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cody8411_186897's avatar
cody8411_186897
Icon for Nimbostratus rankNimbostratus
Jan 25, 2016

Undefined Procedure with elseif when using comments

I am having trouble committing the following iRule when adding in comments above the elseif statements. It always interprets the first comment it finds above an elseif statement as the offender. 0...
application delivery
iRules
LTM
syntax error
  • Vernon_97235's avatar
    Vernon_97235
    Jan 25, 2016

    This results from a peculiarity of Tcl. The language doesn't really have built-ins and it doesn't really have blocks. Rather, it has commands and non-interpolated quoted strings. As such, 

    if
    is a command (just like 
    split
    or 
    HTTP::headers
    ). It is followed by a quoted block (the {}s, in Tcl, are really non-interpolating quoting operators, and not really block delimiters as they are in most languages). The quoted block may then be optionally followed by zero or more 
    elseif
    literals (each followed by a quoted block) and zero or one 
    else
    literals (followed by a quoted block). When a condition is met, the contents of the corresponding quoted block are evaluated.

    Said another way, in Tcl, an 

    if
    /
    elseif
    /
    else
    set is really a single statement. The statement command is 
    if
    . The 
    elseif
    and 
    else
    are just parameters to this command. And because the newlines in the blocks are ignored by the parser (because they are inside of the non-interpolating {} quoting operators), the entire statement is effectively on one line.

    As a side-effect of this, in Tcl you MUST cuddle an 

    if
    /
    elseif
    /
    else
    block, as in:

    if { ... } {
   ...
} else {
   ...
}

    This is not allowed:

    if { ... } {
   ...
}
else {
   ...
}

    However, this trips up most programmers, so iRules (a Tcl dialect) allows non-cuddled 

    if
    blocks. But if you add a comment between the control elements, this effectively inserts a newline. So, with the comments you have above, the parser "sees" this:

    if { ... } 
elseif { ... } 
else { ... }

    which it thinks is really three different commands: 

    if
    , 
    elseif
    and 
    else
    . Only 
    if
    is a command; the others are not (again, they are just parameters of the 
    if
    command).

    Okay, so that's really just a long-winded way of saying: you must move those comments inside the squirly braces ({}). 🙂

Vernon_97235's avatar
Vernon_97235
Historic F5 Account

This results from a peculiarity of Tcl. The language doesn't really have built-ins and it doesn't really have blocks. Rather, it has commands and non-interpolated quoted strings. As such, 

if
is a command (just like 
split
or 
HTTP::headers
). It is followed by a quoted block (the {}s, in Tcl, are really non-interpolating quoting operators, and not really block delimiters as they are in most languages). The quoted block may then be optionally followed by zero or more 
elseif
literals (each followed by a quoted block) and zero or one 
else
literals (followed by a quoted block). When a condition is met, the contents of the corresponding quoted block are evaluated.

Said another way, in Tcl, an 

if
/
elseif
/
else
set is really a single statement. The statement command is 
if
. The 
elseif
and 
else
are just parameters to this command. And because the newlines in the blocks are ignored by the parser (because they are inside of the non-interpolating {} quoting operators), the entire statement is effectively on one line.

As a side-effect of this, in Tcl you MUST cuddle an 

if
/
elseif
/
else
block, as in:

if { ... } {
   ...
} else {
   ...
}

This is not allowed:

if { ... } {
   ...
}
else {
   ...
}

However, this trips up most programmers, so iRules (a Tcl dialect) allows non-cuddled 

if
blocks. But if you add a comment between the control elements, this effectively inserts a newline. So, with the comments you have above, the parser "sees" this:

if { ... } 
elseif { ... } 
else { ... }

which it thinks is really three different commands: 

if
, 
elseif
and 
else
. Only 
if
is a command; the others are not (again, they are just parameters of the 
if
command).

Okay, so that's really just a long-winded way of saying: you must move those comments inside the squirly braces ({}). 🙂

cody8411_186897's avatar
cody8411_186897
Icon for Nimbostratus rankNimbostratus
Jan 25, 2016
Excellent explanation. That definitely makes sense now that I understand how the code is interpreted in Tcl. Moved the comments down inside the commands and worked as expected, thank you!