This macro implements a syntax that emulates Pythons generator-expression syntax in a form more compatible with rusts usual syntax.

This means that there a few small differences between the python syntax and the syntax prvided in this macro:

• The expression in the beginning of the generator expression must end with a semicolon (;).
• The pattern between the for and in tokens is a fully-fledged rust pattern, which can be as simple as a simple token and as complex as struct destructuring.
• The expression defining the iterator after the for token (and potentially before an if token) must evaluate to either an Iterator or an impl IntoIterator, and end with a semicolon (;).
• The conditional expression after the if expression expression (and potentially before a for token) must evaluate to a boolean, and end with a semicolon (;).

The expression replaced by the comp!() macro invocation is a lazy iterator whose lifetime is bound by any references it needs to capture. This means that it can be .collect()ed into any container you like.

Examples

Simple generator expression with a conditional:

use py_comp::comp;

#[derive(Debug, PartialEq, Eq)]
struct Foo(i32);

let arr = &[Foo(11), Foo(12)];

// Notice the semicolons
let comp_vector = comp!(item; for item in arr; if item.0 % 10 == 2;)
.collect::<Vec<&Foo>>();

assert_eq!(comp_vector, vec![&Foo(12)])

Triple cartesian product with conditions and patterns:

use py_comp::comp;

#[derive(Debug, PartialEq, Eq)]
struct Foo(i32);

// These need to be references to arrays because of how the closures
// that the macro expands to capture their environment.
let x = &[(Foo(11), "foo"), (Foo(12), "bar")];
let y = &[Foo(21), Foo(22)];
let z = &[Foo(31), Foo(32)];

let xyz = comp!(
(a, b, c);
for (a, _text) in x;  // You can use any function parameter pattern.
if a.0 % 10 == 2;
for b in y;           // Obviously not every level requires a conditional.
for c in z;
if c.0 % 10 == 2;
)
.collect::<Vec<(&Foo, &Foo, &Foo)>>();

// The result vector here is short for illustration purposes
// but can be as long as long as you need it to be.
assert_eq!(xyz, vec![(&Foo(12), &Foo(21), &Foo(32)), (&Foo(12), &Foo(22), &Foo(32))])