list comprehension

List comprehension in C++!

Requires my Type Utils library to work.

constexpr auto a = var<"a">;
constexpr auto b = var<"b">;
constexpr auto res = ((a, b) | a <- range(0, 4), b <- range(0, 4), a > b);

output:

(1, 0), (2, 0), (2, 1), (3, 0), (3, 1), (3, 2)

Features

So, what can this do?

Normal list comprehension stuff with multiple ranges, some constraints, and the output. In this case the output is a tuple of 3 ints.

constexpr auto r1 = ((a, b, c) | c <- range(1, 11), 
    b <- range(1, c), a <- range(1, b), a*a + b*b == c*c);

Parallel iteration! This is where the magic really comes in. It will iterate in parallel, so the output here will be (0, 0), (1, 1), ...,(4, 4).

constexpr auto r2 = (a + b | (a, b) <- (range(0, 5), range(0, 5)));

Decompose the keys and values from an std::map! You can also combine this with parallel iteration to extract all separate variables.

std::map<int, int> data{ { 1, 2 }, { 3, 4 }, { 5, 6 } };
auto r3 = (value + a | ((key, value), a) <- (data, range(0, 5)));

Calling standard functions in an expression. Most of the functions in the standard have been given an overload for expr<Type>, so you can use them in the constraints or in the result expression. This example takes the pairs from the vector, and results in a vector with the biggest of the 2 values in the tuple.

constexpr auto r4 = (max(a, b) | (a, b) <- 
    std::array<std::pair<int, int>, 3>{ { { 1, 5 }, { 5, 4 }, { 3, 4 } } });

Lazy evaluation! You can create an infinite list

constexpr auto r5 = (a | a <- range(0, inf));

Breaking conditions. You can add a breaking condition to a list comprehension, and it will stop generating results as soon as the condition evaluates to true. So this means the example down below will only generate values until x reaches 100.

constexpr auto r6 = (x | x <- range(0, inf), brk = x == 100);

Variable assignments. You can have intermediate expressions to use in the rest of the list comprehension!

constexpr auto r7 = (a | b <- range(0, 10), a = b * 2);