95 lines
1.8 KiB
Text
95 lines
1.8 KiB
Text
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#lang scribble/manual
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@(require (for-label racket))
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@title{Partial Application}
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Create Functions via Partial Application@section{categories}
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Language
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@section{related}
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Reference/Functions
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Partial application is a way to create a function by passing only some arguments to a method. The
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@racketblock[_:: character stands in for missing arguments and becomes an argument to the created function.
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It only applies to a single method, list, or dictionary call, not to a more complex nested expression.
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for example:
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]
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@racketblock[
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f = _ + 2;
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::
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f is now a function of one argument.
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]
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@racketblock[
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f.value(7); // returns 9
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::
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it is equivalent to having written:
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]
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@racketblock[
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f = {|x| x + 2 };
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::
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(except that there is no name 'x' declared)
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]
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@racketblock[
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g = Point(_, _);
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::
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g is a function of two arguments.
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]
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@racketblock[
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g.value(3, 4);
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::
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Here are some example usages of this in a collect message. Below each is written the equivalent function.
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]
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@racketblock[
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(1..8).collect(_.isPrime);
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(1..8).collect {|x| x.isPrime };
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(1..8).collect(_.hash);
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(1..8).collect {|x| x.hash };
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(1..8).collect([\a, \b, _]);
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(1..8).collect {|x| [\a, \b, x] };
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(1..8).collect((a:_));
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(1..8).collect {|x| (a:x) };
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(1..8).collect(Polar(_, pi));
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(1..8).collect {|x| Polar(x, pi) };
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(1..8).collect((1.._));
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(1..8).collect {|x| (1..x) };
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::
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]
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@racketblock[
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f = (a:_, b:_); // f is a two argument function
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g = f.(_, 5); // g is a partial application of f
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g.(7); // get the answer
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// equivalent to this:
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f = {|x, y| (a:x, b:y) }
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g = {|z| f.(z, 5) };
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g.value(7);
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::
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An example of what you can't do:
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]
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@racketblock[
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(1..8).collect( Point(100 * _, 50) ); // nested expression won't work.
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// only the * gets partially applied, not the surrounding expression.
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(1..8).collect {|x| Point(100 * x, 50) }; // need to use a function for this.
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::
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]
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