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78 lines
1.7 KiB
Markdown
78 lines
1.7 KiB
Markdown
## Summary
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Introduce a form of list comprehension using `n for-in-do` syntax.
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## Motivation
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List comprehensions would bring several benefits and prevent code smell.
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In Lua you are encouraged to not modify tables during traversal.
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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.
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## Design
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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`.
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The `n for-in-do` expression must match ``<identifier> for <identifier> in <expr> do``.
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A ``if <expr> then`` expression might be required.
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Example:
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```lua
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-- normal
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local t = {1,2,3,4,5,6,7,8,9}
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local onlyEven = {}
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for i,n in pairs(t) do
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if n%2 == 0 then
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table.insert(onlyEven, n)
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end
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end
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-- list comprehensions
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local t = {1,2,3,4,5,6,7,8,9}
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local onlyEven = {n for n in t do if n%2 == 0}
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```
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## Drawbacks
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List comprehensions may be misused.
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The list comprehensions have similar drawbacks to the if expression drawbacks.
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https://github.com/Roblox/luau/blob/master/rfcs/syntax-if-expression.md
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## Alternatives
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### C#
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```csharp
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var list2 = from number in Enumerable.Range(0, 10) select 2*number;
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```
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### Rust (using cute)
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```rust
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let vector = c![x, for x in 1..10, if x % 2 == 0];
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```
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### Haskell
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```haskell
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[x * 2 | x <- [0 .. 99], x * x > 3]
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```
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### R
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```r
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x <- 0:100
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S <- 2 * x[x ^ 2 > 3]
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```
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## Function syntax
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List comprehensions could be implemented as functions with ``table.map`` or ``table.filter`` .
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### Examples:
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#### Perl
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```perl
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my @doubles = map {$_*2} 1..9;
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```
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#### Rust
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```rust
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let ns: Vec<_> = (0..100).filter(|x| x * x > 3).map(|x| 2 * x).collect();
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```
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