Fun With Functions Block Scope { let a; { let

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Fun With Functions Block Scope { let a; { let

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Description: Fun With Functions Block Scope let a; let b; a b a Function Scope function green() let a; function yellow() let b; a b a Function Scope function green() let a; function yellow() let b; a

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Presentation Transcript

slide1. Fun With Functions 
slide2. Block Scope {
let a;
{
let b;
… a …
… b …
}
… a …
} 
slide3. Function Scope function green() {
let a;
function yellow() {
let b;
… a …
… b …
}
… a …
} 
slide4. Function Scope function green() {
let a;
function yellow() {
let b;
… a …
… b …
}
… a …
} a 
slide5. Closure function green() {
let a;
function yellow() {
let b;
… a …
… b …
}
… a …
} a b 
slide6. Inner survives the outer function green() {
let a;
return function yellow() {
let b;
… a …
… b …
};
… a …
} 
slide7. Write an identity function that takes an argument and returns that argument. identity(3) // 3 
slide8. function identity(x) {
return x;
}

const identity = function identity(x) {
return x;
}; 
slide9. https://www.crockford.com/pp/fun.html
<!DOCTYPE html><html>
<head><meta charset="UTF-8"></head>
<body><script src="functions.js">
</script></body></html>

https://www.crockford.com/pp/functions.js
function identity(x) {
return x;
}
console.log("identity", 3, identity(3)); 
slide10. The Rules If you have a question, you must ask it.
I won’t debug your stuff.
If you need more time, let me know. 
slide11. Quiz 
slide12. What is x? function funky(o) {
o = null;
}
let x = [];
funky(x);
console.log(x); A.null
B.[]
C.undefined
D.throw 
slide13. What is x? function funky(o) {
o = null;
}
let x = [];
funky(x);
console.log(x); A.null
B.[]
C.undefined
D.throw 
slide14. function funky(o) {
o = null;
}
let x = [];
funky(x);
console.log(x); global [ ] 
slide15. function funky(o) {
o = null;
}
let x = [];
funky(x);
console.log(x); global [ ] funky 
slide16. function funky(o) {
o = null;
}
let x = [];
funky(x);
console.log(x); global [ ] funky 
slide17. What is x? function swap(a, b) {
let temp = a;
a = b;
b = temp;
}
let x = 1;
let y = 2;
swap(x, y);
console.log(x); A.1
B.2
C.undefined
D.throw 
slide18. What is x? function swap(a, b) {
let temp = a;
a = b;
b = temp;
}
let x = 1;
let y = 2;
swap(x, y);
console.log(x); A.1
B.2
C.undefined
D.throw 
slide19. function swap(a, b) {
let temp = a;
a = b;
b = temp;
}
let x = 1;
let y = 2;
swap(x, y);
console.log(x); global 1 2 
slide20. function swap(a, b) {
let temp = a;
a = b;
b = temp;
}
let x = 1;
let y = 2;
swap(x, y);
console.log(x); global 1 2 swap 
slide21. function swap(a, b) {
let temp = a;
a = b;
b = temp;
}
let x = 1;
let y = 2;
swap(x, y);
console.log(x); global 1 2 swap 
slide22. Write three binary functions, add , sub, and mul, that take two numbers and return their sum, difference, and product. add(3, 4) // 7
sub(3, 4) // -1
mul(3, 4) // 12 
slide23. function add(first, second) {
return first + second;
}

function sub(first, second) {
return first - second;
}

function mul(first, second) {
return first * second;
} 
slide24. Write a function identityf that takes an argument and returns a function that returns that argument. const unity = identityf(1);
unity() // 1 
slide25. function identityf(x) {
return function () {
return x;
};
} 
slide26. Write a function addf that adds from two invocations. addf(3)(4) // 7 
slide27. function addf(first) {
return function (second) {
return first + second;
};
} 
slide28. Write a function curry that takes a binary function and an argument, and returns a function that can take a second argument. let add3 = curry(add, 3);
add3(4) // 7

curry(mul, 5)(6) // 30 
slide29. function curry(binary, first) {
return function (second) {
return binary(first, second);
};
} 
slide30. function curry(binary, first) {
return function (second) {
return binary(first, second);
};
}

function curry(func, first) {
return liftf(func)(first);
} currying

schönfinkelisation 
slide31. function curry(func) {
const slice = Array.prototype.slice;
const args = slice.call(arguments, 1);
return function () {
return func.apply(
null,
args.concat(slice.call(arguments, 0))
);
};
}

function curry(func, ...first) {
return function (...second) {
return func(...first, ...second);
};
} 
slide32. function curry(func) {
const slice = Array.prototype.slice;
const args = slice.call(arguments, 1);
return function () {
return func.apply(
null,
args.concat(slice.call(arguments, 0))
);
};
}

function curry(func, ...first) {
return function (...second) {
return func(...first, ...second);
};
} 
slide33. Write a function curryr that takes a binary function and a second argument, and returns a function that can take a first argument. let dec = curryr(sub, 1);
dec(7) // 6 
slide34. function curryr(binary, second) {
return function (first) {
return binary(first, second);
};
} 
slide35. Write a function liftf that takes a binary function, and makes it callable with two invocations. let addf = liftf(add);
addf(3)(4) // 7
liftf(mul)(5)(6) // 30 
slide36. function liftf(binary) {
return function (first) {
return function (second) {
return binary(first, second);
};
};
}

function liftf(binary) {
return function (first) {
return curry(binary, first);
};
} 
slide37. function liftf(binary) {
return function (first) {
return function (second) {
return binary(first, second);
};
};
}

function liftf(binary) {
return function (first) {
return curry(binary, first);
};
} 
slide38. Without writing any new functions, show four ways to create the inc function. let inc = _ _ _ ;
inc(5) // 6
inc(inc(5)) // 7 
slide39. 1. inc = addf(1);

2. inc = curry(add, 1);

3. inc = curryr(add, 1);

4. inc = liftf(add)(1); 
slide40. Write a function twice that takes a binary function and returns a unary function that passes its argument to the binary function twice. add(11, 11) // 22
let double = twice(add);
double(11) // 22
let square = twice(mul);
square(11) // 121 
slide41. function twice(binary) {
return function (a) {
return binary(a, a);
};
} 
slide42. Write reverse, a function that reverses the arguments of a binary function. let bus = reverse(sub);
bus(3, 2) // -1 
slide43. function reverse(binary) {
return function (first, second) {
return binary(second, first);
};
}

function reverse(any) {
return function (...args) {
return any(...args.reverse());
};
} 
slide44. Write a function composeu that takes two unary functions and returns a unary function that calls them both. composeu(double, square)(5) // 100 
slide45. function composeu(f, g) {
return function (a) {
return g(f(a));
};
} 
slide46. function composeu(...functions) {
return function (value) {
return functions.forEach(
function (func) {
value = func(value);
}
);
return value;
};
} 
slide47. Write a function composeb that takes two binary functions and returns a function that calls them both. composeb(add, mul)(2, 3, 7) // 35 
slide48. function composeb(f, g) {
return function (a, b, c) {
return g(f(a, b), c);
};
} 
slide49. Write a limit function that allows a function to be called a limited number of times. let add_ltd = limit(add, 1);
add_ltd(3, 4) // 7
add_ltd(3, 5) // undefined 
slide50. function limit(func, count) {
return function (...args) {
if (count >= 1) {
count -= 1;
return func(...args);
}
return undefined;
};
} 
slide51. Generators Generator factory:
A function that returns a generator function.

Generator function:
A function that returns a series of values. 
slide52. Write a from factory that produces a generator that will produce a series of values. let index = from(0);
index() // 0
index() // 1
index() // 2 
slide53. function from(start) {
return function from_generator() {
let next = start;
start += 1;
return next;
};
} 
slide54. Write a to factory that takes a generator and an end value, and returns a generator that will produce numbers up to but not including that limit. let index = to(from(2), 4);
index() // 2
index() // 3
index() // undefined 
slide55. function to(gen, end) {
return function to_generator() {
let value = gen();
if (value < end) {
return value;
}
};
} 
slide56. Write a fromTo factory that produces a generator that will produce values in a range. let index = fromTo(0, 3);
index() // 0
index() // 1
index() // 2
index() // undefined 
slide57. function fromTo(start, end) {
return to(from(start), end);
} 
slide58. Write an element factory that takes an array and a generator and returns a generator that will produce elements from the array. let ele = element(
["a", "b", "c", "d"],
fromTo(1, 3)
);
ele() // "b"
ele() // "c"
ele() // undefined 
slide59. function element(array, gen) {
return function element_generator() {
let value = gen();
if (value !== undefined) {
return array[value];
}
};
} 
slide60. Modify the element factory so that the generator argument is optional. If a generator is not provided, then each of the elements of the array will be produced. let ele = element(["a", "b", "c", "d"]);
ele() // "a"
ele() // "b"
ele() // "c"
ele() // "d"
ele() // undefined 
slide61. function element(
array,
gen = fromTo(0, array.length)
) {
return function element_generator() {
let value = gen();
if (value !== undefined) {
return array[value];
}
};
} 
slide62. Write a collect factory that takes a generator and an array and produces a generator that will collect the results in the array. let array = [];
let col = collect(fromTo(0, 2), array);
col() // 0
col() // 1
col() // undefined
array // [0, 1] 
slide63. function collect(gen, array) {
return function collect_generator() {
let value = gen();
if (value !== undefined) {
array.push(value);
}
return value;
};
} 
slide64. Write a filter factory that takes a generator and a predicate and produces a generator that produces only the values approved by the predicate. let fil = filter(
fromTo(0, 5),
function divisible_by_3(value) {
return (value % 3) === 0;
}
);
fil() // 0
fil() // 3
fil() // undefined 
slide65. function filter(gen, predicate) {
return function filter_generator() {
let value;
do {
value = gen();
} while (
value !== undefined
&& !predicate(value)
);
return value;
};
} 
slide66. function filter(gen, predicate) {
return function filter_generator() {
let value = gen();
if (
value === undefined
|| predicate(value)
) {
return value;
}
return filter_generator();
};
} 
slide67. Write a concat factory that takes two generators and produces a generator that combines the sequences. let con = concat(fromTo(0, 3), fromTo(0,2));
con() // 0
con() // 1
con() // 2
con() // 0
con() // 1
con() // undefined 
slide68. function concat(gen1, gen2) {
let gen = gen1;
return function concat_generator() {
let value = gen();
if (value !== undefined) {
return value;
}
gen = gen2;
return gen();
};
} 
slide69. function concat(...gens) {
let next = element(gens);
let gen = next();
return function concat_generator() {
if (gen !== undefined) {
let value = gen();
if (value !== undefined) {
return value;
}
gen = next();
return concat_generator();
}
};
} 
slide70. Write a repeat function that takes a generator and calls it until it returns undefined. let array = [];
repeat(collect(fromTo(0, 4), array));
log(array); // 0, 1, 2, 3 
slide71. function repeat(gen) {
let value;
do {
value = gen();
} while (value !== undefined);
}

function repeat(func) {
if (func() !== undefined) {
return repeat(func);
}
} 
slide72. function repeat(gen) {
let value;
do {
value = gen();
} while (value !== undefined);
}

function repeat(gen) {
if (gen() !== undefined) {
return repeat(gen);
}
} 
slide73. Write a map function that takes an array and a unary function, and returns an array containing the result of passing each element to the unary function. Use the repeat function. map([2, 1, 0], inc) // [3, 2, 1] 
slide74. function map(array, unary) {
const ele = element(array);
const result = [];
repeat(collect(
function () {
let value = ele();
if (value !== undefined) {
return unary(value);
}
},
result
));
return result;
} 
slide75. Write a reduce function that takes an array and a binary function, and returns a single value. Use the repeat function. reduce([], add) // undefined
reduce([2], add) // 2
reduce([2, 1, 0], add) // 3 
slide76. function reduce(array, binary) {
const ele = element(array);
let result;
repeat(function () {
let value = ele();
if (value !== undefined) {
result = (
result === undefined
? value
: binary(result, value)
);
}
return value;
});
return result;
} 
slide77. Make a gensymf factory that makes a unique symbol generator. let geng = gensymf("G");
let genh = gensymf("H");
geng() // "G1"
genh() // "H1"
geng() // "G2"
genh() // "H2" 
slide78. function gensymf(prefix) {
let gen = from(1);
return function gensym_generator() {
return prefix + gen();
};
} 
slide79. Write a gensymff factory factory that takes a factory function and a seed and returns a gensymf. let gensymf = gensymff(from, 1);
let geng = gensymf("G");
let genh = gensymf("H");
geng() // "G1"
genh() // "H1"
geng() // "G2"
genh() // "H2" 
slide80. function gensymff(factory, seed) {
return function (prefix) {
let gen = factory(seed);
return function generator() {
return prefix + gen();
};
};
} geng() // "G1"
genh() // "H1"
geng() // "G2"
genh() // "H2" 
slide81. function gensymff(factory, seed) {
let gen = factory(seed);
return function (prefix) {
return function generator() {
return prefix + gen();
};
};
} geng() // "G1"
genh() // "H2"
geng() // "G3"
genh() // "H4" 
slide82. Make a fibonaccif factory that returns a generator that will produce the fibonacci sequence. let fib = fibonaccif(0, 1);
fib() // 0
fib() // 1
fib() // 1
fib() // 2
fib() // 3
fib() // 5 
slide83. function fibonaccif(a, b) {
let i = 0;
return function () {
let next;
if (i === 0) {
i = 1;
return a;
}
if (i == 1) {
i = 2;
return b;
}
next = a + b;
a = b;
b = next;
return next;
};
} 
slide84. function fibonaccif(a, b) {
return function () {
let next = a;
a = b;
b += next;
return next;
};
} 
slide85. const single = composeu(identityf, curryr(limit, 1));

function fibonaccif(a, b) {
return concat(
concat(
single(a),
single(b)
),
function fibonacci() {
let next = a + b;
a = b;
b = next;
return next;
}
);
} 
slide86. function fibonaccif(a, b) {
return concat(
element([a, b]),
function fibonacci() {
let next = a + b;
a = b;
b = next;
return next;
}
);
} 
slide87. Write a counter constructor that returns an object containing two functions that implement an up/down counter, hiding the counter. let object = counter(10);
let up = object.up;
let down = object.down;
up() // 11
down() // 10
down() // 9
up() // 10 
slide88. function counter(value) {
return {
up: function () {
value += 1;
return value;
},
down: function () {
value -= 1;
return value;
}
};
} 
slide89. Make a revocable constructor that takes a function, and returns an object containing an invoke method that can invoke the function, and a revoke method that disables the invoke method. let rev = revocable(add);
let add_rev = rev.invoke;
add_rev(3, 4); // 7
rev.revoke();
add_rev(5, 7); // undefined 
slide90. function revocable(any) {
return {
invoke: function (...args) {
if (any !== undefined) {
return any(...args);
}
},
revoke: function () {
any = undefined;
}
};
} 
slide91. Write a function num that takes a value and an optional source string and returns them in an object. JSON.stringify(num(1))
// {"value": 1, "source": "1"}
JSON.stringify(num(Math.PI, "pi"))
// {"value": 3.14159…, "source": "pi"} 
slide92. https://www.crockford.com/pp/num.js

function num(value, source) {
return {
value,
source: (
typeof source === "string"
? source
: String(value)
)
};
} 
slide93. Write a function addnum that adds two num objects and returns an num object. JSON.stringify(addnum(num(3), num(4)))
// {"value": 7, "source": "(3+4)"}
JSON.stringify(addnum(num(1), num(Math.PI, "pi")))
// {"value": 4.14159…, "source": "(1+pi)"} 
slide94. function addnum(a, b) {
return num(
a.value + b.value,
"(" + a.source + "+" + b.source + ")"
);
} 
slide95. Write a function lifnutm that takes a binary function and a string and returns a function that acts on num objects. let addm = liftnum(add, "+");
JSON.stringify(addm(num(3), num(4)))
// {"value": 7, "source": "(3+4)"}
JSON.stringify(liftnum(mul, "*")(num(3), num(4)))
// {"value": 12, "source": "(3*4)"} 
slide96. function liftnum(binary, op) {
return function (a, b) {
return num(
binary(a.value, b.value),
"(" + a.source + op + b.source + ")"
);
};
} 
slide97. Modify function liftnum so that the functions it produces can accept arguments that are either numbers or m objects. let addnum = liftnum(add, "+");
JSON.stringify(addnum(3, 4))
// {"value": 7, "source": "(3+4)"} 
slide98. function liftnum(binary, op) {
return function (a, b) {
if (typeof a === "number") {
a = num(a);
}
if (typeof b === "number") {
b = num(b);
}
return num(
binary(a.value, b.value),
"(" + a.source + op +
b.source + ")"
);
};
} 
slide99. Write a function exp that evaluates simple array expressions. let sae = [mul, 5, 11];
exp(sae) // 55
exp(42) // 42 
slide100. function exp(value) {
return (
Array.isArray(value)
? value[0](value[1], value[2])
: value
);
} 
slide101. Modify exp to evaluate nested array expressions. let nae = [
Math.sqrt,
[
add,
[square, 3],
[square, 4]
]
];
exp(nae) // 5 
slide102. function exp(value) {
return (
Array.isArray(value)
? value[0](
exp(value[1]),
exp(value[2])
)
: value
);
}

// recursion: a function calls itself 
slide103. Write a function addg that adds from many invocations, until it sees an empty invocation. addg() // undefined
addg(2)() // 2
addg(2)(7)() // 9
addg(3)(0)(4)() // 7
addg(1)(2)(4)(8)() // 15 
slide104. function addg(first) {
function more(next) {
if (next === undefined) {
return first;
}
first += next;
return more;
}
if (first !== undefined) {
return more;
}
}

// retursion: a function returns itself 
slide105. Write a function liftg that will take a binary function and apply it to many invocations. liftg(mul)() // undefined
liftg(mul)(3)() // 3
liftg(mul)(3)(0)(4)() // 0
liftg(mul)(1)(2)(4)(8)() // 64 
slide106. function liftg(binary) {
return function (first) {
if (first === undefined) {
return first;
}
return function more(next) {
if (next === undefined) {
return first;
}
first = binary(first, next);
return more;
};
};
} 
slide107. Write a function arrayg that will build an array from many invocations. arrayg() // []
arrayg(3)() // [3]
arrayg(3)(4)(5)() // [3, 4, 5] 
slide108. function arrayg(first) {
let array = [];
function more(next) {
if (next === undefined) {
return array;
}
array.push(next);
return more;
}
return more(first);
} 
slide109. function arrayg(first) {
if (first === undefined) {
return [];
}
return liftg(
function (array, value) {
array.push(value);
return array;
}
)([first]);
} 
slide110. Make an objectify factory that takes an array of property names and returns a constructor that takes values and returns an object. let make = objectify("date", "marry", "kill");
JSON.stringify(make("butterfly", "unicorn", "monster"))
// {
// "date": "butterfly",
// "marry": "unicorn",
// "kill" : "monster"
// } 
slide111. function objectify(...names) {
return function objectify_constructor(...values) {
const object = Object.create(null);
names.forEach(function (name, name_nr) {
object[name] = values[name_nr];
});
return object;
};
} 
slide112. Make a join factory that takes a function and generators that provide arguments to the function. let fo = join(
objectify("number", "fibonacci"),
from(0),
fibonaccif(4, 5)
);
fo() // {"number": 0, "fibonacci": 4}
fo() // {"number": 1, "fibonacci": 5}
fo() // {"number": 2, "fibonacci": 9} 
slide113. function join(func, ...gens) {
return function join_generator() {
return func(...gens.map(function (gen) {
return gen();
}));
};
} 
slide114. Make a continuize factory that takes a function, and returns a function that takes a callback and an argument. let sqrtc = continuize(Math.sqrt);
sqrtc(console.log, 81); 
slide115. function continuize(any) {
return function (continuation, ...args) {
return continuation(any(...args));
};
} 
slide116. function constructor(spec) {
let {member} = spec;
const reuse = other_constructor(spec);
const method = function () {
// spec, member, reuse, method
};
return Object.freeze({
method,
goodness: reuse.goodness
});
} 
slide117. Make a vector constructor with methods get, store, and append, such that an attacker cannot get access to the private array. myvector = vector();
myvector.append(7);
myvector.store(1, 8);
myvector.get(0) // 7
myvector.get(1) // 8 
slide118. function vector() {
const array = [];

return {
get: function get(i) {
return array[i];
},
store: function store(i, v) {
array[i] = v;
},
append: function append(v) {
array.push(v);
}
};
} 
slide119. function vector() {
const array = [];

return {
get: function get(i) {
return array[i];
},
store: function store(i, v) {
array[i] = v;
},
append: function append(v) {
array.push(v);
}
}; let stash;
} myvector.store("push", function () {
stash = this;
});
myvector.append(); // stash is array 
slide120. function vector() {
const array = [];

return {
get: function (i) {
return array[i];
},
store: function store(i, v) {
array[Number(i)] = v;
},
append: function (v) {
array[array.length] = v;
}
};
} 
slide121. Make a function that makes a publish/subscribe object. It will reliably deliver all publications to all subscribers in the right order. my_pubsub = pubsub();
my_pubsub.subscribe(log);
my_pubsub.publish("It works!");
// log("It works!") 
slide122. function pubsub() {
const subscribers = [];
return {
subscribe: function (subscriber) {
subscribers.push(subscriber);
},
publish: function (publication) {
let i;
const length = subscribers.length;
for (i = 0; i < length; i += 1) {
subscribers[i](publication);
}
}
};
} 
slide123. function pubsub() {
const subscribers = [];
return {
subscribe: function (subscriber) {
subscribers.push(subscriber);
},
publish: function (publication) {
let i;
const length = subscribers.length;
for (i = 0; i < length; i += 1) {
subscribers[i](publication);
}
}
};
}

my_pubsub.subscribe(); 
slide124. function pubsub() {
const subscribers = [];
return {
subscribe: function (subscriber) {
subscribers.push(subscriber);
},
publish: function (publication) {
let i;
const length = subscribers.length;
for (i = 0; i < length; i += 1) {
try {
subscribers[i](publication);
} catch (ignore) {}
}
}
};
} 
slide125. function pubsub() {
const subscribers = [];
return {
subscribe: function (subscriber) {
subscribers.push(subscriber);
},
publish: function (publication) {
let i;
const length = subscribers.length;
for (i = 0; i < length; i += 1) {
try {
subscribers[i](publication);
} catch (ignore) {}
}
}
};
} my_pubsub.publish = undefined; 
slide126. function pubsub() {
const subscribers = [];
return Object.freeze({
subscribe: function (subscriber) {
subscribers.push(subscriber);
},
publish: function (publication) {
let i;
const length = subscribers.length;
for (i = 0; i < length; i += 1) {
try {
subscribers[i](publication);
} catch (ignore) {}
}
}
});
} 
slide127. function pubsub() {
const subscribers = [];
return Object.freeze({
subscribe: function (subscriber) {
subscribers.push(subscriber);
},
publish: function (publication) {
let i;
const length = subscribers.length;
for (i = 0; i < length; i += 1) {
try {
subscribers[i](publication);
} catch (ignore) {}
}
}
}); my_pubsub.subscribe(function () {
} this.length = 0;
}); 
slide128. function pubsub() {
const subscribers = [];
return Object.freeze({
subscribe: function (subscriber) {
subscribers.push(subscriber);
},
publish: function (publication) {
subscribers.forEach(function (subscriber) {
try {
subscriber(publication);
} catch (ignore) {}
});
}
});
} 
slide129. function pubsub() {
const subscribers = [];
return Object.freeze({
subscribe: function (subscriber) {
subscribers.push(subscriber);
},
publish: function (publication) {
subscribers.forEach(function (subscriber) {
try {
subscriber(publication);
} catch (ignore) {}
});
} my_pubsub.subscribe(limit(
}); function () {
} my_pubsub.publish("Out of order");
},
1
)); " 
slide130. function pubsub() {
const subscribers = [];
return Object.freeze({
subscribe: function (subscriber) {
subscribers.push(subscriber);
},
publish: function (publication) {
subscribers.forEach(function (subscriber) {
setTimeout(function () {
subscriber(publication);
}, 0);
});
}
});
}

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