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Avoiding shared data
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Date: -- (:)
From: Brian Hurt <bhurt@s...>
Subject: Re: FP/IP and performance (in general) and Patterns... (Re: [Caml-list] Avoiding shared data)

On Tue, 4 Oct 2005, Oliver Bandel wrote:

> Most often people say, imperative is faster in most of all
> cases, and only in some cases FP might be faster.

The general argument here is that if the FP solution is faster, the 
imperitive language can just implement the FP solution.  My answer to that 
is that imperitive programming languages discourage you from thinking in 
certain ways rather strongly- while the FP solution may be faster, it's 
not "natural".

The big advantage of FP programming IMHO is not performance, but instead 
*correctness*.  With today's multi-gigahertz machines with multi-gigabytes 
of memory, performance isn't as critical as it used to be.  But 
correctness- especially automatically gaurenteed correctness on projects 
spanning hundreds of thousands of lines of code and dozens of developers 
maintained over decades of time- starts becoming critical.  I'd quite 
happily trade off 10% performance for correctness, or even 50% 

FP is a huge gain in correctness, because it allows me to *control 
mutability*.  If I pass a mutable data structure to a block of code there 
is an instant implicit contract between the caller and the callee on how 
(or wether) to modify the mutable data structure.  It doesn't matter what 
the contract is- wether it's to not modify the structure at all, to allow 
optional modification (either unlimited or only in certain ways), or to 
require certain modifications- a dependency between the two different 
peices of code exists.  And this dependency, this contract, is probably 
undocumented and always unchecked by the compiler, which means it's a 
maintaince nightmare waiting to happen.  One peice of code gets modified 
to violate the contract, perhaps even unknowingly, or perhaps due to some 
changing requirement, and untouched code thousands of lines away suddenly 

Note that such a bidirectional dependency can be implemented in functional 
code- the called code returns a new copy of the modified structure to the 
calling code, which the calling code then adopts and uses.  But notice 
that such a bidirectional dependency is explicitly stated in the code, and 
automatically checked by the compiler.  And the calling code has the 
option of wether or not to allow the modification (or to use both the old 
and the new copy).

Java programmers are begining to tumble to this advantage- notice the 
increasing popularity of immutable objects (starting with Int and Float), 
and Java Beans.

> I recently had a discussion about some OO-patterns
> ( so called "GoF"-Book) and tried to solve some of them with
> OCamls module system.
> Adapter and bridge was talked about. Adapter was easy in writing a simple
> wrapper.
> Bridge could be done with a functor. :)

Some of the patterns are doable with modules.  Singleton, for example, is 
a classic module pattern.  Many of the patterns aren't, however.  An 
important comment here is that modules are NOT weird and broken objects. 
A better point of comparison is C++ templates, except that 95+% of the use 
of templates in C++ is to implement universal types, i.e. a C++ programmer 
writes "list<A>" where an Ocaml programmer would write "'a list".

> Another interesting theme in this respect is: If not only some patterns
> were adapted to Fp, but the whole problem solving approach is different,
> which patterns will be unnevcessary then, because the whole structure of
> the software will be different...?!

There are patterns in every programming idiom.  The GoF book was just 
Object Oriented patterns- but there are procedural patterns, and 
functional patterns, and logic patterns, etc.