Language Integrated Query in Rust (created by declarative macros).

• Inspired by LINQ in .NET.
• What's LINQ

This project is under development! API might be changed.

This is an example:

use linq::linq;
use linq::Queryable;

fn try_linq_methods() {
    let x = 1..100;
    let mut y: Vec<i32> = x.clone().filter(|p| p <= &5).collect();
    y.sort_by_key(|t| -t);
    let y: Vec<i32> = y.into_iter().map(|t| t * 2).collect();
    let e: Vec<i32> = x
        .clone()
        .where_by(|p| p <= &5)
        .order_by(|p| -p)
        .select(|p| p * 2)
        .collect();
    assert_eq!(e, y);
}

fn try_linq_expr() {
    let x = 1..100;
    let mut y: Vec<i32> = x.clone().filter(|p| p <= &5).collect();
    y.sort_by_key(|t| -t);
    let y: Vec<i32> = y.into_iter().map(|t| t * 2).collect();
    let e: Vec<i32> =
        linq!(from p in x.clone(), where p <= &5, orderby -p, select p * 2).collect();
    assert_eq!(e, y);
}

If you are familier with LINQ in C#, you will find this is easy to use.

The two imports is necessary:

use linq::linq;         // for `linq!` macro
use linq::iter::Enumerable;    // for LINQ methods and `linq!` macro

The trait linq::Queryable supports LINQ methods on Iterator. You can find the correspondences below.

• Normal items mean they are builtin methods of Iterator in std.
• Bold items mean they are implemented in this project. You can find them in module linq::iter (but they are private so that you can't import them).
• Italic items mean they are not in roadmap. Happy for your suggestions.

• where => where_by => filter
• select => map
• select_many => select_many_single, select_many
• skip
• skip_while
• take
• take_while
• join
• group_join
• concate => chain
• order_by
• order_by_descending
• then_by
• then_by_descending
• reverse => rev
• group_by
• distinct
• union
• intersect
• except
• first => next
• single
• element_at => nth
• all
• any
• contains
• count
• sum
• product
• min
• max
• average
• aggregate => fold

The query expression begins with from clause and ends with select clause. Use , to seperate every clause.

linq!(from x in coll, select x)

Now we supports these keywords:

• from
  • from (select_many_single)
  • zfrom (select_many)
• in
• select
• where
• orderby
• descending
• group_by
• more...

from <id> in <iter expr>,

Also you can enumerate elements of each set in the collection (Attention: for this type, you can't access the value that is in the first from clause in select clause):

let x = 1..5;
let y = vec![0, 0, 1, 0, 1, 2, 0, 1, 2, 3];
let e: Vec<i32> = linq!(from p in x.clone(), from t in 0..p, select t).collect();

assert_eq!(e, y);

If you want to zip or enumerate value-pairs of two sets, use zfrom for the second from:

let x = 1..5;
let y = vec![
    (1, 0),
    (2, 0),
    (2, 1),
    (3, 0),
    (3, 1),
    (3, 2),
    (4, 0),
    (4, 1),
    (4, 2),
    (4, 3),
];
let e: Vec<_> = linq!(from p in x.clone(), zfrom t in 0..p, select (p,t)).collect();

assert_eq!(e, y);

The expression in zfrom recieve the cloned value in the first from, and the elements in two sets will be cloned for select clause.

while <expr>,

You can use where clause in single-from query, and the expression will recieve a variable named the id in from clause. The expression need to return a boolean value.

orderby <expr>,
orderby <expr>, descending,

You can use orderby clause in single-from query. This query will collect the iterator, and sort them by the expression, then return the new iterator.

We need more unit-test samples. If you have any ideas, open issues to tell us.

Since the expression procedural macros is not stable, I only create macros by declarative macros.

$ cargo test