[Haskell-cafe] infinite lists

Albert Y. C. Lai trebla at vex.net
Tue Mar 27 15:17:05 EDT 2007

Matthias Fischmann wrote:
> here is the bug, narrowed to four lines.  a function that only
> sometimes terminates.
> f = do
>     n <- randomRIO (0,5)
>     let l = replicate n '*'
>         i = join $ repeat l  -- infinite extension
>     print (take 12 i)

In this paragraph I speak about just this program, ignoring that it is a 
lossy excerpt from another program for a different specification. This 
program, when corrected, will output the empty string with probability 
1/6, and/or the string of twelve *'s with probability 5/6. This could be 
done more simply and less error-prone by writing "if n==0 then the 
former else the latter". In other words, I wonder why more information 
is produced (in the sense of information theory and entropy), only to be 
discarded, i.e., in 5/6 of the cases, you ask for 1 to 5 *'s, then 
discard that length and make them all 12. Along with the introduction of 
unnecessary work comes the introduction of unnecessary mistakes.

In this paragraph I acknowledge that this program is a lossy excerpt 
from another program for a different specification. Still, if there is a 
sane way of mapping this program back to its origin, there is probably 
also some partial way of mapping my comment above to the original 
specification and program. I advise you to review your specification and 
program, and look for opportunities for eliminating unnecessary or 
indirect work; you may accidentally eliminate some mistakes along the way.

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