[Richard Suchenwirth] 2006-01-25 - Variables are popular parts of many programming languages, and most coders have no resistance to write something like
   i = i+1
which would cause mathematicians to frown. 

In [functional programming], variables are less popular. In [Functional Programming (Backus 1977)] it was even tried to do almost totally without them, by just composing functions instead of data objects. [Forth] and similar [RPN] systems also mostly do without variables: there, the stack is the big thing that varies all the time :^)

Here's experiments with a Tcl in which we don't have variables in the sense that they can vary. To demonstrate that, let's first of all get rid of the [set] command:
 rename set {}
(Seriously, almost all the commands which cause side-effects by changing values, as discussed in [Dangers of creative writing], should be discarded. But this is just experimental...

We still can associate names with values, by using argument-less procs:
 proc pi {} {return 3.141}
and due to Tcl's dynamics, we still can change such associations too
 proc pi {} {return 3}
but let's resolve we won't. Procs as constants are also global, so we don't need to import them into function bodies.

For nicer looks (the idea was borrowed from [Scheme]), let's also have just one unified definer which handles both argument-less and argumented functions:

 proc def args {
     if {[lindex $args 1] eq "="} {
	 proc [lindex $args 0] {} [list return [lindex $args 2]]
     } else {
	 proc [lindex $args 0] [lindex $args 1] [lindex $args 3]
     }
 }

What we also still have is local variables of functions. We can read their value with the ''$name'' notation, but we can't vary their value any more (if we stick to our resolution). Let's rather call them "parameters".

Now experimenting:
 def a = 6
 def b = 7
 def mul {a b} = {expr {$a * $b}}
 
 puts [mul [a] [b]]
 puts [mul 111 6]

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[Arts and crafts of Tcl-Tk programming]