On 30/12/06, Ranjan Bagchi
I guess the consumer's really important (Didn't even occur to me, I was concentrating on how I was generating the list). I was trying to de-lazy the list, I did the following:
bs = [...] recs' = (take 1000000) breakUp bs recs = foldr seq recs' recs' print $ length recs
Would the fold be blowing the stack?
Ranjan
Is there a really good reason to de-lazy the list? That's usually a bad idea if the list is long. If your application can do any kind of stream processing at all, then it's a better idea to allow it to work with the list lazily. If it can't, and you're basically producing some kind of summary of the input data, you're probably better off using a strict left fold and consuming the list in a strict fashion, but not forcing it earlier than actually needed. By strictifying the list, you're making it impossible to work with the first cons without computing the last one. That means that the following consumption of the list by length can't even start until it's finished building the whole thing in memory (you're wasting both time and space). Holding a large list like that in memory all at once has few benefits. If you're currently doing lazy IO and are concerned about the file changing as you're processing it, you're better off copying the whole file into memory as one big block and still building the list lazily from that copy in memory. If you're coming from any sort of imperative background, you can basically think of a list as a loop which has not yet happened. If you can generate the indices for the loop on the fly, you're probably not going to want to waste time putting them all in memory beforehand, regardless of what that loop is doing (how the list is being consumed). There are a few cases where strictifying production of data is good, but it's far more common to want strict consumption and lazy production. - Cale