第十三號艦隊
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Thought about macros

Posted on Edited on

Macro這東西很早就有了，不過各有巧妙不同。
在Assembler的年代就已經有Macro了

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COPY            macro   Dest, Source
                mov     ax, Source
                mov     Dest, ax
                endm

而在C語言的時候，Macro的被使用了更頻繁了

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#define min(a, b) ((a < b) ? a : b)

C語言的Macro也很簡單，就只是作文字的代換而已。Side Effect除外，出錯的話也很難Debug，因此其他語言打算因此作改進。

Define group of functions

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#define CK_PR_FENCE(T, I)                               \
        CK_CC_INLINE static void                        \
        ck_pr_fence_strict_##T(void)                    \
        {                                               \
                __asm__ __volatile__(I ::: "memory");   \
        }

CK_PR_FENCE(atomic, "sfence")
CK_PR_FENCE(atomic_store, "sfence")
CK_PR_FENCE(atomic_load, "mfence")
CK_PR_FENCE(store_atomic, "sfence")
CK_PR_FENCE(load_atomic, "mfence")
CK_PR_FENCE(load, "lfence")

在這情況下 Templateˊ無能為力.

Rust

Macro overload

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macro_rules! sayHello {
    () => {
       println!("sayHello");
    };
    ($name:ident) => {
       println!("sayHello {:?}",  stringify!($name));
    };
}
fn main() {
    sayHello!();
    sayHello!(ABC);
    
}

Pattern Matching

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macro_rules! min {
    // base case
    ($x:expr) => ($x);
    // `$x` followed by at least one `$y,`
    ($x:expr, $($y:expr),+) => (
        // call min! on the tail `$y`
        std::cmp::min($x, min!($($y),+))
    )
}

Recursive Macro

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macro_rules! write_html {
    ($w:expr, ) => (());

    ($w:expr, $e:tt) => (write!($w, "{}", $e));

    ($w:expr, $tag:ident [ $($inner:tt)* ] $($rest:tt)*) => {{
        write!($w, "<{}>", stringify!($tag));
        write_html!($w, $($inner)*);
        write!($w, "</{}>", stringify!($tag));
        write_html!($w, $($rest)*);
    }};
}

fn main() {
    use std::fmt::Write;
    let mut out = String::new();

    write_html!(&mut out,
        html[
            head[title["Macros guide"]]
            body[h1["Macros are the best!"]]
        ]);

    assert_eq!(out,
        "<html><head><title>Macros guide</title></head>\
         <body><h1>Macros are the best!</h1></body></html>");
}

Nim

Reference

– Macros - Rust Documentation
– A Quick Intro to Rust Macros
– Module macros - Nim
– Nim Manual