![var colin = {
clients: [
{
name: "Fred",
investments: [
{
name: "Shares",
markets: [
{
name: "Japan",
value: 7000
},
{
name: "America",
value: 8888
}
]
},
{
name: "Gold",
markets: [
{
name: "China",
value: 3333
},
]
}
]
}
]
};](https://image.slidesharecdn.com/06mapreduce-140227072701-phpapp01/85/06-Map-Reduce-13-320.jpg)
![sumInvestments = (clients) ->
values = []
foreach client in clients:
foreach investment in client.investments
foreach market in investment.markets
values.push(market.balance)
return sum(values)](https://image.slidesharecdn.com/06mapreduce-140227072701-phpapp01/85/06-Map-Reduce-15-320.jpg)
![function sumInvestments (clients) {
var i, j, k;
var investments, markets;
var value = 0;
!
for (i = 0; i < clients.length; i++) {
investments = clients[i].investments;
!
for (j = 0; j < investments.length; j++) {
markets = investments[j].markets;
!
for (k = 0; k < markets.length; k++) {
value += markets[k].value
}
}
}
!
return value;
};](https://image.slidesharecdn.com/06mapreduce-140227072701-phpapp01/85/06-Map-Reduce-16-320.jpg)
![function reduce(iter, memo, fn) {
for (var i = 0; i < iter.length; i++) {
memo = fn(memo, iter[i]);
}
!
return memo;
}
!
function get_unless_null(obj, key) {
if (obj !== null && obj !== undefined) {
return obj[key];
}
return null;
}
!
function findBreeder(person) {
return reduce(['pet', 'mother', ‘owner’], person, get_unless_null);
}](https://image.slidesharecdn.com/06mapreduce-140227072701-phpapp01/85/06-Map-Reduce-23-320.jpg)
![function map(iter, fn) {
var out = [],
val;
!
for (var i = 0; i < iter.length; i++) {
out[i] = fn(iter[i]);
}
!
return out;
}
!
function flat_map(iter, fn) {
iter = map(iter, fn);
return reduce(iter, [], function(memo, item) {
return memo.concat(item);
});
}
!
function sum(list) {
return reduce(list, 0, function(memo, num) {
return memo + num;
});
}](https://image.slidesharecdn.com/06mapreduce-140227072701-phpapp01/85/06-Map-Reduce-26-320.jpg)
![function sumInvestments(clients) {
var v, fn;
!
fn = function(arr, key) {
return flat_map(arr, function(item) {
return get_unless_null(item, key);
});
};
!
v = reduce(clients, ['investments', 'markets', 'value'], fn);
return sum(v);
};](https://image.slidesharecdn.com/06mapreduce-140227072701-phpapp01/85/06-Map-Reduce-27-320.jpg)
![function sumInvestments (clients) {
var i, j, k;
var investments, markets;
var value = 0;
!
!
ugly
for (i = 0; i < clients.length; i++) {
investments = clients[i].investments;
for (j = 0; j < investments.length; j++) {
markets = investments[j].markets;
!
for (k = 0; k < markets.length; k++) {
value += markets[k].value
}
}
!
}
return value
};
!
function sumInvestments(clients) {
var v;
!
!
};
awesome
v = reduce(clients, ['investments', 'markets', 'value'], function(arr, key) {
return flat_map(arr, function(item) {
return item[key];
});
});
return sum(v);](https://image.slidesharecdn.com/06mapreduce-140227072701-phpapp01/85/06-Map-Reduce-29-320.jpg)
![ugly
function findBreeder(person) {
if (!person) {
return null;
}
!
if (!person.pet) {
return null;
}
if (!person.pet.mother) {
return null;
}
!
return person.pet.mother.owner;
}
!
!
!
awesome
function findBreeder(person) {
return reduce(get_unless_null, person, ['pet', 'mother', 'owner']);
}](https://image.slidesharecdn.com/06mapreduce-140227072701-phpapp01/85/06-Map-Reduce-30-320.jpg)
06 Map Reduce
- 1. Let’s talk about lambda
- 2. Why care?
- 3. NULL is troublesome The dog breeder example
- 5. var colleen, richard; ! colleen = { pet: { mother: { owner: "Sarah" } } }; ! richard = { pet: null };
- 6. Given a person, return its pet’s mother’s owner.
- 7. function findBreeder(person) { return person.pet.mother.owner; }
- 8. function findBreeder(person) { return person.pet.mother.owner; } ! >>> findBreeder(colleen); "Sarah" ! >>> findBreeder(richard); Exception: "Can not read property 'mother' of undefined"
- 9. function findBreeder(person) { if (!person) { return null; } ! if (!person.pet) { return null; } if (!person.pet.mother) { return null; } ! return person.pet.mother.owner; }
- 11. LISTS are troublesome The investment broker example
- 13. var colin = { clients: [ { name: "Fred", investments: [ { name: "Shares", markets: [ { name: "Japan", value: 7000 }, { name: "America", value: 8888 } ] }, { name: "Gold", markets: [ { name: "China", value: 3333 }, ] } ] } ] };
- 14. Find the sum of all of an investors’s client’s investments.
- 15. sumInvestments = (clients) -> values = [] foreach client in clients: foreach investment in client.investments foreach market in investment.markets values.push(market.balance) return sum(values)
- 16. function sumInvestments (clients) { var i, j, k; var investments, markets; var value = 0; ! for (i = 0; i < clients.length; i++) { investments = clients[i].investments; ! for (j = 0; j < investments.length; j++) { markets = investments[j].markets; ! for (k = 0; k < markets.length; k++) { value += markets[k].value } } } ! return value; };
- 18. (multi-layer list iteration) vs. (null checking) ! What’s the common thread?
- 19. no code duplication ! lots of concept duplication
- 20. function call_unless_null(val, fn) { if (val !== null) { return fn(val); } ! return null; } ! function findBreeder(person) { return call_unless_null(person, function(person) { return call_unless_null(person.pet, function(pet) { return call_unless_null(pet.mother, function(mother) { return mother.owner; }); }); }); }
- 22. Still too much duplication.
- 23. function reduce(iter, memo, fn) { for (var i = 0; i < iter.length; i++) { memo = fn(memo, iter[i]); } ! return memo; } ! function get_unless_null(obj, key) { if (obj !== null && obj !== undefined) { return obj[key]; } return null; } ! function findBreeder(person) { return reduce(['pet', 'mother', ‘owner’], person, get_unless_null); }
- 25. function sumInvestments(clients) { var values; ! values = flat_map(clients, function(client) { return flat_map(client.investments, function(investment) { return flat_map(investment.markets, function(market) { return market.value; }); }); }); ! return sum(values); };
- 26. function map(iter, fn) { var out = [], val; ! for (var i = 0; i < iter.length; i++) { out[i] = fn(iter[i]); } ! return out; } ! function flat_map(iter, fn) { iter = map(iter, fn); return reduce(iter, [], function(memo, item) { return memo.concat(item); }); } ! function sum(list) { return reduce(list, 0, function(memo, num) { return memo + num; }); }
- 27. function sumInvestments(clients) { var v, fn; ! fn = function(arr, key) { return flat_map(arr, function(item) { return get_unless_null(item, key); }); }; ! v = reduce(clients, ['investments', 'markets', 'value'], fn); return sum(v); };
- 28. What have we accomplished?
- 29. function sumInvestments (clients) { var i, j, k; var investments, markets; var value = 0; ! ! ugly for (i = 0; i < clients.length; i++) { investments = clients[i].investments; for (j = 0; j < investments.length; j++) { markets = investments[j].markets; ! for (k = 0; k < markets.length; k++) { value += markets[k].value } } ! } return value }; ! function sumInvestments(clients) { var v; ! ! }; awesome v = reduce(clients, ['investments', 'markets', 'value'], function(arr, key) { return flat_map(arr, function(item) { return item[key]; }); }); return sum(v);
- 30. ugly function findBreeder(person) { if (!person) { return null; } ! if (!person.pet) { return null; } if (!person.pet.mother) { return null; } ! return person.pet.mother.owner; } ! ! ! awesome function findBreeder(person) { return reduce(get_unless_null, person, ['pet', 'mother', 'owner']); }
- 31. Concept duplication is just masked code duplication
- 32. Repeating patterns are hint
- 33. Higher-order functions make all this possible