This is how I think of it:
lazyIntMult :: Int -> Int -> Int
lazyIntMult 0 _ = 0
lazyIntMult _ 0 = 0
lazyIntMult a b = a * b
*Q> 0 * (5 `div` 0)
*** Exception: divide by zero
*Q> 0 `lazyIntMult` (5 `div` 0)
0
foldr evaluates a `f` (b `f` (c `f` ...))
Only f knows which arguments are strict and in which order to evaluate
them. foldr knows nothing about evaluation order.
Dan
Michael T. Richter wrote:
So, I'm now going over the code in the 'Report with a fine-toothed comb
because a) I'm actually able to read it now pretty fluently and b) I
want to know what's there in detail for a project I'm starting. I
stumbled across this code:
any :: (a -> Bool) -> [a] -> Bool
any p = or . map p
or :: [Bool] -> Bool
or = foldr (||) False
Now I see how this works and it's all elegant and clear and all that.
But I have two nagging problems with it (that are likely related):
1. Using foldr means I'll be traversing the whole list no matter
what. This implies (perhaps for a good reason) that it can only
work on a finite list.
2. I don't see any early bale-out semantics. The way I read this
it's going to expand a whole list of n and perform n comparisons
(including the one with the provided False).
Considering that I only need a single True result to make the whole
expression true, I'd have expected there to be some clever semantics to
allow exactly this. But what I'm seeing, unless I'm really misreading
the code, is that if I give it a list of a million boolean expressions,
it will cheerfully evaluate these million boolean expressions and
perform a million calls to (||) before giving me a result.
Please tell me I'm wrong and that I'm missing something?
--
*Michael T. Richter* <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> (*GoogleTalk:* [EMAIL PROTECTED])
/There are two ways of constructing a software design. One way is to
make it so simple that there are obviously no deficiencies. And the
other way is to make it so complicated that there are no obvious
deficiencies. (Charles Hoare)/
------------------------------------------------------------------------
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
