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eval(3tcl) [debian man page]

eval(3tcl)						       Tcl Built-In Commands							eval(3tcl)

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NAME
eval - Evaluate a Tcl script SYNOPSIS
eval arg ?arg ...? _________________________________________________________________ DESCRIPTION
Eval takes one or more arguments, which together comprise a Tcl script containing one or more commands. Eval concatenates all its argu- ments in the same fashion as the concat command, passes the concatenated string to the Tcl interpreter recursively, and returns the result of that evaluation (or any error generated by it). Note that the list command quotes sequences of words in such a way that they are not further expanded by the eval command. EXAMPLES
Often, it is useful to store a fragment of a script in a variable and execute it later on with extra values appended. This technique is used in a number of places throughout the Tcl core (e.g. in fcopy, lsort and trace command callbacks). This example shows how to do this using core Tcl commands: set script { puts "logging now" lappend $myCurrentLogVar } set myCurrentLogVar log1 # Set up a switch of logging variable part way through! after 20000 set myCurrentLogVar log2 for {set i 0} {$i<10} {incr i} { # Introduce a random delay after [expr {int(5000 * rand())}] update ;# Check for the asynch log switch eval $script $i [clock clicks] } Note that in the most common case (where the script fragment is actually just a list of words forming a command prefix), it is better to | use {*}$script when doing this sort of invocation pattern. It is less general than the eval command, and hence easier to make robust in | practice. The following procedure acts in a way that is analogous to the lappend command, except it inserts the argument values at the start of the list in the variable: proc lprepend {varName args} { upvar 1 $varName var # Ensure that the variable exists and contains a list lappend var # Now we insert all the arguments in one go set var [eval [list linsert $var 0] $args] } However, the last line would now normally be written without eval, like this: | set var [linsert $var 0 {*}$args] | SEE ALSO
catch(3tcl), concat(3tcl), error(3tcl), interp(3tcl), list(3tcl), namespace(3tcl), subst(3tcl), tclvars(3tcl), uplevel(3tcl) KEYWORDS
concatenate, evaluate, script Tcl eval(3tcl)

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uplevel(3tcl)						       Tcl Built-In Commands						     uplevel(3tcl)

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NAME
uplevel - Execute a script in a different stack frame SYNOPSIS
uplevel ?level? arg ?arg ...? _________________________________________________________________ DESCRIPTION
All of the arg arguments are concatenated as if they had been passed to concat; the result is then evaluated in the variable context indi- cated by level. Uplevel returns the result of that evaluation. If level is an integer then it gives a distance (up the procedure calling stack) to move before executing the command. If level consists of # followed by a number then the number gives an absolute level number. If level is omitted then it defaults to 1. Level cannot be defaulted if the first command argument starts with a digit or #. For example, suppose that procedure a was invoked from top-level, and that it called b, and that b called c. Suppose that c invokes the uplevel command. If level is 1 or #2 or omitted, then the command will be executed in the variable context of b. If level is 2 or #1 then the command will be executed in the variable context of a. If level is 3 or #0 then the command will be executed at top-level (only global variables will be visible). The uplevel command causes the invoking procedure to disappear from the procedure calling stack while the command is being executed. In the above example, suppose c invokes the command uplevel 1 {set x 43; d} where d is another Tcl procedure. The set command will modify the variable x in b's context, and d will execute at level 3, as if called from b. If it in turn executes the command uplevel {set x 42} then the set command will modify the same variable x in b's context: the procedure c does not appear to be on the call stack when d is executing. The info level command may be used to obtain the level of the current procedure. Uplevel makes it possible to implement new control constructs as Tcl procedures (for example, uplevel could be used to implement the while construct as a Tcl procedure). The namespace eval and apply commands offer other ways (besides procedure calls) that the Tcl naming context can change. They add a call frame to the stack to represent the namespace context. This means each namespace eval command counts as another call level for uplevel and upvar commands. For example, info level 1 will return a list describing a command that is either the outermost procedure call or the out- ermost namespace eval command. Also, uplevel #0 evaluates a script at top-level in the outermost namespace (the global namespace). EXAMPLE
As stated above, the uplevel command is useful for creating new control constructs. This example shows how (without error handling) it can be used to create a do command that is the counterpart of while except for always performing the test after running the loop body: proc do {body while condition} { if {$while ne "while"} { error "required word missing" } set conditionCmd [list expr $condition] while {1} { uplevel 1 $body if {![uplevel 1 $conditionCmd]} { break } } } SEE ALSO
apply(3tcl), namespace(3tcl), upvar(3tcl) KEYWORDS
context, level, namespace, stack frame, variables Tcl uplevel(3tcl)
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