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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