Re: [Haskell-cafe] How is laziness defined?

On 2/5/07, ajb@spamcop.net wrote:

Quoting TJ :

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I would think that with 100% laziness, nothing would happen until the Haskell program needed to output data to, e.g. the console. Quite obviously that's not it. So how is laziness defined in Haskell?

It means that the program behaves as if "things" are evaluated if and only if they are needed. "Needed" in the Haskell sense, means "needed to do I/O".

So it's just IO which makes things run huh? OK that's basically what I said there. Cool.

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This is one of the things that just boggles my mind everytime I try to wrap it around this thing called Haskell ;)

The cool part is that for the most part, it doesn't matter. It just works. If you ever come across a case where it doesn't just work (e.g. if you need a tilde pattern), you'll be ready for it.

I despise using what I don't understand. It's a big problem but one which is more insurmountable than understanding Haskell, I think... On 2/5/07, Andrew Wagner wrote:

I found it useful to work through an example where lazy evaluation was important, and wrote it up in a tutorial. It may or may not help you, no guarantees, but here it is: http://www.haskell.org/haskellwiki/Haskell/Lazy_Evaluation

With the code from your tutorial, magic :: Int -> Int -> [Int] magic 0 _ = [] magic m n = m : (magic n (m+n)) getIt :: [Int] -> Int -> Int getIt [] _ = 0 getIt (x:xs) 1 = x getIt (x:xs) n = getIt xs (n-1) and the example expression, getIt (magic 1 1) 3 It only gets run (starts pattern matching and all) if I do a print on it, or run it from GHCi (which will do the"print" for me), right? Thanks, TJ