How to work with collections?

[sobolevn]

Currently we don't have any utilities to work with collections.
I need to investigate this use-case.

[sobolevn]

Here's the first way: reduce

from functools import reduce
from typing import List

from returns.result import Success, Failure, Result

items: List[Result[int, int]] = [
    Success(1),
    Success(2),
    Failure(3),
    Success(4),
    Failure(5),
]

print(
    reduce(
        lambda x, y: x.bind(
            lambda v1: y.fix(
                lambda _: 0,
            ).map(
                lambda v2: v1 + v2,
            ),
        ),
        items,
    )
)
# => <Success: 7>

And it is compatible with mypy:

» mypy ex.py
Success: no issues found in 1 source file

Let's try something bad:

# changing `v1 + v2` to `v1 + x`
» mypy ex.py
ex.py:14: note: Revealed type is 'builtins.list[returns.result.Result[builtins.int, builtins.int]]'
ex.py:22: error: Unsupported operand types for + ("int" and "Result[int, int]")
Found 1 error in 1 file (checked 1 source file)

[sobolevn]

Example from fp-ts:

import { IO, io } from 'fp-ts/lib/IO'
import { array } from 'fp-ts/lib/Array'
import { log } from 'fp-ts/lib/Console'

const logGiraffe: IO<void> = log('giraffe')
const logZebra: IO<void> = log('zebra')

const logGiraffeThenZebra: IO<void[]> = array.sequence(io)([logGiraffe, logZebra])

logGiraffeThenZebra() // returns undefined and outputs words "giraffe" and "zebra" to console

https://github.com/gcanti/fp-ts/blob/e08a16e1060437992aae9970e578d53a27b267d8/src/IO.ts

[sobolevn]

Well, we won't support Array or similar. Just use collections as usual.

[sobolevn]

With the addition of Future we need to support collections. Because, it is a popular use-case to spawn multiple coroutines, collect and process their results.

I like this approach: https://github.com/correl/typesafe-monads/blob/master/monads/reader.py#L54-L59

We can add iterable function to converters module. And it has to support all container types.

[sobolevn]

Or maybe sequence() -> Sequence[T] is better?