switch for statements

Summary

This RFC proposes a new switch syntax for the Luau language, adding switch statements for clearer, more concise branching logic.

Motivation

The purpose of the switch statement is to simplify readability in cases where multiple branches depend on the value of one variable. Luau currently supports branching by using chains of if-elseif, but these may become wordy if multiple values are to be checked, or if fall-through behavior is desired. A switch syntax allows a simpler, easier to read structure.

Design

The syntax below, now proposed, is using switch value, then the definition of cases with for blocks. Each case can take a tuple of comma-separated values; its code is inside do and end. A default case is a do block at the end.

Example:

switch value
    for "a" do
        -- Code for case "a"
    end
for "b" do
        -- Code for case "b"
        break
    end
    for "a", "b", "c" do
        -- Code for any of these values
        break
    end
    do
-- Code for default case
    end
end

Drawbacks

This syntax adds a pattern not found in Luau, and may necessitate concepts for developers to learn. The syntax looks like the syntax of for loops but isn't a loop, which can be confusing in some cases.

Alternatives

For instance, in the absence of a syntax for switch, developers depend on if-elseif chains: In the absence of a syntax for switch, developers have to depend on if-elseif chains, which often become cumbersome and harder to read with multiple values to check. To that effect, consider handling the fall-through behavior of switch statements. Here every condition is explicitly repeated; thus, code becomes more verbose and error-prone: ```lua if value == "a" then -- Code for case "a" elseif value == "b" then -- Code for case "b" elseif value == "c" or value == "d" then -- Code for case "c" or "d" else -- Default case end


Using the above chained `if`s, the convenience a `switch` provides is consumed by repeated comparisons and explicit coding of fall-through behavior. This can result in longer branching code and is more annoying to maintain when cases are added or altered.

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