Closed6

Turbofishの小さな物語

yukiyuki

Rust は基本的にはほとんど型注釈のいらない言語だが、ときどき型を明示的に書く必要がある。

use std::net::Ipv4Addr;

let addr = "127.0.0.1".parse::<Ipv4Addr>();

この ::<> を Turbofish (ターボフィッシュ) と呼ぶ。なんか右に魚が進んでいるように見えるからそう呼んでいる。

yukiyuki

そしてこの Turbofish のテストファイルには、コメントで小さな物語が書いてある。

https://github.com/rust-lang/rust/blob/master/src/test/ui/bastion-of-the-turbofish.rs

// Bastion of the Turbofish
// ------------------------
// Beware travellers, lest you venture into waters callous and unforgiving,
// where hope must be abandoned, ere it is cruelly torn from you. For here
// stands the bastion of the Turbofish: an impenetrable fortress holding
// unshaking against those who would dare suggest the supererogation of the
// Turbofish.
//
// Once I was young and foolish and had the impudence to imagine that I could
// shake free from the coils by which that creature had us tightly bound. I
// dared to suggest that there was a better way: a brighter future, in which
// Rustaceans both new and old could be rid of that vile beast. But alas! In
// my foolhardiness my ignorance was unveiled and my dreams were dashed
// unforgivingly against the rock of syntactic ambiguity.
//
// This humble program, small and insignificant though it might seem,
// demonstrates that to which we had previously cast a blind eye: an ambiguity
// in permitting generic arguments to be provided without the consent of the
// Great Turbofish. Should you be so naïve as to try to revolt against its
// mighty clutches, here shall its wrath be indomitably displayed. This
// program must pass for all eternity, fundamentally at odds with an impetuous
// rebellion against the Turbofish.
//
// My heart aches in sorrow, for I know I am defeated. Let this be a warning
// to all those who come after. Here stands the bastion of the Turbofish.

// See https://github.com/rust-lang/rust/pull/53562
// and https://github.com/rust-lang/rfcs/pull/2527
// for context.

ところでなんでこれコンパイル通るかわからないんだけど。どういう文法なんだろ。

fn main() {
    let (oh, woe, is, me) = ("the", "Turbofish", "remains", "undefeated");
    let _: (bool, bool) = (oh<woe, is>(me));
}
yukiyuki

あー、このコードの意味がわかったかも。

let _: (bool, bool) = (oh<woe, is>(me));

このコードのパートは2つにわかれる。

oh<woe

これと、

is>(me)

これ。この2つの比較結果は bool のタプルになって返る、ということか。そもそも &str は比較可能だったか。

このスクラップは2021/02/19にクローズされました