Purpose: discuss general guidelines for gathering input and
then making use of it in Tcl.



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A new user often finds s/he has a problem when dealing with user
input.

Sometimes, the problem is a file name with a space in it.
Sometimes, the problem is a string with special characters like [[ or $ .

On this page, please add notes to help a new programmer avoid
these problems.


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One important thing is to keep the ''type'' of the input string in mind as you code.
While Tcl has the reputation of '''everything is a string''', not every command expects all its arguments to be arbitrary strings.
In the most obvious example, Tcl input or output commands which
require a file handle expects that argument to be a
string returned from an open type command.  Just handing some arbitrary
string is very likely not going to work.

Similarly, handing an arbitrary string to a command which expects an
argument in a [list] format may very well not work either:

 set s "this is a test of [ ; and ]"
 puts [llength $s]

does not result in a number, but instead with the error:

 invalid command name "and"
    while executing
 "and "
    invoked from within
 "set s "this is a test of [ ; and ]""
    (file "/tmp/s1.tcl" line 3)

If in the above example, s was the result of a [gets] or [read] Tcl command,
the developer could find the same error arising from arbitrary user input.

The reason is that [llength]'s interface is documented as requiring a list
as its argument - the input above isn't a properly quoted list. 
 
[DGP] - uhhh... no.  Look at the stack trace above.  We never
reach the [llength] command because the error occurs during
command substitution of the arguments to [set].  This is a
simple error in Tcl syntax having nothing to do with list/string issues.


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