Type assertions for multiple variables fail on function calls returning multiple values

[THENOMNOMMONSTER55 (egg)]

[MercuryBromine0]

Hello! This is intentional behavior, albeit tricky to read.

Type assertion expressions can only return a single value, they always “drop” the rest of the type pack, you can see the same behavior occur if you wrap a function call in parentheses.

local Success, Response = (pcall(..., ...))

If you’re working around typing issues with pcall you could try casting the function to (...any) -> (...any) and then annotating Success and Response.

[THENOMNOMMONSTER55 (egg)]

why is it like this though?
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[THENOMNOMMONSTER55 (egg)]

and no this still doesn’t work by the way, (if i’m doing it the way you wanted me to)

huh well that’s weird

[MercuryBromine0]

Sorry, I should have been clearer:

local Success: boolean, Response: KeyFrameSequence = pcall(KeyframeSequenceProvider.GetkeyFrameSequenceAsync :: (...any) -> (...any), KeyframeSequenceProvider, AnimationId)

… why is it like this though?

Type packs are meant to mimic how “multi-return” in Luau (and Lua before it) work. Part of the semantics of multi-return is that it only “forwards” all of the values in certain positions:

• If you directly assign the result of a multi-return

local function multiret(): (string, number, boolean) --[[ ... ]] end

local str, num, bool = multiret()

• The last value in a return statement:

local function multiret(): (string, number, boolean) --[[ ... ]] end

-- This will return `({}, string, number, boolean)`
local function forward1()
    return {}, multiret()
end

• The last argument to a function call:

local function multiret(): (string, number, boolean) --[[ ... ]] end

-- This will print all returns from `multiret` and the table
print({}, multiret())

In all other cases, we take the “first” element of the type pack, for example:

local function multiret(): (string, number, boolean) --[[ ... ]] end

-- This _only_ prints the first return from `multiret` and "hmmm"
print(multiret(), "hmmm")

When you write a type assertion expression that counts as one of those “all other cases.” The ways I am aware of to do a “type pack assertion expression” are all roundabout.

[THENOMNOMMONSTER55 (egg)]

thank you for the in-depth explanation, i think i had the solution right in front of me the entire time though…

local Success,Response = pcall(function() return KeyframeSequenceProvider:GetKeyframeSequenceAsync(AnimationID)::KeyframeSequence end)