Type inferencing of filter style functions

Given:

---@generic T
---@param f fun(a: T)
---@param t table<any, T>
---@return T[] (table)
local function tbl_filter(f, t)
  return t
end

---@type string[]
local s = {'a', 'b', 'c'}

local r1 = tbl_filter(function(a) end, s)

r1 is inferred as unknown[] because a is unknown despite s being known. However, could we infer a from s and force the typechecker to insist a is a string?

If I remove the type from a:

---@generic T
---@param f fun(a)
---@param t table<any, T>
---@return T[] (table)
local function tbl_filter2(f, t)
  return t
end

local r2 = tbl_filter2(function(_) end, s)

r2 is now inferred as string[] because only s is used to infer the type.

I don't know how to design a strict rule.

Look at all the sources of T
Make sure all the known sources match
For any unknown sources which come anonymous function arguments (or return values) insist it is the same type from the known sources.

Would that work?

I will try it, thank you.

Interestingly, this does work as expected when using list T[] instead of table<any,T> (using version 3.7.3)

code:

---Filter a list
---@generic T
---@param list T[]
---@param p fun(v:T):boolean
---@return T[]
function iter.filter(list, p)
	local filtered = {}
	for _, v in ipairs(list) do
		if p(v) then
			table.insert(filtered, v)
		end
	end
	return filtered
end

-- number[]
local input = { 1, 2, 3, 4, 5 }

local filtered = iter.filter(input, function(v)
    return v % 2 == 0
end)

Screenshots of inferred types

filtered array:

filter function param:

Navigation Menu

Repository navigation

Type inferencing of filter style functions #1883

Screenshots of inferred types

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Type inferencing of filter style functions #1883

Description

Activity

sumneko commented on Feb 6, 2023

lewis6991 commented on Feb 6, 2023

sumneko commented on Feb 6, 2023

itsfrank commented on Dec 23, 2023

Screenshots of inferred types

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions