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rfcs/syntax-list-comprehensions.md

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+## 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>`` expression might be required.
+
+Example:
+```lua
+-- 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#
+```csharp
+var list2 = from number in Enumerable.Range(0, 10) select 2*number;
+```
+
+### Rust (using cute)
+```rust
+let vector = c![x, for x in 1..10, if x % 2 == 0];
+```
+
+### Haskell
+```haskell
+[x * 2 | x <- [0 .. 99], x * x > 3]
+```
+
+### R
+```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
+```perl
+my @doubles = map {$_*2} 1..9;
+```
+
+#### Rust
+```rust
+let ns: Vec<_> = (0..100).filter(|x| x * x > 3).map(|x| 2 * x).collect();
+```