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]
}

main

Answer 2

not sure how well metapost (1) would work for your application, but it is declarative.

Answer 3

Wanting to have two definitions of X is inherently imperative. In a truly declarative language you have a single definition of a variable in a single scope. The behavior you want from Excel corresponds to editing the program.

Answer 4

JavaFX will do that for you if you use bind instead of = for Z

Answer 5

Groovy and the magic of closures.

def (x, y) = [ 10, 20 ]

def z = { x + y }

assert 30 == z()

x = 50

assert 70 == z()

def f = { n -> n + 1 }  // define another closure

def g = { x + f(x) }    // ref that closure in another

assert 101 == g()       // x=50, x + (x + 1)

f = { n -> n + 5 }     // redefine f()

assert 105 == g()      // x=50, x + (x + 5)

It's possible to add automagic memoization to functions too but it's a lot more complex than just one or two lines. http://blog.dinkla.net/?p=10

Answer 6

Two things you can look at are the cells lisp library, and the Modelica dynamic modelling language, both of which have relation/equation capabilities.