retard wrote:
Tue, 22 Dec 2009 14:51:55 -0600, Andrei Alexandrescu wrote:
retard wrote:
I use the Okasaki book as my bible when talking about purely functional
data structures.
It's an awesome book. It's also very frank and tells things as they are,
as opposed to others (*cough*) that patronize the reader by artificially
making it seem so easy, it's amazing how Stoopid People missed the
point. Un-incidentally, Okasaki does not dabble in qsort. He does
describe mergesort - a perfectly sound and advantageous sorting methods
for lists.
Ok, I'm itching to share another gem.
"At this point you might be wondering how it's possible to program
without variables. How can you express something like X = X + 1 in
<language erased to protect the guilty>? The answer is easy. Invent a
new variable whose name hasn't been used before (say X1), and write X1 =
X + 1".
That's a nasty limitation indeed. From my point of view imperative
languages just pass around the global state with the value of the
computation, and the assignment (X=X+1) shadows the previous value of X.
In some cases this allows optimizing compilers to perform the assignment
in-place! :o) In fact, this is exactly what happens in another pure
functional language (name erased to protect the awesomeness):
foo x
# x = x + 1
x = x * 2
x = x - 5
= x
vs
T foo(T)(T x) {
x = x + 1;
x = x * 2;
x = x - 5;
return x;
}
Hm, I guess I should have been more explicit. The problem isn't defining
a few more names, but that the explanation completely neglects any
control flow issues (e.g. conditionals and loops). You can't define a
new name for each instance of a loop, so everything is recursion etc. I
mean the consequences are earth-shattering. There is absolutely no doubt
the author is aware of the fact that single assignment is a lot more
than just defining a few names, so I find it very questionable that he
chose to put things that way. And the book has much more allegations
like that one!
Imagine you'd find this in a book for learning French: "Translating from
English to French is very easy - just replace English words with the
corresponding French words."
Andrei
