RFC: Result type function

docs/result-type-operator.md

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+# `result` type function
+
+## Summary
+
+This RFC proposes the addition of a new type function `result`, which can be used to make result types in luau. That are consistent with the idiom established by the [pcall and xpcall global functions](https://luau.org/library#global-functions) in the language currently.
+
+## Motivation
+
+Currently, in order to properly type out a result one would have to write the following:
+
+```luau
+type Mrow = ((meow: string, mrrp: number) -> (true, string)) &
+	((meow: string, mrrp: number) -> (false, nil))
+
+--[[
+	Note: the function is being cast to any, 
+	as this rfc is being written with the assumption the developer is using --!strict mode
+--]]
+local mrow: Mrow = (function(meow: string, mrrp: string)
+	if math.random() > .5 then
+		return true, `{meow} {mrrp}`
+	else
+		return false, nil
+	end
+end) :: any
+```
+
+Due to to that making the developer have to write an overloaded function, and with overloaded functions not consistently working. Most will instead do this:
+
+```luau
+type Mrow = (meow: string, mrrp: number) -> (boolean, string?)
+```
+
+Leading to having the type burden passed onto the developer using the function, as now they have to cast the result if they want to avoid type errors:
+
+```luau
+local success, result = mrow("cat food", ":3")
+
+if success then
+	local new_result: string = result :: any
+	print(`new food of type: {new_result}!`)
+else
+	error("no food :(")
+end	
+```
+
+Some may take a diffrent approach by making their own result type, where they break from the luau idiom:
+
+```luau
+type Result<S, F = nil> = {
+	ok: true,
+	value: S
+} | {
+	ok: false,
+	value: F
+}
+
+local function mrow(meow: string, mrrp: string): Result<string>
+	if math.random() > .5 then
+		return { ok = true, value = `{meow} {mrrp}` }
+	else
+		return { ok = false, value = nil }
+	end
+end
+```
+
+## Design
+
+The functionality of the `result` type function is special, with it being an exception as it'll create a type-pack union. Thus using the result type function will not require the developer to write an overloaded function type.
+
+```luau
+type result<S..., F...>
+```
+
+```luau
+local function mrow(meow: string, mrrp: string): result<string>
+	if math.random() > .5 then
+		return true, `{meow} {mrrp}` -- wont error
+
+		return true, nil -- Error message: type 'nil' is not of type 'string'
+	else
+		return false, nil
+	end
+end
+```
+
+## Drawbacks
+
+This type function will need to be special cased, complicating maintenance for the type solver. But, having a result type would allow for pcall and xpcall to get proper types in the future. Where calls to `error` within a function being pcalled would set the second typepack in the result type to the type of the value `error` was called with. 
+
+```luau
+local function mrow(meow: string, mrrp: string): string
+	if math.random() > .5 then
+		return `{meow} {mrrp}` 
+	else
+		error("no cats allowed")
+	end
+end
+
+-- inferred as result<string, string>
+local success, result = pcall(mrow, "cat food", ":3")
+```
+
+## Alternatives
+
+Allow for type pack unions to be written by developers, with the syntax probably looking like this:
+
+```luau
+local function mrow(meow: string, mrrp: string): (true, string) | (false, nil)
+	-- code here
+end
+```
+
+With this example breaking backwards compadibility with some types developers may have written already, as today the following is allowed:
+
+```luau
+-- inferred as: ((meow: string, mrrp: string) -> (true, string)) | false 
+type mrrp = (meow: string, mrrp: string) -> (true, string) | false
+```
+
+Do nothing, and leave it up to developers if they want to write overloaded functions, or make their own result type.