Truly declarative language?

Question

Does anyone know of a truly declarative language? The behavior I\'m looking for is kind of what Excel does, where I can define variables and formulas, and have the formula\'s re

Answer 1

You can do this in Tcl, somewhat. In tcl you can set a trace on a variable such that whenever it is accessed a procedure can be invoked. That procedure can recalculate the value on the fly.

Following is a working example that does more or less what you ask:

proc main {} {
    set x 10
    set y 20
    define z {$x + $y}

    puts "z (x=$x): $z"
    set x 50
    puts "z (x=$x): $z"
}


proc define {name formula} {
    global cache
    set cache($name) $formula
    uplevel trace add variable $name read compute
}

proc compute {name _ op} {
    global cache
    upvar $name var
    if {[info exists cache($name)]} {
        set expr $cache($name)
    } else {
        set expr $var
    }
    set var [uplevel expr $expr]
}

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