macro-examples/src/lib.rs

#![allow(unused)]
//#![recursion_limit = "1000000000"]

//! # Examples
//!
//! ```compile_fail
//! macro_rules! ah_ah_ah_didnt_say_the_magic_word {
//!     ($x:ident) => {
//!         let x_mut_1 = &mut $x;
//!         let x_mut_2 = &mut $x;
//!         *x_mut_1 += 1i32;
//!     }
//! }
//! let mut x: i32 = 0;
//! ah_ah_ah_didnt_say_the_magic_word!(x);
//! ```
//!
//! Generic `min` won't compile:
//!
//! ```compile_fail
//! fn min<T>(a: T, b: T) -> T {
//!     if a < b { a } else { b }
//! }
//! ```
//!
//! ```compile_fail
//! macro_rules! add_one { ($x:ident) => { $x + 1i32 } }
//! let b: std::collections::HashMap<String, u32> = Default::default();
//! add_one!(b);
//! ```
//!
//! ```compile_fail
//! macro_rules! add_one { ($x:ident) => { $x + 1i32 } }
//! add_one(42i32);
//! ```
//!
//! ```compile_fail
//! macro_rules! make_adder_fn {
//!     ($n:expr) => {
//!         fn add_$n(rhs: i32) -> i32 { 
//!             $n + rhs
//!         }
//!     }
//! }
//! make_adder_fn!(42);
//! ```

#[cfg(test)]
mod tests {

    #[test]
    fn a_bunch_of_print_lns_showing_var_args() {
        println!();
        println!("{}", 1);
        println!("{} {}", 1, 2);
        println!("{} {} {}", 1, 2, 3);
    }

    #[test]
    fn shows_vec_macro_and_its_equivalent() {
        let xs: Vec<i32> = vec![1, 2, 3];
        let ys: Vec<i32> = {
            let mut out = Vec::with_capacity(3);
            out.push(1);
            out.push(2);
            out.push(3);
            out
        };
        assert_eq!(xs, ys);
    }

    #[test]
    fn macro_that_if_used_would_generate_illegal_code_is_itself_not_illegal() {
        macro_rules! ah_ah_ah_didnt_say_the_magic_word {
            ($x:ident) => {
                let x_mut_1 = &mut $x;
                let x_mut_2 = &mut $x;
                *x_mut_1 += 1i32;
            }
        }
    }

    #[test]
    fn generic_max_macro_compiles_and_works() {
        macro_rules! max {
            ($a:expr, $b:expr) => {
                if $a > $b { $a } else { $b }
            }
        }

        let a: usize = 1;
        let b: usize = 2;
        assert_eq!(max!(a, b), 2);

        let c = f64::INFINITY;
        let d = f64::NEG_INFINITY;
        assert_eq!(max!(c, d), f64::INFINITY);
    }

    #[test]
    fn min_with_partial_ord_will_compile() {
        fn min_that_compiles<T: PartialOrd>(a: T, b: T) -> T {
            if a < b { a } else { b }
        }
        let a: usize = 1;
        let b: usize = 2;
        assert_eq!(min_that_compiles(a, b), 1);
    }

    #[test]
    fn generated_type_errors_the_example_of_something_that_works() {
        macro_rules! add_one {
            ($x:ident) => {
                $x + 1i32
            }
        }
        let a: i32 = 42;

        assert_eq!(add_one!(a), 43);    // Ok: expands to i32 + i32   
    }

    #[test]
    fn im_curious_if_a_randomly_chosen_example_from_little_quote_unquote_black_book_compiles() {
        macro_rules! abacus {
            ((- $($moves:tt)*) -> (+ $($count:tt)*)) => {
                abacus!(($($moves)*) -> ($($count)*))
            };
            ((- $($moves:tt)*) -> ($($count:tt)*)) => {
                abacus!(($($moves)*) -> (- $($count)*))
            };
            ((+ $($moves:tt)*) -> (- $($count:tt)*)) => {
                abacus!(($($moves)*) -> ($($count)*))
            };
            ((+ $($moves:tt)*) -> ($($count:tt)*)) => {
                abacus!(($($moves)*) -> (+ $($count)*))
            };

            // Check if the final result is zero.
            (() -> ()) => { true };
            (() -> ($($count:tt)+)) => { false };
        }

        // this one never finishes
        //macro_rules! abacus2 {
        //    (-) => {-1};
        //    (+) => {1};
        //    ($($moves:tt)*) => {
        //        0 $(+ abacus2!($moves))*
        //    }
        //}

        let equals_zero = abacus!((++-+-+++--++---++----+) -> ());
        assert_eq!(equals_zero, true);

        //let equals_zero = abacus2!((++-+-+++--++---++----+) -> ());
        //assert_eq!(equals_zero, true);
    }

    #[test]
    fn how_about_this_example_from_little_book() {
        macro_rules! parse_unitary_variants {
            (@as_expr $e:expr) => {$e};
            (@as_item $($i:item)+) => {$($i)+};
            
            // Exit rules.
            (
                @collect_unitary_variants ($callback:ident ( $($args:tt)* )),
                ($(,)*) -> ($($var_names:ident,)*)
            ) => {
                parse_unitary_variants! {
                    @as_expr
                    $callback!{ $($args)* ($($var_names),*) }
                }
            };

            (
                @collect_unitary_variants ($callback:ident { $($args:tt)* }),
                ($(,)*) -> ($($var_names:ident,)*)
            ) => {
                parse_unitary_variants! {
                    @as_item
                    $callback!{ $($args)* ($($var_names),*) }
                }
            };

            // Consume an attribute.
            (
                @collect_unitary_variants $fixed:tt,
                (#[$_attr:meta] $($tail:tt)*) -> ($($var_names:tt)*)
            ) => {
                parse_unitary_variants! {
                    @collect_unitary_variants $fixed,
                    ($($tail)*) -> ($($var_names)*)
                }
            };

            // Handle a variant, optionally with an with initialiser.
            (
                @collect_unitary_variants $fixed:tt,
                ($var:ident $(= $_val:expr)*, $($tail:tt)*) -> ($($var_names:tt)*)
            ) => {
                parse_unitary_variants! {
                    @collect_unitary_variants $fixed,
                    ($($tail)*) -> ($($var_names)* $var,)
                }
            };

            // Abort on variant with a payload.
            (
                @collect_unitary_variants $fixed:tt,
                ($var:ident $_struct:tt, $($tail:tt)*) -> ($($var_names:tt)*)
            ) => {
                const _error: () = "cannot parse unitary variants from enum with non-unitary variants";
            };
            
            // Entry rule.
            (enum $name:ident {$($body:tt)*} => $callback:ident $arg:tt) => {
                parse_unitary_variants! {
                    @collect_unitary_variants
                    ($callback $arg), ($($body)*,) -> ()
                }
            };
        }

        /*

        not sure how to invoke this one

        let mut variants: Vec<&'static str> = Vec::new();

        macro_rules! get_unitary_variant_str {
            {() ($($var_names),*) } => { $( variants.push(stringify!($variant)); )*  }
        }

        parse_unitary_variants!(
            enum Abcs {
                A,
                B,
                C,
                D,
                E
            } => get_unitary_variant_str () 
        );
        */
    }

    #[test]
    fn tt_muncher() {
        macro_rules! mixed_rules {
            () => {};
            (trace $name:ident; $($tail:tt)*) => {
                {
                    println!(concat!(stringify!($name), " = {:?}"), $name);
                    mixed_rules!($($tail)*);
                }
            };
            (trace $name:ident = $init:expr; $($tail:tt)*) => {
                {
                    let $name = $init;
                    println!(concat!(stringify!($name), " = {:?}"), $name);
                    mixed_rules!($($tail)*);
                }
            };
        }

        mixed_rules!(trace x = 1;);
    }

    use std::fmt::Display;

    fn i_need_display_not_debug<T: Display>(x: T) -> String {
        format!("{}", x)
    }

    #[test]
    fn debug_display_macro_actually_works() {
        #[derive(Debug)]
        struct Point {
            pub x: f64,
            pub y: f64,
        }

        macro_rules! debug_display {
            ($t:ident) => {
                impl std::fmt::Display for $t {
                    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                        write!(f, "{:?}", self)
                    }
                }
            }
        }

        debug_display!(Point); // poof! it implements Display

        let p = Point { x: 1.234, y: 2.345 };

        i_need_display_not_debug(p);
    }

    #[test]
    fn example_for_why_you_should_use_fully_qualified_names() {
        struct A;

        trait Devious {
            fn abc(&self) -> isize { 42 }
            fn xyz(&self) -> isize { 42 }
        }

        impl Devious for A {}

        let a = A;

        macro_rules! didnt_see_it_coming {
            { $x:ident ;; } => {
                <A as Devious>::xyz(&$x)
            };

            { $x:ident <-> } => {
                <A as Devious>::abc(&$x)
            }
        }

        assert_eq!(didnt_see_it_coming!{ a <-> }, 42);

        /// muhaha
        fn the_upside_down(a: &A) -> isize {
            /// we have our own 'Devious' down here...
            trait Devious {
                fn abc(&self) -> isize { -42 }
                fn xyz(&self) -> isize { -42 }
            }

            impl Devious for A {}

            didnt_see_it_coming!{ a ;; }
        }

        assert_eq!(the_upside_down(&a), -42);

    }

}