RFC: switch for statements

docs/switch-for-statements.md

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+# 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:
+
+```lua
+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.