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Diffstat (limited to 'doc/return.n')
| -rw-r--r-- | doc/return.n | 144 |
1 files changed, 87 insertions, 57 deletions
diff --git a/doc/return.n b/doc/return.n index b08de4a..b59a93d 100644 --- a/doc/return.n +++ b/doc/return.n @@ -45,32 +45,38 @@ However, the \fB\-code\fR option may be used to generate an exceptional return from the procedure. \fICode\fR may have any of the following values: .TP 13 -\fBok (or 0)\fR +\fBok\fR (or \fB0\fR) +. Normal return: same as if the option is omitted. The return code of the procedure is 0 (\fBTCL_OK\fR). .TP 13 -\fBerror (1)\fR +\fBerror\fR (or \fB1\fR) +. Error return: the return code of the procedure is 1 (\fBTCL_ERROR\fR). The procedure command behaves in its calling context as if it -were the command \fBerror \fIresult\fR. See below for additional +were the command \fBerror\fR \fIresult\fR. See below for additional options. .TP 13 -\fBreturn (2)\fR +\fBreturn\fR (or \fB2\fR) +. The return code of the procedure is 2 (\fBTCL_RETURN\fR). The procedure command behaves in its calling context as if it were the command \fBreturn\fR (with no arguments). .TP 13 -\fBbreak (3)\fR +\fBbreak\fR (or \fB3\fR) +. The return code of the procedure is 3 (\fBTCL_BREAK\fR). The procedure command behaves in its calling context as if it were the command \fBbreak\fR. .TP 13 -\fBcontinue (4)\fR +\fBcontinue\fR (or \fB4\fR) +. The return code of the procedure is 4 (\fBTCL_CONTINUE\fR). The procedure command behaves in its calling context as if it were the command \fBcontinue\fR. .TP 13 \fIvalue\fR +. \fIValue\fR must be an integer; it will be returned as the return code for the current procedure. .LP @@ -87,7 +93,6 @@ an invocation of the \fBreturn \-code \fIcode\fR command will cause the return code of \fBsource\fR to be \fIcode\fR. .SH "RETURN OPTIONS" .PP -.VS 8.5 In addition to a result and a return code, evaluation of a command in Tcl also produces a dictionary of return options. In general usage, all \fIoption value\fR pairs given as arguments to \fBreturn\fR @@ -96,13 +101,13 @@ are acceptable except as noted below. The \fBcatch\fR command may be used to capture all of this information \(em the return code, the result, and the return options dictionary \(em that arise from evaluation of a script. -.VE 8.5 .PP As documented above, the \fB\-code\fR entry in the return options dictionary receives special treatment by Tcl. There are other return options also recognized and treated specially by Tcl. They are: .TP \fB\-errorcode \fIlist\fR +. The \fB\-errorcode\fR option receives special treatment only when the value of the \fB\-code\fR option is \fBTCL_ERROR\fR. Then the \fIlist\fR value is meant to be additional information about the error, @@ -114,6 +119,7 @@ to the default value of \fBNONE\fR. The \fB\-errorcode\fR return option will also be stored in the global variable \fBerrorCode\fR. .TP \fB\-errorinfo \fIinfo\fR +. The \fB\-errorinfo\fR option receives special treatment only when the value of the \fB\-code\fR option is \fBTCL_ERROR\fR. Then \fIinfo\fR is the initial stack trace, meant to provide to a human reader additional information @@ -130,8 +136,26 @@ the value of \fB\-errorinfo\fR in a return options dictionary captured by the \fBcatch\fR command (or from the copy of that information stored in the global variable \fBerrorInfo\fR). .TP +\fB\-errorstack \fIlist\fR +.VS 8.6 +The \fB\-errorstack\fR option receives special treatment only when the value +of the \fB\-code\fR option is \fBTCL_ERROR\fR. Then \fIlist\fR is the initial +error stack, recording actual argument values passed to each proc level. The error stack will +also be reachable through \fBinfo errorstack\fR. +If no \fB\-errorstack\fR option is provided to \fBreturn\fR when +the \fB\-code error\fR option is provided, Tcl will provide its own +initial error stack in the entry for \fB\-errorstack\fR. Tcl's +initial error stack will include only the call to the procedure, and +stack unwinding will append information about higher stack levels, but +there will be no information about the context of the error within +the procedure. Typically the \fIlist\fR value is supplied from +the value of \fB\-errorstack\fR in a return options dictionary captured +by the \fBcatch\fR command (or from the copy of that information from +\fBinfo errorstack\fR). +.VE 8.6 +.TP \fB\-level \fIlevel\fR -.VS 8.5 +. The \fB\-level\fR and \fB\-code\fR options work together to set the return code to be returned by one of the commands currently being evaluated. The \fIlevel\fR value must be a non-negative integer representing a number @@ -141,14 +165,12 @@ be \fIcode\fR. If no \fB\-level\fR option is provided, the default value of \fIlevel\fR is 1, so that \fBreturn\fR sets the return code that the current procedure returns to its caller, 1 level up the call stack. The mechanism by which these options work is described in more detail below. -.VE 8.5 .TP \fB\-options \fIoptions\fR -.VS 8.5 +. The value \fIoptions\fR must be a valid dictionary. The entries of that dictionary are treated as additional \fIoption value\fR pairs for the \fBreturn\fR command. -.VE 8.5 .SH "RETURN CODE HANDLING MECHANISMS" .PP Return codes are used in Tcl to control program flow. A Tcl script @@ -174,7 +196,6 @@ of \fBTCL_BREAK\fR or \fBTCL_CONTINUE\fR, the loop command can react in such a way as to give the \fBbreak\fR and \fBcontinue\fR commands their documented interpretation in loops. .PP -.VS 8.5 Procedure invocation also involves evaluation of a script, the body of the procedure. Procedure invocation provides special treatment when evaluation of the procedure body returns the return code @@ -202,20 +223,22 @@ of the \fB\-code\fR option (or \fBTCL_OK\fR by default). Any other value for the \fB\-level\fR option (including the default value of 1) will cause the return code of the \fBreturn\fR command itself to be \fBTCL_RETURN\fR, triggering a return from the enclosing procedure. -.VE 8.5 .SH EXAMPLES +.PP First, a simple example of using \fBreturn\fR to return from a procedure, interrupting the procedure body. +.PP .CS proc printOneLine {} { - puts "line 1" ;# This line will be printed. - \fBreturn\fR - puts "line 2" ;# This line will not be printed. + puts "line 1" ;# This line will be printed. + \fBreturn\fR + puts "line 2" ;# This line will not be printed. } .CE .PP Next, an example of using \fBreturn\fR to set the value returned by the procedure. +.PP .CS proc returnX {} {\fBreturn\fR X} puts [returnX] ;# prints "X" @@ -223,74 +246,81 @@ puts [returnX] ;# prints "X" .PP Next, a more complete example, using \fBreturn -code error\fR to report invalid arguments. +.PP .CS proc factorial {n} { - if {![string is integer $n] || ($n < 0)} { - \fBreturn\fR -code error \e - "expected non-negative integer,\e - but got \e"$n\e"" - } - if {$n < 2} { - \fBreturn\fR 1 - } - set m [expr {$n - 1}] - set code [catch {factorial $m} factor] - if {$code != 0} { - \fBreturn\fR -code $code $factor - } - set product [expr {$n * $factor}] - if {$product < 0} { - \fBreturn\fR -code error \e - "overflow computing factorial of $n" - } - \fBreturn\fR $product + if {![string is integer $n] || ($n < 0)} { + \fBreturn\fR -code error \e + "expected non-negative integer,\e + but got \e"$n\e"" + } + if {$n < 2} { + \fBreturn\fR 1 + } + set m [expr {$n - 1}] + set code [catch {factorial $m} factor] + if {$code != 0} { + \fBreturn\fR -code $code $factor + } + set product [expr {$n * $factor}] + if {$product < 0} { + \fBreturn\fR -code error \e + "overflow computing factorial of $n" + } + \fBreturn\fR $product } .CE .PP Next, a procedure replacement for \fBbreak\fR. +.PP .CS proc myBreak {} { - \fBreturn\fR -code break + \fBreturn\fR -code break } .CE .PP -.VS 8.5 With the \fB\-level 0\fR option, \fBreturn\fR itself can serve -as a replacement for \fBbreak\fR. +as a replacement for \fBbreak\fR, with the help of \fBinterp alias\fR. +.PP .CS interp alias {} Break {} \fBreturn\fR -level 0 -code break .CE .PP An example of using \fBcatch\fR and \fBreturn -options\fR to re-raise a caught error: +.PP .CS proc doSomething {} { - set resource [allocate] - catch { - # Long script of operations - # that might raise an error - } result options - deallocate $resource - \fBreturn\fR -options $options $result + set resource [allocate] + catch { + # Long script of operations + # that might raise an error + } result options + deallocate $resource + \fBreturn\fR -options $options $result } .CE .PP Finally an example of advanced use of the \fBreturn\fR options to create a procedure replacement for \fBreturn\fR itself: +.PP .CS proc myReturn {args} { - set result "" - if {[llength $args] % 2} { - set result [lindex $args end] - set args [lrange $args 0 end-1] - } - set options [dict merge {-level 1} $args] - dict incr options -level - \fBreturn\fR -options $options $result + set result "" + if {[llength $args] % 2} { + set result [lindex $args end] + set args [lrange $args 0 end-1] + } + set options [dict merge {-level 1} $args] + dict incr options -level + \fBreturn\fR -options $options $result } .CE -.VE 8.5 .SH "SEE ALSO" -break(n), catch(n), continue(n), dict(n), error(n), proc(n), source(n), tclvars(n) +break(n), catch(n), continue(n), dict(n), error(n), proc(n), +source(n), tclvars(n), throw(n), try(n) .SH KEYWORDS -break, catch, continue, error, procedure, return +break, catch, continue, error, exception, procedure, result, return +.\" Local Variables: +.\" mode: nroff +.\" End: |