[Haskell-beginners] folds again -- myCycle

7stud bbxx789_05ss at yahoo.com
Sat Mar 14 03:57:57 EDT 2009

I spent a long time working on a solution for an exercise in RWH: if you can, 
use foldr to mimic haskell's cycle function.  At first, I wondered whether
it was even possible.  Then I worked on some ideas, and finally I came up with
a solution.  Surprisingly, my solution ended up being very simple:

myCycle [] = [] 
myCycle xs = helperFunc xs [1..] 
    where helperFunc ys foldrXs = foldr accFunc [] foldrXs 
            where accFunc _ acc = ys ++ acc  

I tested it out, and it worked like a charm:

*Main> let x = myCycle [1, 2, 3]
*Main> take 2 x   
*Main> take 10 x
*Main> take 30 x

After re-examining my solution, I decided that this line:

--where accFunc _ acc = ys ++ acc 

read better like this:

--where accFunc _ acc =  acc ++ ys

The altered function returns a list just fine.  But when I use take on the 
list, I get a stack overflow. What is being thunked in both cases?

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