Partial Application

Partial application is a way to create a function by passing only some arguments to a method. The _ character stands in for missing arguments and becomes an argument to the created function. It only applies to a single method, list, or dictionary call, not to a more complex nested expression.

for example:f = _ + 2;

f is now a function of one argument.f.value(7); // returns 9

it is equivalent to having written:f = {|x| x + 2 };

(except that there is no name 'x' declared)g = Point(_, _);

g is a function of two arguments.g.value(3, 4);

Here are some example usages of this in a collect message. Below each is written the equivalent function.(1..8).collect(_.isPrime); (1..8).collect {|x| x.isPrime }; (1..8).collect(_.hash); (1..8).collect {|x| x.hash }; (1..8).collect([\a, \b, _]); (1..8).collect {|x| [\a, \b, x] }; (1..8).collect((a:_)); (1..8).collect {|x| (a:x) }; (1..8).collect(Polar(_, pi)); (1..8).collect {|x| Polar(x, pi) }; (1..8).collect((1.._)); (1..8).collect {|x| (1..x) }; f = (a:_, b:_); // f is a two argument function g = f.(_, 5); // g is a partial application of f g.(7); // get the answer // equivalent to this: f = {|x, y| (a:x, b:y) } g = {|z| f.(z, 5) }; g.value(7);

An example of what you can't do:(1..8).collect( Point(100 * _, 50) ); // nested expression won't work. // only the * gets partially applied, not the surrounding expression. (1..8).collect {|x| Point(100 * x, 50) }; // need to use a function for this.