Closed OliverJAsh closed 7 years ago
IMO, even the strictest type systems tend to allow unsafe array indexing. I think the reason is that a type system should check things that are within its scope of being proven, but without going into the world of proof assistants and formal verification, virtually all array lookups cannot be proven to the type system to be within bounds, so the only practical result is that xs[i]!
gets written everywhere instead, or lookups are done through a function that throws in the case of out-of-bounds.
In other words, there isn't any benefit having a type checker tell you that you failed to prove something without having any means of proving it.
Duplicate of #9235 See also #5203, 7140#issuecomment-192606629, #6229, and #9842
+1 for this. I've been bitten by this compromise a couple times now so count me in for more safety not less!
the only practical result is that
xs[i]!
gets written everywhere instead
If folks are using an unsafe pattern everywhere, maybe that's exactly what should happen. I don't find it particularly onerous. I guess I'd like to see some data on exactly how much impact this would have on existing TypeScript programs before we write it off as making the language unusable.
!
(this could be automated for existing code)Right now the sentiment I've seen from TS team is that 1 is negligible, 2 is huge, and 3 is prohibitive. Are there numbers to back up these hunches?
Some alternatives to !
proliferating:
for (let item of array)
or array.forEach
instead of for(let i = 0; i < array.length; i++)
.Looping over a string[]
array with for-of
or forEach
(or map
, etc.) would never emit undefined, since each result is guaranteed to be in-bounds.
The same could be argued for Map<string,string>
. map.get('foo')
returns string | undefined
, but map.set('foo', undefined)
would be banned so that when looping over values with forEach
or somesuch, you are guaranteed only to get string
values, not string | undefined
.
Rewrite loops with for (let item of array) or array.forEach instead of for(let i = 0; i < array.length; i++).
+1000
this indeed is a duplicate of #9235. thanks @aluanhaddad.
As noted in #9235, this is an intentional behavior, and without this array access would be fairly unusable under --strictNullChecks
.
If authors enable strictNullChecking
then they should expect some boilerplate overhead in return for increased safety. I'm surprised TypeScript isn't consistent in its approach here—as far as I know, this is the only scenario where the team has decided to go against type safety in favour of reduced boilerplate with regards to strictNullChecking
?
I'm not too familiar with many other type systems, but Scala does have List.headOption
which will return Option<T>
, which would be the same thing as T | undefined
in TypeScript.
Indeed there would be slightly more boilerplate if we did return T | undefined
, as discussed above, but surely the job of the type system is to return the correct type, so the author can handle that correctly.
Without this there will be a whole class of bugs that TypeScript will never catch. 😒 😢
@OliverJAsh Scala is an interesting example but headOption
is a way to get the first element in a collection that may or may not be empty instead of first checking if the collection is empty. But the result of headOption
on some TraversableLike[T]
is still an Option[T]
so we are just deferring the check.
Consider the following Scala:
case class Something(name: String)
var xs = Vector[Something]()
def unsafe = {
xs head
}
def safe = {
xs headOption
}
println(safe match {
case Some(s) => s name
case _ => "explicit default"
})
println(unsafe name)
If you index into xs
via xs(n)
then you get T
just like when you call head
instead of headOption
.
Edit:
I'm currently working around this with a helper function:
const arrayHead = <T>(ts: T[]): T | undefined => (
ts[0]
);
it seems bizarre to me that this code is OK when pretty much all other possible errors are covered:
var myArray: string[];
const firstItem = myArray[0];
console.log(firstItem.length);
I vote for firstItem
here to be string | undefined
TS 2.0.3
The type system doesn't know of care about the length of
xs
, so I would expect any lookup in the array to returnT | undefined
.