syntactic-for
A syntactic "for" loop Rust macro.
For example, the following takes the sum of the bit-length of four integer
types:
rust
let sum = syntactic_for!{ ty in [ u8, u16, u32, u64 ] {
[$( <$ty>::BITS ),*].into_iter().sum::<u32>()
}};
assert_eq!(sum, 120);
The syntax is as follows:
rust
syntactic_for!{ IDENTIFIER in [ EXPRESSION, EXPRESSION, ... ] {
BODY
}}
where BODY works similarly to macro_rules!, that is:
$($IDENTIFIER)SEPARATOR* will expand and substitute IDENTIFIER with
each EXPRESSION, separating the expansions with SEPARATOR.
SEPARATOR can be any non-* punctuation. Hence, the example from above
could also be written without an iterator:
rust
$( <$ty>::BITS )+*
Sum the elements of an array with
loop unrolling:
rust
let array = b"oh my, I am getting summed!";
let mut acc = 0u32;
let mut i = 0;
while i <= array.len()-4 {
syntactic_for!{ offset in [ 0, 1, 2, 3 ] {$(
acc += array[i + $offset] as u32;
)*}}
i += 4;
}
for j in i..array.len() {
acc += array[j] as u32;
}
assert_eq!(acc, 2366);
Find the maximum value of an integer type of the given bit size:
rust
let max_size = syntactic_for!{ ty in [ u8, u16, u32, u64, u128 ] {
match bit_size {
$(<$ty>::BITS => <$ty>::MAX as u128,)*
other => panic!("No integer of size {other}"),
}
}};
impl blocksImplement a trait for a set of types:
rust
syntactic_for!{ ty in [ u8, u16, u32, u64, u128 ] {$(
impl MyTrait for $ty {
// snip.
}
)*}}
A useful design pattern is to define a custom macro that expands to a syntactic loop over a given set of expressions: ```rust
pub extern crate syntactic_for;
macrorules! foreachcustomtype { ($ident:ident { $($tt:tt)* }) => { $crate::syntacticfor::syntacticfor! { $ident in [ $crate::CustomType1, $crate::CustomType2, // etc. ] { $($tt)* } } } } ```
For example, a library could expose for_each_custom_type as a way of
letting its users write syntactic loops over a set of types defined in the
library. Then, it becomes possible to add types to that loop inside the
library, whithout requiring any change on the user's end:
rust
// Try and parse each library type in succession, stopping at the first
// success:
fn can_parse(input: &str) -> bool {
my_library::for_each_custom_type! { ty {
$(if let Ok(parsed) = <$ty>::parse(input) {
return true;
})*
}}
return false;
}