Proposal: Static Typing

[Ranando King]

> - If you allow user-defined types, objects can spontaneously change their
type by mutating their prototype.  Thus, you can declare a variable x to
have type Foo and check that you're assigning an instance of Foo to it, but
the value x can become a Bar (different from a Foo) spontaneously without
any operations on x.

At least for static typing, the engine will need to freeze a copy of the
class definition at the time when the static type is referenced. That
frozen type will be associated with the variable in a fixed way, making all
attempts to change the type of the variable fail. Also, that would need to
divorce the frozen type from the dynamic version so the dynamic version can
still be mutated. That concept should work, but it might prove to be
ridiculously complicated since it risks a proliferation of objects all of
type Foo but with different type definitions. One way to get around that is
to flag a type as static the first time it is used in a statically typed
variable definition. This would cause an error to be thrown should any part
of the class be altered. Not sure how workable that is.

> but that begs the question of what is the type of just ['a', 'b'].

It should be any[]. Typing should be as non-aggressive as possible. Auto
typing should only consider the top level data to determine the type, and
in this case, that's an Array. To make c restricted to string elements, the
type would need to be specified as string[].

> If you follow this to its logical conclusion and think about what the
types of various methods that work on arrays should be, you end up with an
enormous and confusing variety of array and object types, which is
something we explored years ago.

Maybe, but that's always the case with any type system that allows for
user-defined types. The main problem here is the memory requirements for
storing all of those types. If I remember the description of V8 internals,
it seems to have already managed this issue. At least in V8, the challenge
would be in permanently associating a specific evolution of an internal
type to a variable.

On Wed, Apr 3, 2019 at 7:50 PM Waldemar Horwat <waldemar at google.com> wrote:

> On 3/23/19 1:34 PM, IdkGoodName Vilius wrote:
> > This is a proposal for static typing. Here is the github repository
> link: https://github.com/CreatorVilius/Proposal-Static-Typing
> > I think it would be great thing in JS.
>
> We intentionally reserved syntax so that something like that would be
> possible in the future.
>
> I've also spent a lot of time working on past proposals to do such
> things.  A few interesting issues would invariably arise that make both
> static and runtime typing unsound:
>
> - If you allow user-defined types, objects can spontaneously change their
> type by mutating their prototype.  Thus, you can declare a variable x to
> have type Foo and check that you're assigning an instance of Foo to it, but
> the value x can become a Bar (different from a Foo) spontaneously without
> any operations on x.
>
> - Something similar happens with trying to type arrays.  You wrote:
>
> let c: auto = ['a', 'b']  // c is now string[]
>
> but that begs the question of what is the type of just ['a', 'b'].
>
> - Is it string[]?  No, it can't be that because you can replace its second
> element with a number.
> - Is it any[]?  Well, in that case c should have type any[], not string[]
> - Is it object?  In that case c should have type Object.
> and so on.
>
> If you follow this to its logical conclusion and think about what the
> types of various methods that work on arrays should be, you end up with an
> enormous and confusing variety of array and object types, which is
> something we explored years ago.  In some cases you'd want structural
> types, in some cases you'd want 'like' types (an array of anything which
> just happens to hold numbers at the moment), and so on.
>
>      Waldemar
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