1(10)];
let filtered: Result<Vec<_>, _> = array
    .iter()
    .map(matcher!(&Var1(_0) if _0 > 20))
    .collect();
// `Var2` is the first value that doesn't match
assert_eq!(filtered, Err(&Var2));

[yvt]

Unstably implemented in commit 4950789, using the signature try_match!(, $pat) (inspired by GHC tuple sections)

[yvt]

Change of plan: Insta-stabilized the feature in commit 7e3beed. Since it's not an ecosystem kind of crate, we can just increase the major version anytime without causing an ecosystem split.