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.s4
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23 <p class=
"p1"><br></p>
24 <p class=
"p2"><b>Function Creation via Partial Application
</b></p>
25 <p class=
"p1"><br></p>
26 <p class=
"p3">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.
</p>
27 <p class=
"p4"><br></p>
28 <p class=
"p3">for example:
</p>
29 <p class=
"p4"><br></p>
30 <p class=
"p5">f = _ +
2;
</p>
31 <p class=
"p4"><br></p>
32 <p class=
"p3">f is now a function of one argument.
</p>
33 <p class=
"p4"><br></p>
34 <p class=
"p5">f.value(
7);
</p>
35 <p class=
"p4"><br></p>
36 <p class=
"p3">it is equivalent to having written:
</p>
37 <p class=
"p4"><br></p>
38 <p class=
"p5">f = {
<span class=
"s1">|x|
</span> x +
2 };
</p>
39 <p class=
"p4"><br></p>
40 <p class=
"p3">(except that there is no name 'x' declared)
</p>
41 <p class=
"p4"><br></p>
42 <p class=
"p4"><br></p>
43 <p class=
"p5">g =
<span class=
"s1">Point
</span>(
<span class=
"s1">_
</span>,
<span class=
"s1">_
</span>);
</p>
44 <p class=
"p4"><br></p>
45 <p class=
"p3">g is a function of two arguments.
</p>
46 <p class=
"p4"><br></p>
47 <p class=
"p5">g.value(
3,
4);
</p>
48 <p class=
"p4"><br></p>
49 <p class=
"p4"><br></p>
50 <p class=
"p3">Here are some example usages of this in a collect message. Below each is written the equivalent function.
</p>
51 <p class=
"p5">(
1.
.8).collect(
<span class=
"s1">_
</span>.isPrime);
</p>
52 <p class=
"p5">(
1.
.8).collect {
<span class=
"s1">|x|
</span> x.isPrime };
</p>
53 <p class=
"p4"><br></p>
54 <p class=
"p5">(
1.
.8).collect(
<span class=
"s1">_
</span>.hash);
</p>
55 <p class=
"p5">(
1.
.8).collect {
<span class=
"s1">|x|
</span> x.hash };
</p>
56 <p class=
"p4"><br></p>
57 <p class=
"p5">(
1.
.8).collect([
<span class=
"s2">\a
</span>,
<span class=
"s2">\b
</span>,
<span class=
"s1">_
</span>]);
</p>
58 <p class=
"p5">(
1.
.8).collect {
<span class=
"s1">|x|
</span> [
<span class=
"s2">\a
</span>,
<span class=
"s2">\b
</span>, x] };
</p>
59 <p class=
"p4"><br></p>
60 <p class=
"p5">(
1.
.8).collect((a:
<span class=
"s1">_
</span>));
</p>
61 <p class=
"p5">(
1.
.8).collect {
<span class=
"s1">|x|
</span> (a:x) };
</p>
62 <p class=
"p4"><br></p>
63 <p class=
"p5">(
1.
.8).collect(
<span class=
"s1">Polar
</span>(
<span class=
"s1">_
</span>, pi));
</p>
64 <p class=
"p5">(
1.
.8).collect {
<span class=
"s1">|x|
</span> <span class=
"s1">Polar
</span>(x, pi) };
</p>
65 <p class=
"p4"><br></p>
66 <p class=
"p5">(
1.
.8).collect((
1..
<span class=
"s1">_
</span>));
</p>
67 <p class=
"p5">(
1.
.8).collect {
<span class=
"s1">|x|
</span> (
1..x) };
</p>
68 <p class=
"p4"><br></p>
69 <p class=
"p6"><span class=
"s3">f = (a:
</span><span class=
"s1">_
</span><span class=
"s3">, b:
</span><span class=
"s1">_
</span><span class=
"s3">);
</span>// f is a two argument function
</p>
70 <p class=
"p6"><span class=
"s3">g = f.(
</span><span class=
"s1">_
</span><span class=
"s3">,
5);
<span class=
"Apple-converted-space"> </span></span>// g is a partial application of f
</p>
71 <p class=
"p6"><span class=
"s3">g.(
7);
<span class=
"Apple-converted-space"> </span></span>// get the answer
</p>
72 <p class=
"p4"><br></p>
73 <p class=
"p6">// equivalent to this:
</p>
74 <p class=
"p5">f = {
<span class=
"s1">|x, y|
</span> (a:x, b:y) }
</p>
75 <p class=
"p5">g = {
<span class=
"s1">|z|
</span> f.(z,
5) };
</p>
76 <p class=
"p5">g.value(
7);
</p>
77 <p class=
"p4"><br></p>
78 <p class=
"p4"><br></p>
79 <p class=
"p3">An example of what you can't do:
</p>
80 <p class=
"p4"><br></p>
81 <p class=
"p5">(
1.
.8).collect(
<span class=
"s1">Point
</span>(
100 *
<span class=
"s1">_
</span>,
50) );
<span class=
"s4">// nested expression won't work.
</span></p>
82 <p class=
"p6">// only the * gets partially applied, not the surrounding expression.
</p>
83 <p class=
"p4"><br></p>
84 <p class=
"p6"><span class=
"s3">(
1.
.8).collect {
</span><span class=
"s1">|x|
</span><span class=
"s3"> </span><span class=
"s1">Point
</span><span class=
"s3">(
100 * x,
50) };
</span>// need to use a function for this.
</p>
85 <p class=
"p4"><br></p>
