[Feature Request] Interfaces and Implementors

[Perodactyl]

The current class system is alright, but imagine this: I have a Stream class which is extended by ReadStream, WriteStream, SeekableStream, etc.

---@class Stream
---@field close fun() Deallocates or hangs up a stream.

---@class SeekableStream: Stream
---@field seek fun(whence: SeekWhence, offset: integer): integer

---@class ReadStream: Stream
---@field read fun(self: ReadStream, length: integer): string|nil

---@class WriteStream: Stream
---@field write fun(data: string): integer

---@class StringStream: ReadStream, SeekableStream Streams data out of a string.

Now I have a function which takes any Seekable, Readable string. How do I specify this? Either we need a conjunction type (ReadStream & SeekableStream, which has keys from both), or we need (preferably) interfaces, which are generic and can be extended by classes. The reason I suggest interfaces is because if I write this:

---@param length integer
---@return string|nil
function StringStream:read(length)
	if self.buffer == nil then
		error("Stream is closed", 2)
	end
	if self.cursor > #self.buffer then
		return nil
	end
	local out = string.sub(self.buffer, self.cursor, math.min(#self.buffer, self.cursor + length))
	self.cursor = self.cursor + #out
	return out
end

and I omit the type annotation, it assumes read is already defined because StringStream extends ReadStream, and it doesn't validate types (arguments and return type are any).

[tomlau10]

This is already supported to a certain extend by author in v3.10.6

• Infer the parameter types of a same-named function in the subclass based on the parameter types in the superclass function. https://github.com/LuaLS/lua-language-server/blob/802e13ccac1153c32b90e6f203fa482fff7a603e/changelog.md?plain=1#L156-L160
• at commit: https://github.com/LuaLS/lua-language-server/commit/7c481f57c407f7e4d0b359a3cfbce66add99ec2f

example

---@class ReadStream
local ReadStream

---@param length integer
---@return string|nil
function ReadStream:read(length)    --> length: integer
end

---@class StringStream: ReadStream

---@class StringStream
local StringStream

function StringStream:read(length)  --> length: integer
end

StringStream:read("")   --> Cannot assign `string` to parameter `integer`.

but may be bugged when using @field syntax ...

From your reproduction code snippet, I believe there maybe some bugs when defining the interface using @field syntax 😕 Because when you use @field to define the read() method, no type inference on the implemented method param.

---@class ReadStream
---@field read fun(self: ReadStream, length: integer): string|nil
local ReadStream

---@class StringStream: ReadStream

---@class StringStream
local StringStream

function StringStream:read(length)  --> length: any
end

StringStream:read("")   --> no type checking

current workaround

So the current workaround is to define the interface using function style (maybe you already have a separate @meta file?), instead of @field, before this issue is fixed.