1Syntax::Keyword::Try(3)User Contributed Perl DocumentatioSnyntax::Keyword::Try(3)
2
6 "Syntax::Keyword::Try" - a "try/catch/finally" syntax for perl
7
9 use Syntax::Keyword::Try;
10 sub foo
12 {
13 try {
14 attempt_a_thing();
15 return "success";
16 }
17 catch {
18 warn "It failed - $@";
19 return "failure";
20 }
21 }
22
24 This module provides a syntax plugin that implements exception-handling
25 semantics in a form familiar to users of other languages, being built
26 on a block labeled with the "try" keyword, followed by at least one of
27 a "catch" or "finally" block.
28 As well as providing a handy syntax for this useful behaviour, this
30 module also serves to contain a number of code examples for how to
31 implement parser plugins and manipulate optrees to provide new syntax
32 and behaviours for perl code.
33
35 try
36 try {
37 STATEMENTS...
38 }
39 ...
40 A "try" statement provides the main body of code that will be invoked,
42 and must be followed by either a "catch" statement, a "finally"
43 statement, or both.
44 Execution of the "try" statement itself begins from the block given to
46 the statement and continues until either it throws an exception, or
47 completes successfully by reaching the end of the block. What will
48 happen next depends on the presence of a "catch" or "finally" statement
49 immediately following it.
50 The body of a "try {}" block may contain a "return" expression. If
52 executed, such an expression will cause the entire containing function
53 to return with the value provided. This is different from a plain "eval
54 {}" block, in which circumstance only the "eval" itself would return,
55 not the entire function.
56 The body of a "try {}" block may contain loop control expressions
58 ("redo", "next", "last") which will have their usual effect on any
59 loops that the "try {}" block is contained by.
60 The parsing rules for the set of statements (the "try" block and its
62 associated "catch" and "finally") are such that they are parsed as a
63 self- contained statement. Because of this, there is no need to end
64 with a terminating semicolon.
65 Note (especially to users of Try::Tiny and similar) that the "try {}"
67 block itself does not necessarily stop exceptions thrown inside it from
68 propagating outside. It is the presence of a later "catch {}" block
69 which causes this to happen. A "try" with only a "finally" and no
70 "catch" will still propagate exceptions up to callers as normal.
71 catch
73 ...
74 catch {
75 STATEMENTS...
76 }
77 A "catch" statement provides a block of code to the preceding "try"
79 statement that will be invoked in the case that the main block of code
80 throws an exception. The "catch" block can inspect the raised exception
81 by looking in $@ in the usual way.
82 Presence of this "catch" statement causes any exception thrown by the
84 preceding "try" block to be non-fatal to the surrounding code. If the
85 "catch" block wishes to optionally handle some exceptions but not
86 others, it can re-raise it (or another exception) by calling "die" in
87 the usual manner.
88 As with "try", the body of a "catch {}" block may also contain a
90 "return" expression, which as before, has its usual meaning, causing
91 the entire containing function to return with the given value. The body
92 may also contain loop control expressions ("redo", "next" or "last")
93 which also have their usual effect.
94 If a "catch" statement is not given, then any exceptions raised by the
96 "try" block are raised to the caller in the usual way.
97 finally
99 ...
100 finally {
101 STATEMENTS...
102 }
103 A "finally" statement provides a block of code to the preceding "try"
105 statement (or "try/catch" pair) which is executed afterwards, both in
106 the case of a normal execution or a thrown exception. This code block
107 may be used to provide whatever clean-up operations might be required
108 by preceding code.
109 Because it is executed during a stack cleanup operation, a "finally {}"
111 block may not cause the containing function to return, or to alter the
112 return value of it. It also cannot see the containing function's @_
113 arguments array (though as it is block scoped within the function, it
114 will continue to share any normal lexical variables declared up until
115 that point). It is protected from disturbing the value of $@. If the
116 "finally {}" block code throws an exception, this will be printed as a
117 warning and discarded, leaving $@ containing the original exception, if
118 one existed.
119
121 Warning: the feature described in this section is experimental.
122 This experiment may be stablised in a later version, or may be
123 altered or removed without further notice. It is present here for
124 testing and evaluation purposes.
125 Additionally, on perl versions 5.18 and later, it will produce a
127 warning in the "experimental" category.
128 The syntax provided by this module may be used as a value-yielding
130 expression. Because this syntax is new, experimental, and somewhat
131 surprising, it must be specifically requested by name "try_value":
132 use Syntax::Keyword::Try qw( try try_value );
134 my $result = try do { ... } catch { ... };
136 Also, on Perl versions 5.24 and later:
138 my $result = try do { ... } finally { ... };
140 my $result = try do { ... } catch { ... } finally { ... };
142 Specifically, note that the expression must be spelled as "try do { ...
144 }" so that the syntax is distinct from that used by control-flow
145 statements. The interposed "do" keyword reminds the reader, and
146 instructs the syntax parser, that this will be an expression, not a
147 statement. It is not necessary to similarly notate the "catch" or
148 "finally" blocks.
149 In this case, the syntax behaves syntactically like an expression, and
151 may appear anywhere a normal expression is allowed. It follows similar
152 semantics to the purely control-flow case; if the code in the "try"
153 block does not throw an exception, then the expression as a whole
154 yields whatever value the "try" expression did. If it fails, then the
155 "catch" block is executed and the expression yields its resulting value
156 instead. A "finally" block, if present, will be evaluated for side-
157 effects before the rest of the expression returns.
158 Remember that, as in the control-flow case, the "return" keyword will
160 cause the entire containing function to return, not just the "try"
161 block.
162
164 There are already quite a number of modules on CPAN that provide a
165 "try/catch"-like syntax for Perl.
166 · Try
168 · TryCatch
170 · Try::Tiny
172 · Syntax::Feature::Try
174 They are compared here, by feature:
176 True syntax plugin
178 Like Try and Syntax::Feature::Try, this module is implemented as a true
179 syntax plugin, allowing it to provide new parsing rules not available
180 to simple functions. Most notably here it means that the resulting
181 combination does not need to end in a semicolon.
182 In comparison, Try::Tiny is plain perl and provides its functionality
184 using regular perl functions; as such its syntax requires the trailing
185 semicolon.
186 TryCatch is a hybrid that uses Devel::Declare to parse the syntax tree.
188 @_ in a try or catch block
190 Because the "try" and "catch" block code is contained in a true block
191 rather than an entire anonymous subroutine, invoking it does not
192 interfere with the @_ arguments array. Code inside these blocks can
193 interact with the containing function's array as before.
194 This feature is unique among these modules; none of the others listed
196 have this ability.
197 "return" in a try or catch block
199 Like TryCatch and Syntax::Feature::Try, the "return" statement has its
200 usual effect within a subroutine containing syntax provided by this
201 module. Namely, it causes the containing "sub" itself to return.
202 In comparison, using Try or Try::Tiny mean that a "return" statement
204 will only exit from the "try" block.
205 "next"/"last"/"redo" in a try or catch block
207 The loop control keywords of "next", "last" and "redo" have their usual
208 effect on dynamically contained loops.
209 Syntax::Feature::Try documents that these do not work there. The other
211 modules make no statement either way.
212 Value Semantics
214 Like Try and Syntax::Feature::Try, the syntax provided by this module
215 only works as a syntax-level statement and not an expression when the
216 experimental "try_value" feature described above has not been enabled.
217 You cannot assign from the result of a "try" block. Additionally,
218 final-expression value semantics do not work, so it cannot be contained
219 by a "do" block to yield this value.
220 In comparison, the behaviour implemented by Try::Tiny can be used as a
222 valued expression, such as assigned to a variable or returned to the
223 caller of its containing function. Such ability is provided by this
224 module if the experimental "try_value" feature is enabled, though it
225 must be spelled differently as "try do { ... }".
226 "try" without "catch"
228 Like Syntax::Feature::Try, the syntax provided by this module allows a
229 "try" block to be followed by only a "finally" block, with no "catch".
230 In this case, exceptions thrown by code contained by the "try" are not
231 suppressed, instead they propagate as normal to callers. This matches
232 the behaviour familiar to Java or C++ programmers.
233 In comparison, the code provided by Try and Try::Tiny always suppress
235 exception propagation even without an actual "catch" block.
236 The TryCatch module does not allow a "try" block not followed by
238 "catch".
239 Typed "catch"
241 Like Try and Try::Tiny, this module makes no attempt to perform any
242 kind of typed dispatch to distinguish kinds of exception caught by
243 "catch" blocks.
244 TryCatch and Syntax::Feature::Try both attempt to provide a kind of
246 typed dispatch where different classes of exception are caught by
247 different blocks of code, or propagated up entirely to callers.
248 The author considers the lack of such ability in this module to be a
250 feature. That kind of dispatch on type matching of a controlling
251 expression is too useful a behaviour to be constrained to exception
252 catching. If the language is to provide such a facility, it should be
253 more universally applicable as a stand-alone independent ability.
254
256 Future::AsyncAwait
257 As of "Future::AsyncAwait" version 0.10 and Syntax::Keyword::Try
258 version 0.07, cross-module integration tests assert that basic
259 "try/catch" blocks inside an "async sub" work correctly, including
260 those that attempt to "return" from inside "try".
261 use Future::AsyncAwait;
263 use Syntax::Keyword::Try;
264 async sub attempt
266 {
267 try {
268 await func();
269 return "success";
270 }
271 catch {
272 return "failed";
273 }
274 }
275
277 Thread-safety at load time cannot be assured before perl 5.16
278 On perl versions 5.16 and above this module is thread-safe.
279 On perl version 5.14 this module is thread-safe provided that it is
281 "use"d before any additional threads are created.
282 However, when using 5.14 there is a race condition if this module is
284 loaded late in the program startup, after additional threads have been
285 created. This leads to the potential for it to be started up multiple
286 times concurrently, which creates data races when modifying internal
287 structures and likely leads to a segmentation fault, either during load
288 or soon after when more code is compiled.
289 As a workaround, for any such program that creates multiple threads,
291 loads additional code (such as dynamically-discovered plugins), and has
292 to run on 5.14, it should make sure to
293 use Syntax::Keyword::Try;
295 early on in startup, before it spins out any additional threads.
297 (See also <https://rt.cpan.org/Public/Bug/Display.html?id=123547>)
299 $@ is not local'ised by "try do" before perl 5.24
301 On perl versions 5.24 and above, or when using only control-flow
302 statement syntax, $@ is always correctly "local"ised.
303 However, when using the experimental value-yielding expression version
305 "try do {...}" on perl versions 5.22 or older, the "local"isation of $@
306 does not correctly apply around the expression. After such an
307 expression, the value of $@ will leak out if a failure happened and the
308 "catch" block was invoked, overwriting any previous value that was
309 visible there.
310 (See also <https://rt.cpan.org/Public/Bug/Display.html?id=124366>)
312
314 With thanks to "Zefram", "ilmari" and others from "irc.perl.org/#p5p"
315 for assisting with trickier bits of XS logic.
316
318 Paul Evans <leonerd@leonerd.org.uk>
319perl v5.30.0 2019-09-09 Syntax::Keyword::Try(3)