git.iko.soy/rowtype-yoga/purescript-native
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Delete blocks.rst
28a663aPhil Freeman
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1−Blocks
2−======
3−
4−Blocks are collections of statements wrapped in braces ``{ ... }``. Blocks must return a value of the same type on every branch of execution.
5−
6−The following types of statement are supported:
7−
8−- Variable introduction
9−- Variable assignment
10−- For loops
11−- While loops
12−- If-Then-Else statements
13−
14−Here is an example of a power function defined using a block::
15−
16− pow n p = {
17− var m = n;
18− for (i <- 0 until p) {
19− m = m * n;
20− }
21− return m;
22− }
23−
24−Blocks enable local mutation of their variables, but mutation is not allowed in general. The type system prevents mutable variables from escaping their scope.
25−
26−That is, while the example above is valid, the following does not compile::
27−
28− incr n = {
29− n = n + 1;
30− return n;
31− }
32−
33−The variable ``n`` is not mutable, and so the assignment in the first line of the block is not allowed.
34−
35−This function can be rewritten as follows::
36−
37− incr n = {
38− var m = n;
39− m = m + 1;
40− return m;
41− }
42−
43−For Loops
44−---------
45−
46−For loops iterate over a range of numbers::
47−
48− total = {
49− var n = 0;
50− for (i <- 0 until 10) {
51− n = n + i;
52− }
53− return n;
54− }
55−
56−The bounds `0` and `10` are inclusive and exclusive respectively.
57−
58−While Loops
59−------------
60−
61−While loops repeat a set of statements while a boolean expression evaluates to ``true``::
62−
63− log2 n = {
64− var count = 0;
65− var m = n;
66− while (m > 1) {
67− m = m / 2;
68− count = count + 1;
69− }
70− return count;
71− }
72−
73−If-Then-Else Statements
74−-----------------------
75−
76−Else branches are optional, and may contain further ``if`` statements, just as in Javascript::
77−
78− collatz n = {
79− var count = 0;
80− var m = n;
81− while (m > 1) {
82− if (m % 2 == 0) {
83− m = m / 2;
84− } else {
85− m = m * 3 + 1;
86− }
87− count = count + 1;
88− }
89− return count;
90− }