refine type of array access to non-undefined given proper for-loop header

Maybe Hegel could check loop headers, and if the iteration variables are written properly, then it can narrow the types of array access to non-undefined.

For example, in the code

let nums: Array<number> = [0,1,2,3]

for (let i=0; i<nums.length; i++) {
  const n = nums[i] // must be a number
  useNum(n)
}

it will not go out of bounds, so there is no check needed like n != undefined && useNum(n).

Sorry for opening so many issues! There are lots of ideas. 😃

[peter-leonov]

Would it be useful to have two inbuilt types Array and SparseArray to represent this ?

I also like the idea of having two distinct types here. Following the rule of the least surprise I’d say that the Array type should stay sparse by default and a more refined NonSparseArray (or Vector) can be a nice addition to the standard type library.

[trusktr]

I had no idea forEach skipped holes. 😆 That’s neat! After all these years, it never mattered (or I wasn’t aware that it mattered).