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[Caml-list] C++ STL and template features compared with OCaml parametric polymorphism and OO features
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Date: 2004-09-26 (13:10)
From: Jon Harrop <jon@j...>
Subject: Re: [Caml-list] C++ STL and template features compared with OCaml parametric polymorphism and OO features
On Sunday 26 September 2004 06:31, Radu Grigore wrote:
> The resulting C++ code is still uglyer than its OCaml counterpart. And
> longer. But you should notice that the same accumulate function is
> used for three different containers, while in OCaml you use one
> function for each container type.

My "sum" function was used for all container types. Indeed, that was the 
point... :-)

If you don't want to supply the container-specific fold at compile-time (e.g. 
with "sum List.fold_left ...") then you can use some form of RTTI, most 
obviously objects:

let sum c = c#fold_left ( +. ) 0. c

But, essentially, this is a problem of factoring code. We wish to factor the 
commonality of operations over different types of container. Iterators are 
only one way to do this and this approach is only suitable in the absence of 
HOFs (IMHO) and in the presence of imperative style. If you have HOFs, you 
just write my "sum" function and get on with something more interesting.

> OTOH, something that is maybe less 
> obvious in the code above, I do use conainer specific functions in
> C++: namely begin and end.

Also, note that the C++ version uses "accumulate" which is effectively 
equivalent to "folding with addition". In OCaml, you can fold using any 
function you want, or write a higher-order function which folds over a given 
function. This is possible but, IMHO, not practicable in C++.

> So, in short: In OCaml you don't have iterators so the fold method is
> container specific. In C++ you have iterators that allow a
> container-independent "fold-like" function to be implemented. The C++
> program is more verbose.

My example is also borderline. Virtually anything more complicated that this 
will widen the gap between C++ and OCaml. Consider computing the variance of 
a given container of integers using rational arithmetic, or computing the 
"n"th nearest neighbours in a graph, or writing a function minimisation 
algorithm, the maximum entropy method...

> Using C++'s <functional> header (or Boost.Lambda for that matter) is
> sure to give you a headache after programming a bit in a functional
> language like OCaml. But the same can be said about writting
> imperative code in OCaml.

I'd contest that. I use mutables for things like testing that some operation 
was done at least once by an otherwise functional function. I haven't had any 

> I have recently compared two implementations of the same small program
> in C++ and OCaml, both written by me. The OCaml one was 45% the size
> of the C++ one (byte count). After compression (with bzip2) it was
> 67%. And it was kind of imperative job so C++ should have been in
> advantage there. And I know C++ much better than OCaml, so this should
> have been another advantage..

I'm finding that OCaml code is much more concise than that. My 
imperative-style vector graphics engine in about 1/4 the size than the C++ 
version and includes much more functionality. My other work simply cannot 
feasibly be written in C/C++ due to the incidental complexity of lexing, 
parsing and interpreting in such languages.

Also, for any >1 month programming tasks, my OCaml code is _always_ faster 
than my C++ equivalents were.


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