On 02/14/2012 12:34 AM, James Miller wrote:
On 14 February 2012 06:25, Timon Gehr<timon.g...@gmx.ch> wrote:
On 02/13/2012 03:19 PM, James Miller wrote:
On 11 February 2012 10:45, Jonathan M Davis<jmdavisp...@gmx.com> wrote:
On Friday, February 10, 2012 13:32:56 Marco Leise wrote:
I know that feeling. I had no exposure to functional programming and
options like chain never come to my head. Although "map" is a concept
that
I made friends with early.
It would benefit your programming in general to learn a functional
programming
language and become reasonably proficient in it, even if you don't intend
to
program in it normally. It'll increase the number of tools in your
programming
toolbox and improve your programming in other programming languages. It's
something that not enough programmers get sufficient exposure to IMHO.
- Jonathan M Davis
I found that learning Haskell made me significantly better at what I
do. New paradigms are good for reminding you to think outside the box,
I also learnt Prolog for a university course (AI) and that was an
interesting challenge. Logical programming, where you define the
boundaries of the program and then it works out the possible answers
for you, amazingly useful for BNF grammars and similar constructs.
If fact it's got to the point where I feel hamstrung if I can't do at
least function passing (fortunately C, C++ and D can do this), and I
prefer to work with languages that support closures and anonymous
functions, since you can do wonders with simple constructs like map,
fold (reduce) and filter. In fact a naive implementation of quicksort
can be done succinctly in any language that supports filter.
T[] sort(T)(T[] array) {
pivot = array[array.length/2];
return sort(filter!("a< "~pivot)(array)~pivot~sort(filter!("a
"~pivot)(array));
}
(Disclaimer, this is probably a very slow implementation, possibly
very broken, may cause compiler demons to possess your computer, DO
NOT USE!)
I have left out some details for brevity, and it probably won't work
in alot of situations, but it demonstrates the power of functional
programming, quicksort in 4 lines (sort of, its not like Haskell's
"quicksort in 2 lines" is any better mind you, its slow as balls
because of all the memory allocation it has to do).
Anyway, yay for functional programming and thread derailment.
James
If it is slow and uses an awful lot of auxiliary memory it is not quicksort
as much as it may conceptually resemble quicksort. Try to implement in-place
quicksort in Haskell. It will look like C code. Also see:
http://stackoverflow.com/questions/5268156/how-do-you-do-an-in-place-quicksort-in-haskell
It is still conceptually quicksort, the divide-and-conquer method
based on partitioning the list.
Hoare's original quicksort algorithm is more detailed than what is
sketched here. The main point is the in-place partition operation with
the two pointers approaching each other.
I wasn't writing it to show a valid
implementation (I didn't even test it, it probably doesn't compile), I
even warned of compiler demons! Its a demonstration of the
succinctness of functional techniques for certain problems, not a show
that functional languages "are teh awesum and all other langauges
suck".
The approach given does not solve the problem (it does not implement
Quicksort). Quicksort in Haskell looks like Quicksort in D, because the
algorithm depends on destructive updates. Functional techniques can be
succinct for certain problems, but implementing Quicksort is not one of
them.
Haskell is almost a pure functional language, therefore, under
normal circumstances, every change to the array will allocate a new
array,
Haskell can do destructive array updates that look like pure operations
just fine.
http://hackage.haskell.org/packages/archive/array/0.2.0.0/doc/html/Data-Array-MArray.html
this is because of the whole immutability thing that it has going on.
This is confusing the abstraction with its implementation. It is
impossible to recreate Haskell's execution semantics in D using only
immutable types.
Of course I would never use such an implementation in real
life, and Haskellers tend to avoid algorithms that do these kinds of
things, using sorts like mergesort instead.
Mostly lazy mergesort if I'm not mistaken. And they don't usually use it
to sort arrays, they sort lists. Haskell arrays ought to be sorted with
introsort if the comparison operation is cheap.
Saying "it is not quicksort as much as it may conceptually resemble
quicksort" is kinda odd, its like saying "it is not a car, as much as
it may conceptually resemble a car" because it doesn't run on petrol
or gas, but instead runs on environment destroying orphan tears.
It is more like saying "a handcart is not a car, as much as it may
conceptually resemble a car" (the engine is missing!).
