05_higher_order.txt

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= Higher-Order Functions =

ifdef::interactive_target[]

[chapterquote="true"]
[quote, Master Yuan-Ma, The Book of Programming]
____
Tzu-li and Tzu-ssu were
boasting about the size of their latest programs. ‘Two-hundred
thousand lines,’ said Tzu-li, ‘not counting comments!’ Tzu-ssu
responded, ‘Pssh, mine is almost a *million* lines already.’ Master
Yuan-Ma said, ‘My best program has five hundred lines.’ Hearing this,
Tzu-li and Tzu-ssu were enlightened.
____

endif::interactive_target[]

[chapterquote="true"]
[quote, C.A.R. Hoare, 1980 ACM Turing Award Lecture]
____
(((Hoare+++,+++ C.A.R.)))There are two ways of constructing a software
design: One way is to make it so simple that there are obviously no
deficiencies, and the other way is to make it so complicated that
there are no obvious deficiencies.
____

(((program size)))A large program is a costly program, and not just
because of the time it takes to build. Size almost always involves
((complexity)), and complexity confuses programmers. Confused
programmers, in turn, tend to introduce mistakes (_((bug))s_) into
programs. A large program also provides a lot of space for these bugs
to hide, making them hard to find.

(((summing example)))Let's briefly go back to the final two example
programs in the introduction. The first is self-contained and six
lines long.

[source,javascript]
----
var total = 0, count = 1;
while (count <= 10) {
  total += count;
  count += 1;
}
console.log(total);
----

The second relies on two external functions and is one line long.

[source,javascript]
----
console.log(sum(range(1, 10)));
----

Which one is more likely to contain a bug?

(((program size)))If we count the size of the definitions of `sum` and
`range`, the second program is also big—even bigger than the first.
But still, I'd argue that it is more likely to be correct.

(((abstraction)))(((domain-specific language)))It is more likely to
be correct because the solution is expressed in a ((vocabulary)) that
corresponds to the problem being solved. Summing a range of
numbers isn't about loops and counters. It is about ranges and sums.

The definitions of this vocabulary (the functions `sum` and `range`)
will still involve loops, counters, and other incidental details. But
because they are expressing simpler concepts than the program as a
whole, they are easier to get right.

== Abstraction ==

In the context of programming, these kinds of vocabularies are usually
called _((abstraction))s_. Abstractions hide details and give us the
ability to talk about problems at a higher (or more abstract) level.

(((recipe analogy)))(((pea soup)))As an analogy, compare these two
recipes for pea soup:

____
Put 1 cup of dried peas per person into a container. Add water until
the peas are well covered. Leave the peas in water for at least 12 hours.
Take the peas out of the water and put them in a cooking pan. Add 4
cups of water per person. Cover the pan and keep the peas
simmering for two hours. Take half an onion per person. Cut it into
pieces with a knife. Add it to the peas. Take a stalk of celery per
person. Cut it into pieces with a knife. Add it to the peas. Take a
carrot per person. Cut it into pieces. With a knife! Add it to the
peas. Cook for 10 more minutes.
____

And the second recipe:
____
Per person: 1 cup dried split peas, half a chopped onion, a stalk of
celery, and a carrot.

Soak peas for 12 hours. Simmer for 2 hours in 4 cups of water
(per person). Chop and add vegetables. Cook for 10 more minutes.
____

(((vocabulary)))The second is shorter and easier to interpret. But
you do need to understand a few more cooking-related words—__soak__,
_simmer_, _chop_, and, I guess, _vegetable_.

When programming, we can't rely on all the words we need to be waiting
for us in the dictionary. Thus, you might fall into the pattern of the
first recipe—work out the precise steps the computer has to perform,
one by one, blind to the higher-level concepts that they express.

(((abstraction)))It has to become second nature, for a programmer, to
notice when a concept is begging to be abstracted into a new word.

== Abstracting array traversal ==

(((array)))Plain functions, as we've seen them so far, are a good
way to build abstractions. But sometimes they fall short.

(((for loop)))In the link:04_data.html#data[previous chapter], this
type of `for` ((loop)) made several appearances:

[source,javascript]
----
var array = [1, 2, 3];
for (var i = 0; i < array.length; i++) {
  var current = array[i];
  console.log(current);
}
----

(((length property,for
array)))(((array,indexing)))(((readability)))It's trying to say, “For
each element in the array, log it to the console”. But it uses a
roundabout way that involves a counter variable `i`, a check against
the array's length, and an extra variable declaration to pick out the
current element. Apart from being a bit of an eyesore, this provides a
lot of space for potential mistakes. We might accidentally reuse the
`i` variable, misspell `length` as `lenght`, confuse the `i` and `current`
variables, and so on.

So let's try to abstract this into a function. Can you think of a way?

Well, it's easy to write a function that goes over an array and calls
`console.log` on every element.

[source,javascript]
----
function logEach(array) {
  for (var i = 0; i < array.length; i++)
    console.log(array[i]);
}
----

[[forEach]]
indexsee:[higher-order function,function+++,+++ higher-order](((function,higher-order)))(((loop)))(((array,traversal)))(((function,as value)))(((forEach method)))But what
if we want to do something other than logging the elements? Since
“doing something” can be represented as a function and functions are
just values, we can pass our action as a function value.

[source,javascript]
----
function forEach(array, action) {
  for (var i = 0; i < array.length; i++)
    action(array[i]);
}

forEach(["Wampeter", "Foma", "Granfalloon"], console.log);
// → Wampeter
// → Foma
// → Granfalloon
----

Often, you don't pass a predefined function to `forEach` but create
a function value on the spot instead.

[source,javascript]
----
var numbers = [1, 2, 3, 4, 5], sum = 0;
forEach(numbers, function(number) {
  sum += number;
});
console.log(sum);
// → 15
----

(((loop body)))(((curly braces)))This looks quite a lot like the
classical `for` loop, with its body written as a block below it.
However, now the body is inside the function value, as well as
inside the ((parentheses)) of the call to `forEach`. This is why it
has to be closed with the closing brace _and_ closing parenthesis.

(((local variable)))(((parameter)))Using this pattern, we can
specify a variable name for the current element (`number`), rather
than having to pick it out of the array manually.

(((array,methods)))(((function,higher-order)))(((forEach
method)))(((array)))In fact, we don't need to write `forEach`
ourselves. It is available as a standard method on arrays. Since the
array is already provided as the thing the method acts on, `forEach`
takes only one required argument: the function to be executed for each
element.

To illustrate how helpful this is, let's look back at a function
from link:04_data.html#analysis[the previous chapter]. It contains two
array-traversing ((loop))s.

[source,javascript]
----
function gatherCorrelations(journal) {
  var phis = {};
  for (var entry = 0; entry < journal.length; entry++) {
    var events = journal[entry].events;
    for (var i = 0; i < events.length; i++) {
      var event = events[i];
      if (!(event in phis))
        phis[event] = phi(tableFor(event, journal));
    }
  }
  return phis;
}
----

(((forEach method)))Working with `forEach` makes it slightly shorter
and quite a bit cleaner.

[source,javascript]
----
function gatherCorrelations(journal) {
  var phis = {};
  journal.forEach(function(entry) {
    entry.events.forEach(function(event) {
      if (!(event in phis))
        phis[event] = phi(tableFor(event, journal));
    });
  });
  return phis;
}
----

== Higher-order functions ==

(((function,higher-order)))(((function,as value)))Functions that
operate on other functions, either by taking them as arguments or by
returning them, are called _higher-order functions_. If you have
already accepted the fact that functions are regular values, there is
nothing particularly remarkable about the fact that such functions
exist. The term comes from ((mathematics)), where the distinction
between functions and other values is taken more seriously.

(((abstraction)))Higher-order functions allow us to abstract over
_actions_, not just values. They come in several forms. For example,
you can have functions that create new functions.

[source,javascript]
----
function greaterThan(n) {
  return function(m) { return m > n; };
}
var greaterThan10 = greaterThan(10);
console.log(greaterThan10(11));
// → true
----

And you can have functions that change other functions.

[source,javascript]
----
function noisy(f) {
  return function(arg) {
    console.log("calling with", arg);
    var val = f(arg);
    console.log("called with", arg, "- got", val);
    return val;
  };
}
noisy(Boolean)(0);
// → calling with 0
// → called with 0 - got false
----

You can even write functions that provide new types of ((control flow)).

[source,javascript]
----
function unless(test, then) {
  if (!test) then();
}
function repeat(times, body) {
  for (var i = 0; i < times; i++) body(i);
}

repeat(3, function(n) {
  unless(n % 2, function() {
    console.log(n, "is even");
  });
});
// → 0 is even
// → 2 is even
----

(((inner function)))(((nesting,of functions)))((({}
(block))))(((local variable)))(((closure)))The ((lexical scoping))
rules that we discussed in link:03_functions.html#scoping[Chapter 3]
work to our advantage when using functions in this way. In the previous example, the `n` variable is a ((parameter)) to the outer function.
Because the inner function lives inside the environment of the outer
one, it can use `n`. The bodies of such inner functions can access the
variables around them. They can play a role similar to the `{}` blocks
used in regular loops and conditional statements. An important
difference is that variables declared inside inner functions do not
end up in the environment of the outer function. And that is usually a
good thing.

== Passing along arguments ==

(((function,wrapping)))(((arguments object)))The `noisy` function
defined earlier, which wraps its argument in another function, has a rather
serious deficit.

[source,javascript]
----
function noisy(f) {
  return function(arg) {
    console.log("calling with", arg);
    var val = f(arg);
    console.log("called with", arg, "- got", val);
    return val;
  };
}
----

If `f` takes more than one ((parameter)), it gets only the first one.
We could add a bunch of arguments to the inner function (`arg1`,
`arg2`, and so on) and pass them all to `f`, but it is not clear how many
would be enough. This solution would also deprive `f` of the
information in `arguments.length`. Since we'd always pass the same
number of arguments, it wouldn't know how many arguments were
originally given.

(((apply method)))(((array-like object)))(((function,application)))For
these kinds of situations, JavaScript functions have an `apply`
method. You pass it an array (or array-like object) of arguments, and
it will call the function with those arguments.

[source,javascript]
----
function transparentWrapping(f) {
  return function() {
    return f.apply(null, arguments);
  };
}
----

(((null)))That's a useless function, but it shows the pattern we are
interested in—the function it returns passes all of the given
arguments, and only those arguments, to `f`. It does this by passing
its own `arguments` object to `apply`. The first argument to `apply`,
for which we are passing `null` here, can be used to simulate a
((method)) call. We will come back to that in the
link:06_object.html#call_method[next chapter].

== JSON ==

(((array)))(((function,higher-order)))(((forEach method)))(((data
set)))Higher-order functions that somehow apply a function to the
elements of an array are widely used in JavaScript. The `forEach`
method is the most primitive such function. There are a number of
other variants available as methods on arrays. To familiarize
ourselves with them, let's play around with another data set.

(((ancestry example)))A few years ago, someone crawled through a lot
of archives and put together a book on the history of my family name
(Haverbeke—meaning Oatbrook). I opened it hoping to find
knights, pirates, and alchemists ... but the book turns out to be
mostly full of Flemish ((farmer))s. For my amusement, I extracted the
information on my direct ancestors and put it into a
computer-readable format.

(((data format)))(((JSON)))The file I created looks something like
this:

[source,application/json]
----
[
  {"name": "Emma de Milliano", "sex": "f",
   "born": 1876, "died": 1956,
   "father": "Petrus de Milliano",
   "mother": "Sophia van Damme"},
  {"name": "Carolus Haverbeke", "sex": "m",
   "born": 1832, "died": 1905,
   "father": "Carel Haverbeke",
   "mother": "Maria van Brussel"},
  … and so on
]
----

indexsee:[JavaScript Object Notation,JSON](((World Wide Web)))This format is called JSON (pronounced “Jason”),
which stands for JavaScript Object Notation. It is widely used as a
data storage and communication format on the Web.

(((array)))(((object)))(((quoting,in JSON)))(((comment)))JSON is similar to
JavaScript's way of writing arrays and objects, with a few
restrictions. All property names have to be surrounded by double quotes, and
only simple data expressions are allowed—no function calls, 
variables, or anything that involves actual computation. Comments are not
allowed in JSON.

(((JSON.stringify function)))(((JSON.parse
function)))(((serialization)))(((deserialization)))(((parsing)))JavaScript
gives us functions, `JSON.stringify` and `JSON.parse`, that convert
data from and to this format. The first takes a JavaScript value and
returns a JSON-encoded string. The second takes such a string and
converts it to the value it encodes.

[source,javascript]
----
var string = JSON.stringify({name: "X", born: 1980});
console.log(string);
// → {"name":"X","born":1980}
console.log(JSON.parse(string).born);
// → 1980
----

(((ANCESTRY_FILE data set)))The variable `ANCESTRY_FILE`, available in
the ((sandbox)) for this chapter and in
http://eloquentjavascript.net/code/ancestry.js[a downloadable file] on
the website(!book (http://eloquentjavascript.net/code#5[_eloquentjavascript.net/code#5_])!), contains the
content of my ((JSON)) file as a string. Let's decode it and see how
many people it contains.

// include_code strip_log

[source,javascript]
----
var ancestry = JSON.parse(ANCESTRY_FILE);
console.log(ancestry.length);
// → 39
----

== Filtering an array ==

(((array,methods)))(((array,filtering)))(((filter
method)))(((function,higher-order)))(((predicate function)))To find
the people in the ancestry data set who were young in 1924, the
following function might be helpful. It filters out the elements in an
array that don't pass a test.

[source,javascript]
----
function filter(array, test) {
  var passed = [];
  for (var i = 0; i < array.length; i++) {
    if (test(array[i]))
      passed.push(array[i]);
  }
  return passed;
}

console.log(filter(ancestry, function(person) {
  return person.born > 1900 && person.born < 1925;
}));
// → [{name: "Philibert Haverbeke", …}, …]
----

(((function,as value)))(((function,application)))This uses the
argument named `test`, a function value, to fill in a “gap” in the
computation. The `test` function is called for each element, and its
return value determines whether an element is included in the returned
array.

(((ancestry example)))Three people in the file were alive and young in
1924: my grandfather, grandmother, and great-aunt.

(((filter method)))(((pure function)))(((side effect)))Note how the
`filter` function, rather than deleting elements from the existing
array, builds up a new array with only the elements that pass the
test. This function is _pure_. It does not modify the array it is
given.

Like `forEach`, `filter` is also a ((standard)) method on arrays. The
example defined the function only in order to show what it does
internally. From now on, we'll use it like this instead:

[source,javascript]
----
console.log(ancestry.filter(function(person) {
  return person.father == "Carel Haverbeke";
}));
// → [{name: "Carolus Haverbeke", …}]
----

== Transforming with map ==

(((array,methods)))(((map method)))(((ancestry example)))Say we
have an array of objects representing people, produced by filtering
the `ancestry` array somehow. But we want an array of names, which is
easier to read.

(((function,higher-order)))The `map` method transforms an array by
applying a function to all of its elements and building a new array
from the returned values. The new array will have the same length as
the input array, but its content will have been “mapped” to a new form
by the function.

// test: join

[source,javascript]
----
function map(array, transform) {
  var mapped = [];
  for (var i = 0; i < array.length; i++)
    mapped.push(transform(array[i]));
  return mapped;
}

var overNinety = ancestry.filter(function(person) {
  return person.died - person.born > 90;
});
console.log(map(overNinety, function(person) {
  return person.name;
}));
// → ["Clara Aernoudts", "Emile Haverbeke",
//    "Maria Haverbeke"]
----

Interestingly, the people who lived to at least 90 years of age are the
same three people who we saw before—the people who were young in the
1920s, which happens to be the most recent generation in my data set.
I guess ((medicine)) has come a long way.

Like `forEach` and `filter`, `map` is also a standard method on
arrays.

== Summarizing with reduce ==

(((array,methods)))(((summing example)))(((reduce method)))(((ancestry
example)))Another common pattern of computation on arrays is computing
a single value from them. Our recurring example, summing a collection
of numbers, is an instance of this. Another example would be finding
the person with the earliest year of birth in the data set.

(((function,higher-order)))(((fold function)))The higher-order
operation that represents this pattern is called _reduce_ (or
sometimes _fold_). You can think of it as folding up the array, one
element at a time. When summing numbers, you'd start with the number
zero and, for each element, combine it with the current sum by adding
the two.

The parameters to the `reduce` function are, apart from the array, a
combining function and a start value. This function is a little less
straightforward than `filter` and `map`, so pay careful attention.

[source,javascript]
----
function reduce(array, combine, start) {
  var current = start;
  for (var i = 0; i < array.length; i++)
    current = combine(current, array[i]);
  return current;
}

console.log(reduce([1, 2, 3, 4], function(a, b) {
  return a + b;
}, 0));
// → 10
----

(((reduce method)))The standard array method `reduce`, which of course
corresponds to this function, has an added convenience. If your array
contains at least one element, you are allowed to leave off the
`start` argument. The method will take the first element of the array
as its start value and start reducing at the second element.

(((ancestry example)))(((minimum)))To use `reduce` to find my most
ancient known ancestor, we can write something like this:

// test: no

[source,javascript]
----
console.log(ancestry.reduce(function(min, cur) {
  if (cur.born < min.born) return cur;
  else return min;
}));
// → {name: "Pauwels van Haverbeke", born: 1535, …}
----

== Composability ==

(((loop)))(((minimum)))(((ancestry example)))Consider how we would
have written the previous example (finding the person with the
earliest year of birth) without higher-order functions. The code is
not that much worse.

// test: no

[source,javascript]
----
var min = ancestry[0];
for (var i = 1; i < ancestry.length; i++) {
  var cur = ancestry[i];
  if (cur.born < min.born)
    min = cur;
}
console.log(min);
// → {name: "Pauwels van Haverbeke", born: 1535, …}
----

There are a few more ((variable))s, and the program is two lines
longer but still quite easy to understand.

[[average_function]]
(((average
function)))(((composability)))(((function,higher-order)))Higher-order
functions start to shine when you need to _compose_ functions. As an
example, let's write code that finds the average age for men and for
women in the data set.

// test: clip

[source,javascript]
----
function average(array) {
  function plus(a, b) { return a + b; }
  return array.reduce(plus) / array.length;
}
function age(p) { return p.died - p.born; }
function male(p) { return p.sex == "m"; }
function female(p) { return p.sex == "f"; }

console.log(average(ancestry.filter(male).map(age)));
// → 61.67
console.log(average(ancestry.filter(female).map(age)));
// → 54.56
----

(((plus function)))(((+ operator)))(((function,as value)))(It's a bit
silly that we have to define `plus` as a function, but operators in
JavaScript, unlike functions, are not values, so you can't pass them
as arguments.)

(((abstraction)))(((vocabulary)))Instead of tangling the logic into a
big ((loop)), it is neatly composed into the concepts we are
interested in—determining sex, computing age, and averaging numbers. We
can apply these one by one to get the result we are looking for.

This is _fabulous_ for writing clear code. Unfortunately, this clarity
comes at a cost.

== The cost ==

(((efficiency)))(((optimization)))In the happy land of elegant code
and pretty rainbows, there lives a spoil-sport monster called
_inefficiency_.

(((elegance)))(((array,creation)))(((pure
function)))(((composability)))A program that processes an array is most
elegantly expressed as a sequence of cleanly separated steps that each
do something with the array and produce a new array. But building up
all those intermediate arrays is somewhat expensive.

(((readability)))(((function,application)))(((forEach
method)))(((function,as value)))Likewise, passing a function to
`forEach` and letting that method handle the array iteration for us is
convenient and easy to read. But function calls in JavaScript are
costly compared to simple loop bodies.

(((abstraction)))And so it goes with a lot of techniques that help
improve the clarity of a program. Abstractions add layers between the
raw things the computer is doing and the concepts we are working with
and thus cause the machine to perform more work. This is not an iron
law—there are programming languages that have better support for
building abstractions without adding inefficiencies, and even in
JavaScript, an experienced programmer can find ways to write abstract
code that is still fast. But it is a problem that comes up a lot.

(((profiling)))Fortunately, most computers are insanely fast. If you
are processing a modest set of data or doing something that has
to happen only on a human time scale (say, every time the user clicks a
button), then it _does not matter_ whether you wrote a pretty solution
that takes half a millisecond or a super-optimized solution that takes
a tenth of a millisecond.

(((nesting,of loops)))(((inner loop)))(((complexity)))It is helpful to
roughly keep track of how often a piece of your program is going to
run. If you have a ((loop)) inside a loop (either directly or through
the outer loop calling a function that ends up performing the inner
loop), the code inside the inner loop will end up running __N__×__M__
times, where _N_ is the number of times the outer loop repeats and
_M_ is the number of times the inner loop repeats within each iteration
of the outer loop. If that inner loop contains another loop that makes
_P_ rounds, its body will run __M__×__N__×__P__ times, and so on. This
can add up to large numbers, and when a program is slow, the problem
can often be traced to only a small part of the code, which sits inside an inner loop.

== Great-great-great-great-... ==

(((ancestry example)))My ((grandfather)), Philibert Haverbeke, is
included in the data file. By starting with him, I can trace my
lineage to find out whether the most ancient person in the data,
Pauwels van Haverbeke, is my direct ancestor. And if he is, I would
like to know how much ((DNA)) I theoretically share with him.

(((byName object)))(((map)))(((data structure)))(((object,as
map)))To be able to go from a parent's name to the actual object that
represents this person, we first build up an object that associates
names with people.

// include_code strip_log

[source,javascript]
----
var byName = {};
ancestry.forEach(function(person) {
  byName[person.name] = person;
});

console.log(byName["Philibert Haverbeke"]);
// → {name: "Philibert Haverbeke", …}
----

Now, the problem is not entirely as simple as following the `father`
properties and counting how many we need to reach Pauwels. There are
several cases in the family ((tree)) where people married their second
cousins (tiny villages and all that). This causes the branches of the
family tree to rejoin in a few places, which means I share more than
1/2^_G_^ of my genes with this person, where _G_ for the number of
generations between Pauwels and me. This formula comes from the idea
that each generation splits the gene pool in two.

(((reduce method)))(((data structure)))A reasonable way to think about
this problem is to look at it as being analogous to `reduce`, which
condenses an array to a single value by repeatedly combining
values, left to right. In this case, we also want to condense our data
structure to a single value but in a way that follows family
lines. The _shape_ of the data is that of a family tree, rather than a
flat list.

The way we want to reduce this shape is by computing a value for a
given person by combining values from their ancestors. This can be
done recursively: if we are interested in person _A_, we have to
compute the values for __A__’s parents, which in turn requires us to
compute the value for __A__’s grandparents, and so on. In principle,
that'd require us to look at an infinite number of people, but since
our data set is finite, we have to stop somewhere. We'll allow a
((default value)) to be given to our reduction function, which will be
used for people who are not in the data. In our case, that value is
simply zero, on the assumption that people not in the list don't share
DNA with the ancestor we are looking at.

(((recursion)))(((reduceAncestors function)))Given a person, a
function to combine values from the two parents of a given person, and
a default value, `reduceAncestors` condenses a value from a family
tree.

// include_code

[source,javascript]
----
function reduceAncestors(person, f, defaultValue) {
  function valueFor(person) {
    if (person == null)
      return defaultValue;
    else
      return f(person, valueFor(byName[person.mother]),
                       valueFor(byName[person.father]));
  }
  return valueFor(person);
}
----

(((function,higher-order)))The inner function (`valueFor`) handles a
single person. Through the ((magic)) of recursion, it can simply call
itself to handle the father and the mother of this person. The
results, along with the person object itself, are passed to `f`, which
returns the actual value for this person.

We can then use this to compute the amount of ((DNA)) my
((grandfather)) shared with Pauwels van Haverbeke and divide that by
four.

// start_code bottom_lines: 2
// test: clip
// include_code top_lines: 6

[source,javascript]
----
function sharedDNA(person, fromMother, fromFather) {
  if (person.name == "Pauwels van Haverbeke")
    return 1;
  else
    return (fromMother + fromFather) / 2;
}
var ph = byName["Philibert Haverbeke"];
console.log(reduceAncestors(ph, sharedDNA, 0) / 4);
// → 0.00049
----

The person with the name Pauwels van Haverbeke obviously shared 100 percent
of his DNA with Pauwels van Haverbeke (there are no people who share
names in the data set), so the function returns 1 for him. All other
people share the average of the amounts that their parents share.

So, statistically speaking, I share about 0.05 percent of my ((DNA)) with
this 16th-century person. It should be noted that this is only a
statistical approximation, not an exact amount. It is a rather small
number, but given how much genetic material we carry (about 3 billion
base pairs), there's still probably some aspect in the biological
machine that is me that originates with Pauwels.

(((ancestry example)))(((reduceAncestors
function)))(((abstraction)))We could also have computed this number
without relying on `reduceAncestors`. But separating the general
approach (condensing a family tree) from the specific case (computing
shared DNA) can improve the clarity of the code and allows us to
reuse the abstract part of the program for other cases. For example,
the following code finds the percentage of known ancestors, for a
given person, who lived past 70:

// test: clip

[source,javascript]
----
function countAncestors(person, test) {
  function combine(person, fromMother, fromFather) {
    var thisOneCounts = test(person);
    return fromMother + fromFather + (thisOneCounts ? 1 : 0);
  }
  return reduceAncestors(person, combine, 0);
}
function longLivingPercentage(person) {
  var all = countAncestors(person, function(person) {
    return true;
  });
  var longLiving = countAncestors(person, function(person) {
    return (person.died - person.born) >= 70;
  });
  return longLiving / all;
}
console.log(longLivingPercentage(byName["Emile Haverbeke"]));
// → 0.145
----

Such numbers are not to be taken too seriously, given that
our data set contains a rather arbitrary collection of people. But the
code illustrates the fact that `reduceAncestors` gives us a useful
piece of ((vocabulary)) for working with the family tree data
structure.

== Binding ==

(((bind method)))(((partial
application)))(((function,application)))The `bind` method, which all
functions have, creates a new function that will call the original
function but with some of the arguments already fixed.

(((filter method)))(((function,as value)))The following code shows an
example of `bind` in use. It defines a function `isInSet` that
tells us whether a person is in a given set of strings. To call
`filter` in order to collect those person objects whose names are in a
specific set, we can either write a function expression that makes a
call to `isInSet` with our set as its first argument or _partially
apply_ the `isInSet` function.

[source,javascript]
----
var theSet = ["Carel Haverbeke", "Maria van Brussel",
              "Donald Duck"];
function isInSet(set, person) {
  return set.indexOf(person.name) > -1;
}

console.log(ancestry.filter(function(person) {
  return isInSet(theSet, person);
}));
// → [{name: "Maria van Brussel", …},
//    {name: "Carel Haverbeke", …}]
console.log(ancestry.filter(isInSet.bind(null, theSet)));
// → … same result
----

The call to `bind` returns a function that will call `isInSet` with
`theSet` as first argument, followed by any remaining arguments given
to the bound function.

(((null)))The first argument, where the example passes `null`, is used
for ((method call))s, similar to the first argument to `apply`. I'll
describe this in more detail in the
link:06_object.html#call_method[next chapter].

== Summary ==

Being able to pass function values to other functions is not just a
gimmick but a deeply useful aspect of JavaScript. It allows us to
write computations with “gaps” in them as functions and have the code
that calls these functions fill in those gaps by providing function
values that describe the missing computations.

Arrays provide a number of useful higher-order methods—`forEach`
to do something with each element in an array, `filter` to build a new
array with some elements filtered out, `map` to build a new array
where each element has been put through a function, and `reduce` to
combine all an array's elements into a single value.

Functions have an `apply` method that can be used to call them with an
array specifying their arguments. They also have a `bind` method,
which is used to create a partially applied version of the function.

== Exercises ==

=== Flattening ===

(((flattening (exercise))))(((reduce method)))(((concat
method)))(((array)))Use the `reduce` method in combination with
the `concat` method to “flatten” an array of arrays into a single
array that has all the elements of the input arrays.

ifdef::interactive_target[]

// test: no

[source,javascript]
----
var arrays = [[1, 2, 3], [4, 5], [6]];
// Your code here.
// → [1, 2, 3, 4, 5, 6]
----
endif::interactive_target[]

=== Mother-child age difference ===

(((ancestry example)))(((age difference (exercise))))(((average
function)))Using the example data set from this chapter, compute the
average age difference between mothers and children (the age of the
mother when the child is born). You can use the `average` function
defined link:05_higher_order.html#average_function[earlier] in this
chapter.

(((byName object)))Note that not all the mothers mentioned in the data
are themselves present in the array. The `byName` object, which makes
it easy to find a person's object from their name, might be useful
here.

ifdef::interactive_target[]

// test: no
// include_code

[source,javascript]
----
function average(array) {
  function plus(a, b) { return a + b; }
  return array.reduce(plus) / array.length;
}

var byName = {};
ancestry.forEach(function(person) {
  byName[person.name] = person;
});

// Your code here.

// → 31.2
----
endif::interactive_target[]

!!hint!!

(((age difference (exercise))))(((filter method)))(((map
method)))(((null)))(((average function)))Because not all elements in
the `ancestry` array produce useful data (we can't compute the age
difference unless we know the birth date of the mother), we will have
to apply `filter` in some manner before calling `average`. You could
do it as a first pass, by defining a `hasKnownMother` function and
filtering on that first. Alternatively, you could start by calling
`map` and in your mapping function return either the age difference
or `null` if no mother is known. Then, you can call `filter` to remove
the `null` elements before passing the array to `average`.

!!hint!!

=== Historical life expectancy ===

(((life expectancy (exercise))))When we looked up all the people in
our data set that lived more than 90 years, only the latest
generation in the data came out. Let's take a closer look at that
phenomenon.

(((average function)))Compute and output the average age of the people
in the ancestry data set per century. A person is assigned to a
((century)) by taking their year of death, dividing it by 100,
and rounding it up, as in `Math.ceil(person.died / 100)`.

ifdef::interactive_target[]

// test: no

[source,javascript]
----
function average(array) {
  function plus(a, b) { return a + b; }
  return array.reduce(plus) / array.length;
}

// Your code here.

// → 16: 43.5
//   17: 51.2
//   18: 52.8
//   19: 54.8
//   20: 84.7
//   21: 94
----
endif::interactive_target[]

!!hint!!

(((life expectancy (exercise))))The essence of this example lies in
((grouping)) the elements of a collection by some aspect of
theirs—splitting the array of ancestors into smaller arrays with the
ancestors for each century.

(((array)))(((map)))(((object,as map)))During the grouping
process, keep an object that associates ((century)) names (numbers)
with arrays of either person objects or ages. Since we do not know in
advance what categories we will find, we'll have to create them on the
fly. For each person, after computing their century, we test whether
that century was already known. If not, add an array for it. Then add
the person (or age) to the array for the proper century.

(((for/in loop)))(((average function)))Finally, a `for`/`in` loop can
be used to print the average ages for the individual centuries.

!!hint!!

(((grouping)))(((map)))(((object,as map)))(((groupBy function)))For
bonus points, write a function `groupBy` that abstracts the grouping
operation. It should accept as arguments an array and a function that
computes the group for an element in the array and returns an object
that maps group names to arrays of group members.

=== Every and then some ===

(((predicate function)))(((every and some (exercise))))(((every
method)))(((some method)))(((array,methods)))(((&& operator)))(((||
operator)))Arrays also come with the standard methods `every` and
`some`. Both take a predicate function that, when called with an array
element as argument, returns true or false. Just like `&&` 
returns a true value only when the expressions on both sides are true,
`every` returns true only when the predicate returns true for _all_
elements of the array. Similarly, `some` returns true as soon as the
predicate returns true for _any_ of the elements. They do not process
more elements than necessary—for example, if `some` finds that the
predicate holds for the first element of the array, it will not look
at the values after that.

Write two functions, `every` and `some`, that behave like these
methods, except that they take the array as their first argument
rather than being a method.

ifdef::interactive_target[]

// test: no

[source,javascript]
----
// Your code here.

console.log(every([NaN, NaN, NaN], isNaN));
// → true
console.log(every([NaN, NaN, 4], isNaN));
// → false
console.log(some([NaN, 3, 4], isNaN));
// → true
console.log(some([2, 3, 4], isNaN));
// → false
----
endif::interactive_target[]

!!hint!!

(((every and some (exercise))))(((short-circuit evaluation)))(((return
keyword)))The functions can follow a similar pattern to the
link:05_higher_order.html#forEach[definition] of `forEach` at the
start of the chapter, except that they must return immediately (with
the right value) when the predicate function returns false—or true.
Don't forget to put another `return` statement after the loop so that
the function also returns the correct value when it reaches the end of
the array.

!!hint!!