Tcl Tutorial Lesson 33

More command construction - format, list

There may be some unexpected results when you try to compose command strings for eval.

For instance:

eval puts OK

would print the string OK. However,

eval puts Not OK

will generate an error.

The reason that the second command generates an error is that the eval uses concat to merge its arguments into a command string. This causes the two words Not OK to be treated as two arguments to puts. If there is more than one argument to puts, the first argument must be a file pointer.

Correct ways to write the second command include these:

eval [list puts {Not OK}]
eval [list puts "Not OK"]
set cmd "puts" ; lappend cmd {Not OK}; eval $cmd

As long as you keep track of how the arguments you present to eval will be grouped, you can use many methods of creating the strings for eval, including the string commands and format.

The recommended methods of constructing commands for eval is to use the list and lappend commands. These commands become difficult to use, however if you need to put braces in the command, as was done previously.

The earlier example is re-implemented in the example code using lappend.

The completeness of a command can be checked with info complete. info complete can also be used in an interactive program to determine if the line being typed in is a complete command, or the user just entered a newline to format the command better.

info complete string
If string has no unmatched brackets, braces or parentheses, then a value of 1 is returned, else 0 is returned.


Example, part 1 (will produce an error):

set cmd "OK"
eval puts $cmd

set cmd "puts" ; lappend cmd {Also OK}; eval $cmd

set cmd "NOT OK"
eval puts $cmd

  Resulting output
Also OK
can not find channel named "NOT"
    while executing
"puts NOT OK"
    ("eval" body line 1)
    invoked from within
"eval puts $cmd"
    (file "xx.tcl" line 7)

Example, part 2:

eval [format {%s "%s"} puts "Even This Works"]

set cmd "And even this can be made to work"

eval [format {%s "%s"} puts $cmd ]

set tmpFileNum 0;

set cmd {proc tempFileName }
lappend cmd ""
lappend cmd "global num; incr num
lappend cmd "return \"/tmp/TMP.[pid].\$num\""
eval  $cmd

puts "\nThis is the body of the proc definition:"
puts "[info body tempFileName]\n"

set cmd {puts "This is Cool!}

if {[info complete $cmd]} {
    eval $cmd
} else {
    puts "INCOMPLETE COMMAND: $cmd"

  Resulting output
Even This Works
And even this can be made to work

This is the body of the proc definition:
global num; incr num
return "/tmp/TMP.17520.$num"

INCOMPLETE COMMAND: puts "This is Cool!