The EVAL command

It's more that eval and {*} do different things. Eval takes a string and executes it as a command. {*} is used to expand a Tcl list into separate arguments ("stringifying" it, essentially), typically for use in calling a command.

For example, if you had a variable called $headers which was a Tcl list of header names and values, which looked like:

{ Set-Cookie "foo=bar" Set-Cookie "baz=quux" X-Server "BIG-IP iRule" }

you might want to do something like:

HTTP::respond 302 Location http://new.example.com $headers

to craft an HTTP response with all of those headers. But that wouldn't work, because Tcl would pass in those headers as one big parameter, confusing the HTTP::respond command. With {*}, you could make it work:

HTTP::respond 302 Location http://new.example.com {*}$headers

which would be the same as:

HTTP::respond 302 Location http://new.example.com Set-Cookie "foo=bar" Set-Cookie "baz=quux" X-Server "BIG-IP iRule"

However, the {*} syntax was introduced in Tcl 8.5, and BIG-IP is still using Tcl 8.4. So you need to use eval as a workaround:

eval "HTTP::respond 302 Location http://new.example.com $headers"

This will expand the $headers variable as a string, and parse the contents as separate arguments to the HTTP::respond command.

So eval is an old command, but no more than the rest of Tcl. 🙂