44
55= SUBTITLE Statements used to control the flow of execution
66
7+ = head2 X < statements|control flow >
8+
9+ Perl 6 programs consists of one or more statements. Simple statements
10+ are separated by semicolons. The following program will say "Hello"
11+ and then say "World" on the next line.
12+
13+ say "Hello";
14+ say "World";
15+
16+ In most places where spaces appear in a statement, and before the
17+ semicolon, it may be split up over many lines. Also, multiple statements
18+ may appear on the same line. It would be awkward, but the above could
19+ also be written as:
20+
21+ say
22+ "Hello"; say "World";
23+
724= head2 X < blocks|control flow >
825
9- Like many languages, Perl6 uses C < blocks > delimited by C < { > and C < } >
10- to compartmentalize code. When a block stands alone as a statement,
11- it will be entered immediately after the statement before it finishes,
12- and the statements inside it will be executed. Otherwise, a block
13- simply creates a closure, which may be executed at a later time:
26+ Like many languages, Perl6 uses C < blocks > enclosed by C < { > and C < } > to turn
27+ multiple statements into a single statement. It is ok to skip the semicolon
28+ between the last statement in a block and the closing C < } > .
29+
30+ { say "Hello"; say "World" }
31+
32+ When a block stands alone as a statement, it will be entered immediately
33+ after the previous statement finishes, and the statements inside it will be
34+ executed.
35+
36+ say 1; # says "1"
37+ { say 2; say 3 }; # says "2" then says "3"
38+ say 4; # says "4"
39+
40+ Unless it stands alone as a statement, a block simply creates a closure. The
41+ statements inside are not executed immediately. Closures are another topic
42+ and how they are used is explained
43+ L < elsewhere|language/functions#Blocks and Lambdas > . For now it is just
44+ important to understand when blocks run and when they do not:
1445
1546 say "We get here"; { say "then here." }; { say "not here"; 0; } or die;
1647
17- In the above example, after running the first statement, the first
18- block stands alone as a second statement, so we run the statement inside
19- it. The second block does not stand alone as a statement, so it instantiates
20- an object of type C < Block > , but does not run it. Since any object instance
21- is true, the code does not die, even though that block would evaluate to 0,
22- were it to be executed.
48+ In the above example, after running the first statement, the first block stands
49+ alone as a second statement, so we run the statement inside it. The second
50+ block does not stand alone as a statement, so instead, it makes an object of
51+ type C < Block > but does not run it. Object instances are usually considered to
52+ be true, so the code does not die, even though that block would evaluate to 0,
53+ were it to be executed. The example does not say what to do with the C < Block >
54+ object, so it just gets thrown away.
2355
24- Most of the flow control constructs covered below are just ways to
25- tell perl6 when, how, and how many times, to enter blocks like that
26- second block.
56+ Most of the flow control constructs covered below are just ways to tell perl6
57+ when, how, and how many times, to enter blocks like that second block.
2758
2859Before we go into those, an important side-note on syntax: If there is
2960nothing (or nothing but comments) on a line after a closing curly brace where
@@ -116,19 +147,13 @@ run the block, or it will return the value which the block produces:
116147 my $c = 0; say (1, (if 1 { $c += 42; 2; }), 3, $c); # says "1 2 3 42"
117148 my $d = 0; say (1, (if 0 { $d += 42; 2; }), 3, $d); # says "1 3 0"
118149
119- Implementation note: Currently, Rakudo will say "1 Nil 3 0" for the last
120- example because it is not caught up to this part of the design yet.
121-
122150For the statement modifier it is the same, except you have the value
123151of the statement instead of a block:
124152
125153 say (1, (42 if True) , 2); # says "1 42 2"
126154 say (1, (42 if False), 2); # says "1 2"
127155 say (1, 42 if False , 2); # says "1 42" because "if False, 2" is true
128156
129- Implementation note: Currently, Rakudo will say "1 Nil 2" for the second
130- example because it is not caught up to this part of the design yet.
131-
132157The C < if > does not change the topic (C < $_ > ) by default. In order to access
133158the value which the conditional expression produced, you have to ask
134159for it more strongly:
@@ -200,9 +225,6 @@ or the value produced by the block that did run:
200225 (if 0 { $d += 42; "two"; } elsif False { $d += 43; 2; }),
201226 3, $d); # says "1 3 0"
202227
203- Implementation note: Currently, Rakudo will say "1 Nil 3 0" for the last
204- example because it is not caught up to this part of the design yet.
205-
206228It's possible to obtain the value of the previous expression inside an
207229C < else > , which could be from C < if > or the last C < elsif > if any are
208230present:
@@ -234,9 +256,6 @@ two differences C<unless> works the same as L<if>:
234256 my $c = 0; say (1, (unless 0 { $c += 42; 2; }), 3, $c); #-> says "1 2 3 42"
235257 my $c = 0; say (1, (unless 1 { $c += 42; 2; }), 3, $c); #-> says "1 3 0"
236258
237- Implementation note: Currently, Rakudo will say "1 Nil 3 0" for the last
238- example because it is not caught up to this part of the design yet.
239-
240259= head3 X < with, orwith, without|control flow,with orwith without >
241260
242261The C < with > statement is like C < if > but tests for definedness rather than
@@ -394,7 +413,8 @@ as though the rest of the statements in the block are skipped.
394413 }
395414 # The above block evaluates to 43
396415
397- A C < when > statement will also do this.
416+ A C < when > statement will also do this (but a C < when > statement modifier
417+ will I < not > .)
398418
399419In addition, C < when > statements C < smartmatch > the topic (C < $_ > ) against
400420a supplied expression such that it is possible to check against values,
@@ -417,6 +437,20 @@ C<when> statements. The following code says C<"Int"> not C<42>.
417437 }
418438 #-> Int
419439
440+ When a C < when > statement or C < default > statement causes the outer
441+ block to return, nesting C < when > or C < default > blocks do not count
442+ as the outer block, so you can nest these statements and still
443+ be in the same "switch" just so long as you do not open a new block:
444+
445+ given 42 {
446+ when Int {
447+ when 42 { say 42 }
448+ say "Int"
449+ }
450+ default { say "huh?" }
451+ }
452+ #-> 42
453+
420454= head3 X < proceed and succeed|control flow,proceed succeed >
421455
422456Both C < proceed > and C < succeed > are meant to be used only from inside C < when >
@@ -479,6 +513,22 @@ specify a final value for the block.
479513 }
480514 #-> Int
481515
516+ If you are not inside a when or default block, it is an error to try
517+ to use C < proceed > or C < succeed > . Also remember, the C < when > statement
518+ modifier form does not cause any blocks to be left, and any C < succeed >
519+ or C < proceed > in such a statement applies to the surrounding clause,
520+ if there is one:
521+
522+ given 42 {
523+ { say "This says" } when Int;
524+ "This says too".say;
525+ when * > 41 {
526+ { "And this says".say; proceed } when * > 41;
527+ "This never says".say;
528+ }
529+ "This also says".say;
530+ }
531+
482532= head2 X < loop|control flow >
483533
484534The C < loop > statement is the C-style C < for > loop in disguise:
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