Sealing a table type

[giucam]

Suppose i have some code like

--!strict

local function foo<T>(n: string, t: T): T
    ...do something...
    return t
end

type A = {a:number}
local bar:A = foo("bar", { a = 2 })
bar.b = 2

local bar2 = foo("bar", { a = 2 })
bar2.b = 2

Typechecking this will complain abount the bar.b = 2 because A doesn't have a b field, but will say nothing about bar2.b = 2 because, i guess, T in foo is unsealed so it returns an unsealed table. Is there a way to return a sealed T from foo?

[asajeffrey]

Some possibilities...

• Make foo have type (string, A) -> A.
• Declare local x:A = { a= 2 } and call foo("bar", x).
• Write a constrictor function A.new and use that rather than a table literal.

I don't know if these will help with your use case?

[giucam]

• Make foo have type (string, A) -> A.

Do you mean explicitly A, without using the generic T? That won't work for me.

• Declare local x:A = { a= 2 } and call foo("bar", x).
• Write a constrictor function A.new and use that rather than a table literal.

I wanted to avoid having to define a type for this, though, because it would be used just in one call to foo.