The terminology used by dry-python is hard to follow for me, unfortunately (e.g. "monadic" does nothing for me :-).

However there are some people on this list who are working on proposals for tensor types. I don't know if it would apply to the builtin List/list type, but using those proposals you could probably define a function like this
```
def zeros(n: int) -> List[n]:
    return [0]*n
```
The actual notation being proposed is a bit more complex, but you get the idea.

On Tue, Sep 22, 2020 at 10:58 AM Никита Соболев <n.a.sobolev@gmail.com> wrote:
SizedList list is not just a type, it is a dependent type (in a way that the actual type is dependent on the concrete value).
That's something very few languages have. And using a language with dependent types is a nightmare for a regular developer.

There are also two projects that can partially solve this problem (based on phantom types, which are just shallow / phantom wrappers for actual types):
- https://github.com/antonagestam/phantom-types
- https://pypi.org/project/phantoms/ (WIP in our dry-python family)

They have a notation like `NonEmpty[List[int]]`, that guarantees that list has at least one element.
And can be extended to use something like `Sized[List[int], 5]` which guarantees that list has specifically 5 elements.

One more thing that might be helpful: monadic `NonEmptyList` type.
I am not sure that there are any actual implementations in Python,
but we are tracking this potential feature for dry-python/returns here: https://github.com/dry-python/returns/issues/257


вт, 22 сент. 2020 г. в 20:24, Teddy Sudol via Typing-sig <typing-sig@python.org>:
To be clear, my point was that the problem is a lot harder than it looks at first, not just saying your example was bad!

Do other languages have something like SizedList? I can't recall any off the top of my head.

-- Teddy


On Tue, Sep 22, 2020 at 10:21 AM Brett Cannon <brett@python.org> wrote:


On Mon, Sep 21, 2020 at 1:23 PM Teddy Sudol <tsudol@google.com> wrote:
Are you asking for a higher-kinded (I believe?) `split` that can specify in its return type how many elements it returns? A problem with that is `some_str.split(a_delimiter, 2)` is only guaranteed to return an array of up to 3 (maxsplit+1) elements. (In fact, your example only works by accident -- it should be `split("a/b", 1)` to guarantee the call will always succeed.)

Yep, I basically screwed up by misremembering that the count argument to str.split() is a max count, not a guaranteed count. Sorry about the noise on this (although I guess the question somewhat can still be asked had I chosen to create a function that guarantees the length of a list).
 

-- Teddy


On Mon, Sep 21, 2020 at 12:38 PM Guido van Rossum <guido@python.org> wrote:
But in a non-toy example how do you expect a static checker to know how many items "a/b".split("/", 2) returns? Hopefully you're not expecting the checker to evaluate "a/b".split("/", 2) at compile time?

If str.split() worked the way my brain told me it worked, I would have asked if `str.split(self, sep: str, maxsplit: int = Literal[2]) -> SizedList[2, str]: ...` made sense.

-Brett
 

I could only see making this pass by using cast(Tuple[str, str], "a/b".split("/", 2)).

On Mon, Sep 21, 2020 at 12:15 PM Brett Cannon <brett@python.org> wrote:
Let's say I have the following function:

```python
def spam(a: str, b: str, c: str, d: int = 42): ...
```

Now if I call that with:
```python
spam(*"a/b".split("/", 2), "hello")
```

That will execute fine and actually result in appropriate types for those arguments.

Unfortunately both Pylance and mypy say that call is incompatible because of `str.split(...) -> List`. And since 'typing' lacks an concept of a sized list like it does for tuples, I don't know how to make this pass.

Am I overlooking something? Or is this a gap due to there not being an equivalent of 'SizedList' in 'typing'?
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