Hi Bahtijar! I'm not going to answer your question directly, but let me give you some background that will hopefully help you discover the pretty solution yourself.
Consider this definition: ones = 1 : ones -- This list is essentially [1,1,1,...] And this expression: take 5 ones Now, suppose for some reason we need to evaluate this expression to find its value. It would seem that this evaluation would take an infinite amount of time. But because Haskell only evaluates what's necessary, it does something like this: take 5 ones take 5 (1 : ones) take 5 (1 : (1 : ones)) take 5 (1 : (1 : (1 : ones))) take 5 (1 : (1 : (1 : (1 : ones)))) take 5 (1 : (1 : (1 : (1 : (1 : ones))))) [1,1,1,1,1] So, it only looks at the first five elements of the infinite list, because that's all it needs. Now let's take a slightly harder (but more classical) example: fibs = 0:1:zipWith (+) fibs (tail fibs) zipWith (+) just takes the first element of each of two lists and adds them together. (tail fibs) gives fibs, but without the first element. So, it will add the first element to the second element, the second to the third (which was calculated as needed by adding the first two), and so on. So, one way to define an infinite list of negative numbers is to say "the first negative number is -1, and the rest of the list is what you get if you subtract -1 from every number in the list". Then, the second number will be the first number in the "rest of the list", so it will be the number you get by subtracting -1 from the first number in the list. The third number will be the first of the rest of the rest of the list (say that ten times fast), and will be the first number of the list you get by adding -1 to the list starting at the second number. Etc., etc. One final note: Max's solution also works, but this is a good way to practice working with infinite lists, lazy evaluation, and list functions. Don't miss the opportunity! Please feel free to email me or the list with any questions. Andrew On 2/5/07, Bahtijar Vogel <[EMAIL PROTECTED]> wrote:
Hi, How am I supposed to use infinite lists to define the list of all negative integers? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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