Brandon S. Allbery KF8NH wrote:
On May 27, 2007, at 17:23 , Andrew Coppin wrote:
Personally, I try to avoid ever using list comprehensions. But every
now and then I discover an expression which is apparently not
expressible without them - which is odd, considering they're only
"sugar"...
They are. But they're sugar for monadic operations in the list monad,
so you have to use (>>=) and company to desugar them. [x | filter
even x, x <- [1..10]] becomes do { x <- [1..10]; return (filter even
x) } becomes ([1..10] >>= return . filter even).
(Aren't you glad you asked?)
Mmm... LOL!
Actually, I recently read something in a book called The Fun of
Programming. It basically explains how to implement Prolog in Haskell,
including presenting an extremely inefficient way to factorise integers:
factors n = do
x <- [1..100]
y <- [1..100]
if x * y == n
then return (x,y)
else []
I should point out that desugaring do-blocks is possibly one of the
hardest things for a Haskell newbie to figure out, but I think I've
mastered it now. What we actually have above is
factors n =
[1..100] >>= \x ->
[1..100] >>= \y ->
if x * y == n
then [(x,y)]
else []
which makes it quite clear why "x <- [1..100]" actually causes x to
range over all the numbers in the list. (That's the great thing about
monads - sometimes it can be hard to wrap your mind around what they
actually do when used in nontrivial ways...)
Of course, the book explains how to write a new monad that allows
infinite lists to be used, and yet still find all factors in finite
time. (This involves the Evil ******* Function From Hell I mentioned a
while ago.) And then they go on to explain how to implement unification
- something I would never have worked out in a million years. And after
reading the whole chapter, finally I understand how it is possible for
Prolog to exist, even though computers aren't intelligent...
Then again, later on in the very same book there's a chapter entitled
"Fun with Phantom Types", which made precisely no sense at all...
(I find this a lot with Haskell. There is stuff that is clearly written,
fairly easily comprehensible, and extremely interesting. And then
there's stuff that no matter how many times you read it, it just makes
no sense at all. I'm not sure exactly why that is.)
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