Intro to Functional Programming

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An intro to Functional Programming Arusha Coders July 6, 2016 by Reuben Cummings @reubano #arushacoders

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a new way of thinking @reubano #arushacoders

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pure functions

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pure functions function input output outside world

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imperative version @reubano #arushacoders

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class Rectangle(object): def __init__(self, length, width): self.length = length self.width = width @property def area(self): return self.length * self.width def grow(self, amount): length = self.length * amount self.length = length

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>>> r = Rectangle(2, 3) >>> r.length 2 >>> r.area 6 >>> r.grow(2) >>> r.length 4 >>> r.area 12

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>>> r = Rectangle(2, 3) >>> r.length 2 >>> r.area 6 >>> r.grow(2) >>> r.length 4 >>> r.area 12 same commands, different results!?

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>>> r = Rectangle(2, 3) >>> r.length 2 >>> r.area 6 >>> r.grow(2) >>> r.length 4 >>> r.area 12 same commands, different results!?

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functional version @reubano #arushacoders

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def make_rect(length, width): return (length, width) def grow_rect(rect, amount): return (rect[0] * amount, rect[1]) def get_length (rect): return rect[0] def get_area (rect): return rect[0] * rect[1]

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>>> r = make_rect(2, 3) >>> get_length(r) 2 >>> grow_rect(r, 2) (4, 3) >>> get_area(r) 6 >>> get_length(r) 2 >>> get_area(r) 6 returned a new object

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>>> r = make_rect(2, 3) >>> get_length(r) 2 >>> grow_rect(r, 2) (4, 3) >>> get_area(r) 6 >>> get_length(r) 2 >>> get_area(r) 6 attributes didn’t change!

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>>> r = make_rect(2, 3) >>> get_length(r) 2 >>> grow_rect(r, 2) (4, 3) >>> get_area(r) 6 >>> get_length(r) 2 >>> get_area(r) 6 attributes didn’t change!

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>>> big_r = grow_rect(r, 2) assign output of the grow function to a new object

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>>> big_r = grow_rect(r, 2) >>> get_area(big_r) 12 >>> get_length(big_r) 4 attributes of new object are as expected

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so what?? @reubano #arushacoders

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from time import sleep class ExpensiveRectangle(Rectangle): @property def area(self): sleep(5) return self.length * self.width >>> r = ExpensiveRectangle(2, 3) >>> r.area 6 expensive!! >>> r.area 6

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from functools import lru_cache @lru_cache() def exp_get_area (rect): sleep(5) return rect[0] * rect[1] >>> r = make_rect(2, 3) >>> exp_get_area(r) >>> exp_get_area(r) 6 6 expensive!! memoization

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lazy evaluation

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eager version @reubano #arushacoders

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def double(ints): doubled = [] for i in ints: doubled.append(i * 2) return doubled >>> some_ints = range(1, 11) >>> double(some_ints) [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

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lazy version @reubano #arushacoders

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def gen_double(ints): for i in ints: yield i * 2 >>> gen_double(some_ints) >>> gen = gen_double(some_ints) >>> next(gen) 2 >>> next(gen) 4

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can we do better? @reubano #arushacoders

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def gen_double(ints): >>> gen_double(some_ints) >>> gen = gen_double(some_ints) >>> next(gen) 2 >>> next(gen) 4 return (i * 2 for i in ints)

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who cares?? @reubano #arushacoders

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def exp_double(ints): doubled = [] for i in ints: sleep(5) doubled.append(i * 2) return doubled def exp_gen_double(ints): for i in ints: sleep(5) yield i * 2 expensive!! expensive!!

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>>> exp_double(some_ints) >>> gen = gen_exp_double(some_ints) >>> next(gen) >>> next(gen) [2, 4, 6, 8, 10, 12, 14, 16, 18, 20] 2 4 >>> next(gen) 6

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but wait, there’s more! @reubano #arushacoders

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>>> from itertools import count >>> >>> inf_ints = count(1) >>> double(inf_ints) inﬁnite iterator!!!

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>>> from itertools import count >>> >>> inf_ints = count(1) >>> double(inf_ints) ... >>> gen = gen_double(inf_ints) >>> next(gen) 42220156 >>> next(gen) 42220158 inﬁnite iterator!!!

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stream processing

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>>> stream = range(1, 10) >>> doubled = [2 * x for x in stream] >>> doubled [2, 4, 6, 8, 10, 12, 14, 16, 18] >>> old = [x for x in doubled if x > 10] >>> old [12, 14, 16, 18] >>> divided = [x / 3 for x in old] >>> divided [4.0, 4.6667, 5.3333, 6.0] transform stream at each stage

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mini riko demo @reubano #arushacoders

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extracting web data

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RSS feeds

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RSS feeds

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>>> from riko.modules import ( ... fetch, truncate, sort, ... xpathfetchpage as xfetchpage) >>> >>> url = 'news.ycombinator.com/rss' >>> stream = fetch.pipe( ... conf={'url': url}) >>> stream_l = list(stream) >>> len(stream_l) 30 >>> stream_l[0] {'comments': 'http...', 'title': 'Think...', 'published': 'Tue, 5 Ju...'} convert generator into a list

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>>> truncated = truncate.pipe( ... stream_l, conf={'count': 5}) >>> truncated_l = list(truncated) >>> len(truncated_l) 5 >>> sorted = sort.pipe( ... truncated_l, conf={ ... 'rule': { ... 'sort_key': 'title'}}) >>> >>> sorted_l = list(sorted) >>> sorted_l[0] {'title': 'Building a BitTorre...', ...}

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>>> xpath = '/html/body/center/table...' >>> xconf = { ... 'url': {'subkey': 'comments'}, ... 'xpath': xpath} >>> comments = [ ... xfetchpage.pipe(s, conf=xconf) ... for s in sorted_l] >>> item = next(comments[0]) >>> item.keys() ['p', 'div', 'class', 'content'] >>> item['content'] "Great work! Quite annoying actually..."

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pipeline version @reubano #arushacoders

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from riko.collections import sync Pipe = sync.SyncPipe sconf = {'rule': {'sort_key': 'title'}} comments = ( Pipe('fetch', conf={'url': url}) .truncate(conf={'count': '5'}) .sort(conf=sconf) .xpathfetchpage(conf=xconf) .output) item = next(comments)

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head to head functional imperative cacheable ✔ testable ✔ ? composable ✔ simple ✔ stateful ✔ @reubano #arushacoders

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Resources https:/ /docs.python.org/3.5/library/itertools.html https:/ /docs.python.org/3.5/library/functools.html https:/ /lodash.com/ http:/ /coffeescript.org/ https:/ /www.youtube.com/watch?v=7aYS9PcAITQ https:/ /github.com/nerevu/riko @reubano #arushacoders

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Resources https:/ /github.com/reubano/meza http:/ /ocw.mit.edu/courses/electrical-engineering- and-computer-science/6-001-structure-and- interpretation-of-computer-programs- spring-2005/ https:/ /www.infoq.com/presentations/Simple- Made-Easy @reubano #arushacoders

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Thanks! Reuben Cummings @reubano @reubano #arushacoders