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From: rwobben@hotmail.com To: haskell@phirho.com Subject: RE: [Haskell-beginners] another list comprehesion error Date: Fri, 22 Jul 2011 06:46:03 +0000
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Date: Thu, 21 Jul 2011 23:40:52 +0200 From: haskell@phirho.com To: beginners@haskell.org Subject: Re: [Haskell-beginners] another list comprehesion error
Hi!
On 21.07.2011 21:29, Roelof Wobben wrote:
roelof n x = [x | y<- [1..n]]
Have you tried this? What do you get for, say "roelof 4 5" ? Why?
I get [5,5,5,,5] And that the right answer according to the exercise
Now try this instead: roelof' n = [x | x <- [1..n]] What do you get for "roelof' 4" ?
A error message that a instance of print is missing.
A few suggestions (without guards): The list of the first 5 natural numbers. The list of the first 5 odd numbers. The list of all pairs (a, b) where a,b > 0, a <= b and b <= 5
Now you can try the last two with guards.
without guards 1) [x | x <- [1..5]] 2) cannot be done without guards and list comprehession generator [ 2,4 ..10] does not work 3) cannot be done withut guards and list comprehession because of the a<=b with guards. 1) has no need for guards. 2) [x | x <-[1..10], even x] 3] [ (a,b) | a<- [1..4], b<- [1..5], a<=b] Roelof