Version 3 of C-like syntax for numbers

Updated 2007-02-05 04:59:56

slebetman: This implements what I'd call C-like syntax for numbers in Tcl. Basically, this means that not only can you use infix assignment like x = $y * 2 but you also need to declare the variables before using them. There are other implementations of this available somewhere in this wiki, mostly by Richard Suchenwirth. Prompted by GWM I thought I'd dig up my very simple implementation which have been floating around usenet (google "syntax expressiveness"). Anyway, here's the code:

    proc cleanupVar {name1 name2 op} {
        if {![uplevel 1 [list info exists $name1]]} {
            rename $name1 {}
        }
    }

    proc var {name {= =} args} {
        upvar 1 $name x
        if {[llength $args]} {
            set x [expr $args]
        } else {
            set x {}
        }
        proc $name args [subst -nocommands {
            upvar 1 $name $name
            if {[llength \$args]} {
                set $name [expr [lrange \$args 1 end]]
            } else {
                return $$name
            }
        }]
        uplevel 1 [list trace add variable $name unset cleanupVar]
    }

The following is an example of how to use var:

    proc test {} {
        var x
        var y = 10

        x = $y*2

        return $x
    }
    puts [test]

Another feature is that the variables actually exists in local scope even though their associated commands exists in global scope. This means that the variables can be used recursively:

    proc recursiveTest {x} {
        var y = $x - 1

        if {$y > 0} {
            recursiveTest $y
        }
        puts $y
    }
    recursiveTest 10

should output the numbers 0 to 9. Another test:

    proc test2 {} {
        var x = 10
        puts "this x belongs to test2 = $x"
    }

    proc test3 {} {
        var x = 100
        test2
        puts "this x belongs to test3 = $x"
    }

    test3

output:

  this x belongs to test2 = 10
  this x belongs to test3 = 100

Larry Smith All of this would be so much tidier if it were possible to declare local procs.

See also infix, Gadgets, Radical Language Modification and Let unknown know

escargo 4 Feb 2007 - Might it make more sense for the default value of a declare var to be 0 (or, since these are supposed to be reals, 0.0)?

slebetman: I prefer a default value that is not a number. If you want to initialise it to 0 then declare it as:

  var x = 0

Besides, these aren't supposed to be reals. They're regular Tcl variables with expr built-in. It's important to remember that because:

  var x = 1 / 2

is not 0.5 but 0.

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Category Mathematics