Create arrow-functions.md

rfcs/arrow-functions.md

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+# Arrow functions
+
+## Summary
+Reduce boilerplate and improve readability by introducing an arrow function syntax
+
+## Motivation
+Luau is a very flexible language where functions are often used as values. Many libraries use that extensively:
+- [roblox-lua-promise](https://github.com/evaera/roblox-lua-promise)
+- [Fusion](https://github.com/dphfox/Fusion)
+
+Unfortunately, heavy usage of functions as values leads to a lot of unnecessary boilerplate:
+```lua
+Promise.new(function(resolve)
+		somethingThatYields()
+		resolve(1)
+	end)
+	:andThen(function(x)
+		return Promise.new(function(resolve)
+			somethingThatYields()
+			resolve(x + 1)
+		end)
+	end)
+	:andThen(print) --> 2
+```
+
+## Design
+This RFC proposes the addition of the arrow function syntax, which exists in many languages such as [javascript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions), [C#](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/lambda-operator), etc.
+
+The syntax goes as follows:
+```lua
+(...) => expression
+
+(...) =>
+	statements
+end
+```
+
+The only difference is the syntax. Under the hood these still evaluate to regular functions:
+```lua
+function(...)
+	return expression
+end
+```
+
+For single expression arrow functions, you don't need the `end` keyword. They also return the expression:
+
+```lua
+local myArrowFunc = (n) => n * 2
+
+print(myArrowFunc(3)) -- 6
+```
+
+Adding multiple expressions or statements should cause an error. When you want your arrow function to do more, you can insert the `end` keyword. This effectively makes it so that the body of the arrow function is treated as a regular function:
+
+```lua
+local myOtherArrowFunc = (n) =>
+	n *= 2
+	n -= 1
+	return n
+end
+
+print(myOtherArrowFunc(5)) -- 9
+```
+
+### Preview of the arrow function in real use cases
+Promises
+```lua
+Promise.new((resolve) =>
+		somethingThatYields()
+		resolve(1)
+	end)
+	:andThen((x) => Promise.new((resolve) =>
+			somethingThatYields()
+			resolve(x + 1)
+		end)
+	end)
+	:andThen(print) --> 2
+```
+
+Fusion
+```lua
+local text = Value("hello")
+local upperText = Computed((use) => use(text):upper())
+
+New "TextButton" {
+	Text = upperText,
+
+	[OnEvent "Activated"] = () =>
+		print(":)")
+		print(peek(upperText))
+	end
+}
+```
+
+## Drawbacks
+I'm not aware of any drawbacks other than slightly increasing the complexity of the language. The current proposed syntax is invalid, so it should be fully backwards compatible.
+
+## Alternatives
+Instead of `=>`, `->` could be used. Personally I'm not a big fan of that because `->` overlaps with the function type syntax. Arrow functions could potentially also work without the arrow, but I reckon it would be a nightmare to parse.