[Haskell-cafe] Re: [off-topic] OOPer?

On 11/17/08 18:24, Daniel Yokomizo wrote:

On Mon, Nov 17, 2008 at 9:49 PM, Maurí­cio mailto:briqueabraque@yahoo.com> wrote:

> (...) 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.

[snip]

> -- Arithmetic expression forms data Expr = Num Int | Add Expr Expr > > -- Evaluate expressions > eval :: Expr -> Int > (...)

> public abstract class Expr { > public abstract int eval (); > public abstract void modn(int v);

[snip]

when the first standard was set, and I can tell you for sure

that, even

at that time, no one who knows at least the basics of C++

would ever

write that problem like this.

Mauri, I'm not sure what you mean. Do you mean: 1) No C++er would ever "structure" the problem like: -- 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 If so, then I'm unsure what you could mean since the closest counterpart to: date Expr = Num Int | Add Expr Expr in c++ is an abstract Expr class with derived classes, Int and Add, just as shown in Greg's Java counterpart to the haskell Expr. 2) No C++er would every solve the problem with a heirarchy of Expr classes with virtual functions. If so, then I'm really confused because that's exactly the way I would do it *except* if I wanted to avoid the overhead of virtual function dispatch. In this case, I would use template metaprogramming (WARNING: not for c++ template metaprogramming novices): http://www.boost.org/doc/libs/1_37_0/doc/html/proto/users_guide.html#boost_p... In the proto metaprogramming, AFAICT, the 1st element of the proto::expr template, the tag, corresponds to Expr constructor's, Num and Add, of Greg's haskell example code. The | separating the Expr constructor variants corresponds to the proto::or_ template. So, if template metaprogramming were used, then there are some very good c++ programmer which would structure their c++ code like the haskell code (although, as seen by the #boost_proto.users_guide.intermediate_form.expression_introspection.defining_dsel_grammars reference, it's "a bit obscured" by all the "scaffolding" needed to make it work). Another reference which *may* reflect the haskell structure is: http://research.microsoft.com/~akenn/generics/gadtoop.pdf I must admit I don't really understand it, but it seems to have some similarities to haskell's structure. The author even uses haskell code to compare with his corresponding extension to c#. In particular, page 9 contains an example use of his proposed switch statement extension which looks very similar to the way haskell would pattern match on an expression to dispatch to the appropriate case. [snip]