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

Have you seen Resolver One? It's like Excel with a real programming language behind it.

Answer 2

Here is Daniel's example in Python, since I noticed you said you tried it in Python.

x = 10
y = 10

z = lambda: x + y

# Output: 20
print z()

x = 20

# Output: 30
print z()

Answer 3

It's not what you're looking for, but Hardware Description Languages are, by definition, "declarative".

Answer 4

This F# code should do the trick. You can use lazy evaluation (System.Lazy object) to ensure your expression will be evaluated when actually needed, not sooner.

let mutable x = 10;
let y = 20;

let z = lazy (x + y);
x <- 30;

printf "%d" z.Value

Answer 5

It looks like you just want to make Z store a function instead of a value. In C#:

var X = 10;    // define and assign two variables
var Y = 20;

Func<int> Z = () => X + Y;  // declare a formula that uses these two variables

Console.WriteLine(Z());

X = 50;     // change one of the input variables

Console.WriteLine(Z());

So the equivalent of your ?-prefix syntax is a ()-suffix, but otherwise it's identical. A lambda is a "formula" in your terminology.

Behind the scenes, the C# compiler builds almost exactly what you presented in your C# conceptual example: it makes X into a field in a compiler-generated class, and allocates an instance of that class when the code block is entered. So congratulations, you have re-discovered lambdas! :)

Answer 6

There is a Lisp library with this sort of behaviour:

http://common-lisp.net/project/cells/