1Syntax::Keyword::Defer(U3spemr)Contributed Perl DocumentSaytnitoanx::Keyword::Defer(3pm)
2
6 "Syntax::Keyword::Defer" - execute code when leaving a block
7
9 use Syntax::Keyword::Defer;
10 {
12 my $dbh = DBI->connect( ... ) or die "Cannot connect";
13 defer { $dbh->disconnect; }
14 my $sth = $dbh->prepare( ... ) or die "Cannot prepare";
16 defer { $sth->finish; }
17 ...
19 }
20
22 This module provides a syntax plugin that implements a block which
23 executes when the containing scope has finished.
24 It similar to features provided by other languages; Swift, Zig, Jai,
26 Nim and Odin all provide this. Note that while Go also provides a
27 "defer" keyword, the semantics here are not the same. Go's version
28 defers until the end of the entire function, rather than the closest
29 enclosing scope as is common to most other languages, and this module.
30 The operation can be considered a little similar to an "END" block, but
32 with the following key differences:
33 • A "defer" block runs at the time that execution leaves the block it
35 is declared inside, whereas an "END" block runs at the end time of
36 the entire program regardless of its location.
37 • A "defer" block is invoked at the time its containing scope has
39 finished, which means it might run again if the block is entered
40 again later in the program. An "END" block will only ever run once.
41 • A "defer" block will only take effect if execution reaches the line
43 it is declared on; if the line is not reached then nothing happens.
44 An "END" block will always be invoked once declared, regardless of
45 the dynamic extent of execution at runtime.
46 "defer" blocks are primarily intended for cases such as resource
48 finalisation tasks that may be conditionally required.
49 For example in the synopsis code, after normal execution the statement
51 handle will be finished using the "$sth->finish" method, then the
52 database will be disconnected with "$dbh->disconnect". If instead the
53 prepare method failed then the database will still be disconnected, but
54 there is no need to finish with the statement handle as the second
55 "defer" block was never encountered.
56
58 defer
59 defer {
60 STATEMENTS...
61 }
62 The "defer" keyword introduces a block which runs its code body at the
64 time that its immediately surrounding code block finishes.
65 When the "defer" statement is encountered, the body of the code block
67 is pushed to a queue of pending operations, which is then flushed when
68 the surrounding block finishes for any reason - either by implicit
69 fallthrough, or explicit termination by "return", "die" or any of the
70 loop control statements "next", "last" or "redo".
71 sub f
73 {
74 defer { say "The function has now returned"; }
75 return 123;
76 }
77 If multiple "defer" statements appear within the same block, they are
79 pushed to the queue in LIFO order; the last one encountered is the
80 first one to be executed.
81 {
83 defer { say "This happens second"; }
84 defer { say "This happens first"; }
85 }
86 A "defer" block will only take effect if the statement itself is
88 actually encountered during normal execution. This is in direct
89 contrast to an "END" phaser which always occurs. This makes it ideal
90 for handling finalisation of a resource which was created on a nearby
91 previous line, where the code to create it might have thrown an
92 exception instead. Because the exception skipped over the "defer"
93 statement, the code body does not need to run.
94 my $resource = Resource->open( ... );
96 defer { $resource->close; }
97 Unlike as would happen with e.g. a "DESTROY" method on a guard object,
99 any exceptions thrown from a "defer" block are still propagated up to
100 the caller in the usual way.
101 use Syntax::Keyword::Defer;
103 sub f
105 {
106 my $count = 0;
107 defer { $count or die "Failed to increment count"; }
108 # some code here
110 }
111 f();
113 $ perl example.pl
115 Failed to increment count at examples.pl line 6.
116 However, if a "defer" block is being run because of exceptional return
118 of its scope, any further exceptions it attempts to raise are turned
119 into warnings. This ensures that the original exception which caused
120 the stack-unwind to run the block in the first place does not get
121 overwritten on the way.
122 Because a "defer" block is a true block (e.g. in the same way something
124 like an "if () {...}" block is), rather than an anonymous sub, it does
125 not appear to caller() or other stack-inspection tricks. This is useful
126 for calling croak(), for example.
127 sub g
129 {
130 my $count = 0;
131 defer { $count or croak "Expected some items"; }
132 $count++ for @_;
134 }
135 Here, croak() will correctly report the caller of the g() function,
137 rather than appearing to be called from an "__ANON__" sub invoked at
138 the end of the function itself.
139
141 This module contains a unit test file copied and edited from my core
142 perl branch to provide the same syntax. Several test cases are
143 currently commented out because this implementation does not yet handle
144 them:
145 • Detection logic of defer-during-throw is currently based on the
147 truth of the "ERRSV" ($@), which means it is liable to false
148 positives. There may not be much that can be done about this.
149 • Try to fix the double-exception test failure on Perl versions
151 before v5.20. (Test currently skipped on those versions)
152 • Try to detect and forbid nonlocal flow control ("goto",
154 "next/last/redo") from leaving the "defer" block.
155 E.g. currently the following will crash the interpreter:
157 sub func { last ITEM }
159 ITEM: foreach(1..10) {
161 say;
162 defer { func() }
163 }
164
166 Paul Evans <leonerd@leonerd.org.uk>
167perl v5.38.0 2023-07-21 Syntax::Keyword::Defer(3pm)