add running

[ebonnal]

Co-author: @pierrenodet

Add the behavior of itertools.accumulate (aka .scan in Scala/Haskell).

Examples

from streamable import running

cumulative_sum = (
    Stream(range(10))
    .map(running(lambda cumsum, i: cumsum + i, initial=0))
)

assert list(cumulative_sum) == [0, 1, 3, 6, 10, 15, 21, 28, 36, 45]

fibonacci = (
    Stream(range(10))
    .map(running(lambda acc, _: (acc[1], acc[0] + acc[1]), initial=(0, 1)))
    .map(itemgetter(0))
)

assert list(fibonacci) == [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

TODO:

• validate that this is the most elegant way to add this behavior
• validate that the signature follows conventions existing elsewhere
• more unit tests (e.g. are we happy with how it behaves in case exceptions are thrown by the binary func?)
• find a concise yet explicit description for the README and the docstring (we can get inspiration from the itertools.accumulate's doc)
• validate the choice of the example in the README

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b172b49) to head (1424ca2).
Report is 26 commits behind head on main.

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