luau/rfcs/syntax-list-comprehensions.md

Summary

Introduce a form of list comprehension using n for-in-do syntax.

Motivation

List comprehensions would bring several benefits and prevent code smell. In Lua you are encouraged to not modify tables during traversal.

When traversing a table to exclude all the odd numbers you'd be creating a large statement to get rid of them, with list comprehensions this could be shortened and cleaner.

Design

To solve these problems, I propose a n for-in-do expression form that is syntactically similar to a for statement, but lacks terminating end.

The n for-in-do expression must match <identifier> for <identifier> in <expr> do. A if <expr> then expression might be required.

Example:

-- normal
local t = {1,2,3,4,5,6,7,8,9}
local onlyEven = {}
for i,n in pairs(t) do
    if n%2 == 0 then 
      table.insert(onlyEven, n)
    end
end

-- list comprehensions
local t = {1,2,3,4,5,6,7,8,9}
local onlyEven = {n for n in t do if n%2 == 0} 

Drawbacks

List comprehensions may be misused.

The list comprehensions have similar drawbacks to the if expression drawbacks. https://github.com/Roblox/luau/blob/master/rfcs/syntax-if-expression.md

Alternatives

C#

var list2 = from number in Enumerable.Range(0, 10) select 2*number;

Rust (using cute)

let vector = c![x, for x in 1..10, if x % 2 == 0];

Haskell

[x * 2 | x <- [0 .. 99], x * x > 3]

R

 x <- 0:100
 S <- 2 * x[x ^ 2 > 3]

Function syntax

List comprehensions could be implemented as functions with table.map or table.filter .

Examples:

Perl

my @doubles = map {$_*2} 1..9;

Rust

let ns: Vec<_> = (0..100).filter(|x| x * x > 3).map(|x| 2 * x).collect();