# example.tcl --
#     Example of using the proposed command s-expr
#

# s-expr --
#     Mock up version of the proposed command
#
# Arguments:
#     expression     An - in principle - arbitrary arithmetic expression
#
# Result:
#     A prefix version of the expression.
#     In this mock-up just "+ $x $y"
#
proc s-expr {expression} {
    return [list {+ $x $y} x y]
}

# complex --
#     Namespace defining complex number operations
#
namespace eval ::complex {
    variable expressions
}

# number --
#     Construct a complex number
#
# Arguments:
#     real        Real part
#     imaginary   Imaginary part
#
proc ::complex::number {real imaginary} {
    return [list $real $imaginary]
}

# + --
#     Complex addition
#
# Arguments:
#     z1       First argument
#     z2       Second argument
#
# Result:
#     Complex sum of the two arguments
#
proc ::complex::+ {z1 z2} {

    lassign $z1 x1 y1
    lassign $z2 x2 y2

    return [list [expr {$x1+$x2}] [expr {$y1+$y2}]]
}

# cexpr -- 
#     Evaluate complex arithmetic expressions
#
# Arguments:
#     expression     Arbitrary arithmetic expression
#
# Result:
#     Value of the expression
#
proc ::complex::cexpr {expression} {
    variable expressions
        
    #
    # Construct a new procedure if the expression has not been handled before
    #

    if { ![info exists expressions($expression)] } {
        set parsed [s-expr $expression]

        set prefix [lindex $parsed 0]
        set vars   [lrange $parsed 1 end]

        set expressions($expression) 1

        set upvars {}
        foreach var $vars {
            append upvars "upvar 2 $var $var\n"
        }

        proc $expression {} "$upvars$prefix"
     }

     $expression
}

# main --
#     Test this example
#
set x [::complex::number 0 1]
set y [::complex::number 1 0]

puts "Sum of i and 1: "
puts [::complex::cexpr {$x + $y}]