I wrote:
I don't recall where I found the following example
My apologies to Ralf Lammel and Ondrej Rypacek. Five seconds on
Google tells me I had copied that code verbatim from their paper, "The
expression lemma."
http://www.uni-koblenz.de/~laemmel/expression/long.pdf
Great paper, by the way!
-Greg
On Mon, Nov 17, 2008 at 11:00 AM, Greg Fitzgerald
Jonathan Cast wrote:
[Functional and object-oriented programming] have points of similarity, but on net the best plan is to simply never reason analogically from one to the other.
Coming from the OO world, I found it very useful to see how the same solution is modeled using different paradigms. I don't recall where I found the following example, but copied it locally as compelling evidence that the functional solution can be much clearer and shorter than the same solution modeled with objects and inheritance.
-- Arithmetic expression forms data Expr = Num Int | Add Expr Expr
-- Evaluate expressions eval :: Expr -> Int eval (Num i) = i eval (Add l r ) = eval l + eval r
-- Modify literals modulo v modn :: Expr -> Int -> Expr modn (Num i) v = Num (i 'mod' v) modn (Add l r) v = Add (modn l v) (modn r v)
public abstract class Expr { public abstract int eval (); public abstract void modn(int v); }
public class Num extends Expr { private int value; public Num(int value) { this.value = value; } public int eval () { return value; } public void modn(int v) { this.value = this.value % v; }
public class Add extends Expr { private Expr left, right; public Add(Expr left, Expr right ) { this.left = left; this.right = right; } public int eval () { return left.eval () + right.eval (); } public void modn(int v) { left.modn(v); right.modn(v); } }
-Greg