1try(n) Forward compatibility implementation of [try] try(n)
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6
8 try - try - Trap and process errors and exceptions
9
11 package require Tcl 8.5
12 package require try ?1?
14 ::try body ?handler...? ?finally script?
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18
20 This package provides a forward-compatibility implementation of Tcl
21 8.6's try/finally command (TIP 329), for Tcl 8.5. The code was directly
22 pulled from Tcl 8.6 revision ?, when try/finally was implemented as Tcl
23 procedure instead of in C.
24 ::try body ?handler...? ?finally script?
26 This command executes the script body and, depending on what the
27 outcome of that script is (normal exit, error, or some other ex‐
28 ceptional result), runs a handler script to deal with the case.
29 Once that has all happened, if the finally clause is present,
30 the script it includes will be run and the result of the handler
31 (or the body if no handler matched) is allowed to continue to
32 propagate. Note that the finally clause is processed even if an
33 error occurs and irrespective of which, if any, handler is used.
34 The handler clauses are each expressed as several words, and
36 must have one of the following forms:
37 on code variableList script
39 This clause matches if the evaluation of body completed
40 with the exception code code. The code may be expressed
41 as an integer or one of the following literal words: ok,
42 error, return, break, or continue. Those literals corre‐
43 spond to the integers 0 through 4 respectively.
44 trap pattern variableList script
46 This clause matches if the evaluation of body resulted in
47 an error and the prefix of the -errorcode from the inter‐
48 preter's status dictionary is equal to the pattern. The
49 number of prefix words taken from the -errorcode is equal
50 to the list-length of pattern, and inter-word spaces are
51 normalized in both the -errorcode and pattern before com‐
52 parison.
53 The variableList word in each handler is always inter‐
55 preted as a list of variable names. If the first word of
56 the list is present and non-empty, it names a variable
57 into which the result of the evaluation of body (from the
58 main try) will be placed; this will contain the human-
59 readable form of any errors. If the second word of the
60 list is present and non-empty, it names a variable into
61 which the options dictionary of the interpreter at the
62 moment of completion of execution of body will be placed.
63 The script word of each handler is also always inter‐
65 preted the same: as a Tcl script to evaluate if the
66 clause is matched. If script is a literal - and the han‐
67 dler is not the last one, the script of the following
68 handler is invoked instead (just like with the switch
69 command).
70 Note that handler clauses are matched against in order,
72 and that the first matching one is always selected. At
73 most one handler clause will selected. As a consequence,
74 an on error will mask any subsequent trap in the try.
75 Also note that on error is equivalent to trap {}.
76 If an exception (i.e. any non-ok result) occurs during
78 the evaluation of either the handler or the finally
79 clause, the original exception's status dictionary will
80 be added to the new exception's status dictionary under
81 the -during key.
82
84 Ensure that a file is closed no matter what:
85 set f [open /some/file/name a]
87 try {
88 puts \$f "some message"
89 # ...
90 } finally {
91 close \$f
92 }
93 Handle different reasons for a file to not be openable for reading:
96 try {
98 set f [open /some/file/name]
99 } trap {POSIX EISDIR} {} {
100 puts "failed to open /some/file/name: it's a directory"
101 } trap {POSIX ENOENT} {} {
102 puts "failed to open /some/file/name: it doesn't exist"
103 }
104
107 This document, and the package it describes, will undoubtedly contain
108 bugs and other problems. Please report such in the category try of the
109 Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Please also
110 report any ideas for enhancements you may have for either package
111 and/or documentation.
112 When proposing code changes, please provide unified diffs, i.e the out‐
114 put of diff -u.
115 Note further that attachments are strongly preferred over inlined
117 patches. Attachments can be made by going to the Edit form of the
118 ticket immediately after its creation, and then using the left-most
119 button in the secondary navigation bar.
120
122 catch(n), error(n), return(n), throw(n)
123
125 cleanup, error, exception, final, resource management
126
128 Utility
129
131 Copyright (c) 2008 Donal K. Fellows, BSD licensed
132tcllib 1 try(n)