# Crate syntactic_for

Expand description

A syntactic for loop.

For example, the following takes the sum of the bit-length of four integer types:

let sum = syntactic_for!{ ty in [ u8, u16, u32, u64 ] {
[\$( <\$ty>::BITS ),*].into_iter().sum::<u32>()
}};
assert_eq!(sum, 120);

## Usage

The syntax is as follows:

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:

\$( <\$ty>::BITS )+*

## Examples

### Loop unrolling

Sum the elements of an array with loop unrolling:

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);

### Matching

Find the maximum value of an integer type of the given bit size:

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 blocks

Implement a trait for a set of types:

syntactic_for!{ ty in [ u8, u16, u32, u64, u128 ] {\$(
impl MyTrait for \$ty {
// snip.
}
)*}}

### Custom syntactic loop

A useful design pattern is to define a custom macro that expands to a syntactic loop over a given set of expressions:

#[doc(hidden)]
pub extern crate syntactic_for;

#[macro_export]
macro_rules! for_each_custom_type {
(\$ident:ident { \$(\$tt:tt)* }) => {
\$crate::syntactic_for::syntactic_for! { \$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:

// 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;
}

## Macros

Iterate over a list of (syntactic) expressions.