Functions and callbacks

Functions, default and named arguments, arrow functions, and map, filter and reduce.

Functions

function greet( name, greeting = "Hello" ) {
    return greeting & ", " & name;
}

println( greet( "Ada" ) );
println( greet( "Ada", "Hi" ) );
println( greet( greeting = "Hey", name = "Linus" ) );

Output:

Hello, Ada
Hi, Ada
Hey, Linus

A parameter with = has a default. You can also pass arguments by name, in any order.

Functions can call themselves:

function factorial( n ) {
    return n <= 1 ? 1 : n * factorial( n - 1 );
}

println( factorial( 10 ) );

Output:

3628800

Arrow functions

An arrow function is a short, nameless function. It is often passed to another function.

double = n => n * 2;
add = ( a, b ) => a + b;

println( double( 4 ) );
println( add( 2, 3 ) );

Output:

8
5

Map, filter and reduce

Arrays come with three workhorses. map transforms every item, filter keeps some of them, and reduce folds the array down to one value.

words = [ "box", "lang", "match" ];

println( words.map( w => w.ucase() ).toList( " " ) );
println( words.filter( w => w.len() > 3 ).toList( "," ) );
println( words.reduce( ( total, w ) => total + w.len(), 0 ) );

nums = [ 1, 2, 3, 4, 5, 6 ];
sumOfSquares = nums
    .filter( n => n % 2 == 0 )
    .map( n => n * n )
    .reduce( ( a, n ) => a + n, 0 );
println( sumOfSquares );

Output:

BOX LANG MATCH
lang,match
12
56

each, every and some work the same way. every is true when the callback is true for all items, some when it is true for at least one.

println( [ 1, 2, 3 ].every( n => n > 0 ) );
println( [ 1, 2, 3 ].some( n => n > 2 ) );
[ "a", "b" ].each( v => println( "item " & v ) );

Output:

true
true
item a
item b

Functions as values

A function is a value, so you can pass one to another function.

function apply( fn, value ) {
    return fn( value );
}

println( apply( s => s.ucase(), "shout" ) );
println( apply( n => n + 1, 41 ) );

Output:

SHOUT
42

Try it

Write an arrow function isLong that returns true for words longer than 4 letters, then use it with filter on the words array.