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Functional ProgrammingAn Introduction
2015.05.21
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Things to know
> Ask questions > node > babeljs.io
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Reduce/Map/Filter in Javascript
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Without State
Without Mutation
What is functional programming?
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What are we going to focus on?
> Immutability > No state > Higher-order functions > Lots of code
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What aren’t we going to focus on?
> Types > Monads > Being tricky to feel smart
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Why even care?
> Explicitness > Concurrency / Parallelism > Understandability
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Reduce/Map/Filter in Javascript
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Without State
> Without Mutation
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let a = 1;a = 2;
console.log(a);// => 2// :(
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let a = 1;a = 2;
console.log(a);// => 2// :(
Enforced via discipline :)
Reduce/Map/Filter in Javascript
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> Without State
Without Mutation
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class Foo attr_accessor :bar
def initialize(bar) @bar = bar end
def increment(i) i + 1 endend
foo = Foo.new(1)
# things happen here# .# ..# ...
# what does this return?p foo.bar
# what does this return?p foo.increment(2)
Let’s add some numbers!
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let sumIter = function (n) { let sum = 0;
for (let i = 0; i <= n; i++) { sum = sum + i; }
return sum;}
console.log(sumIter(50));// => 1275
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let sumIter = function (n) { let sum = 0;
for (let i = 0; i <= n; i++) { sum = sum + i; }
return sum;}
console.log(sumIter(50));// => 1275
Recursion!
> Base case > Recursive call
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let sumRec = function (n) { if (n === 0) { return 0; } else { return n + sumRec(n - 1); }}
console.log(sumRec(50));// => 1275
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let sumRec = function (n) { if (n === 0) { return 0; } else { return n + sumRec(n - 1); }}
console.log(sumRec(50000000));// => RangeError: Maximum call stack size exceeded
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// sumRec(5)// 1 + sumRec(4)// 1 + (1 + sumRec(3))// 1 + (1 + (1 + sumRec(2)))// 1 + (1 + (1 + (1 + sumRec(1))))// 1 + (1 + (1 + (1 + (1 + sumRec(0)))))// 1 + (1 + (1 + (1 + (1 + 0))))// 1 + (1 + (1 + (1 + 1)))// 1 + (1 + (1 + 2))// 1 + (1 + 3)// 1 + 4// 5
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let sumTail = function (n, acc=0) { if (n === 0) { return acc; } else { return sumTail(n - 1, acc + n); }}
console.log(sumTail(50000000));// => 1250000025000000
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// sumTail(5)// sumTail(5, 0)// sumTail(4, 1)// sumTail(3, 2)// sumTail(2, 3)// sumTail(1, 4)// sumTail(0, 5)// 5
> Reduce/Map/Filter in Javascript
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Without State
Without Mutation
Higher-order functions!
> Functions as values > Take functions as input > Return functions
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Higher-order functions!
YOU KNOW THIS!!
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File.open("/tmp/setec_astronomy", "w") do |f| # do stuff with fend
[1, 2, 3].map do |i| i + 1end
# => [2, 3, 4]
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// take a function as an argument
let callMeMaybe = function (f) { if (Math.random() < 0.5) { return f(); } else { return undefined; }}
for (var i of [1, 2, 3, 4, 5]) { console.log(`Try ${i}`);
callMeMaybe(function () { console.log("I GOT CALLED!"); });}
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Try 1 I GOT CALLED!Try 2 Try 3 I GOT CALLED!Try 4 I GOT CALLED!Try 5
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// return a function
let incrementor = function (n) { return function (i) { return i + n; }}
let add1 = incrementor(1);console.log(add1(2));// => 3
OK! Let’s implement reduce!
> Tail recursion > Higher-order functions > No state > No mutation
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let coll = [1, 2, 3, 4, 5];
let result = reduce(coll, 0, function (i, sum) { return sum + i;});
console.log(result);// => 15
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let reduce = function (coll, acc, fn) { if (coll.length === 0) { return acc; }
let head = coll[0]; let rest = coll.slice(1, coll.length);
let newAcc = fn(head, acc);
return reduce(rest, newAcc, fn);}
OK! Let’s implement map!
> ONLY USING REDUCE!!?!??!
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let coll = [1, 2, 3, 4, 5];
let result = map(coll, function (i) { return i + 1;});
console.log(result);// => [2, 3, 4, 5, 6]
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let map = function (coll, fn) { return reduce(coll, [], function (i, acc) { return acc.concat(fn(i)); });}
OK! Let’s implement filter!
> ONLY USING REDUCE!!?!??! > This time less shockingly revealed!!
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let coll = [1, 2, 3, 4, 5];
let result = filter(coll, function (i) { return i < 4;});
console.log(result);// => [1, 2, 3]
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let filter = function (coll, fn) { return reduce(coll, [], function (i, acc) { if (fn(i)) { return acc.concat(i); } else { return acc; } });}
Reduce/Map/Filter in Javascript
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Without State
Without Mutation
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Things to check out
> Functional Javascript (book) > The Little Schemer (book) > Elixir (language)
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braintreepayments.com
End Slide_
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Thanks! Questions?
