**More command construction - format, list**


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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***

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
======

<<discussion>> Resulting output
======none
OK
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)
======
<<enddiscussion>>

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"
}
======

<<discussion>> Resulting output
======none
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!
======
<<enddiscussion>>
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