Editing 99 questions/Solutions/39 (section)

='''Solution 3.'''=

Another way to compute the claimed list is done by using the ''Sieve of Eratosthenes''. 
<haskell>
primesR :: Integral a => a -> a -> [a]
primesR a b = takeWhile (<= b) $ dropWhile (< a) $ sieve [2..]
  where sieve (n:ns) = n:sieve [ m | m <- ns, m `mod` n /= 0 ] 
</haskell>

The <code>sieve [2..]</code> function call generates a list of all (!) prime numbers using this algorithm and <code>primesR</code> filters the relevant range out. [But this way is very slow and I only presented it because I wanted to show how nicely the ''Sieve of Eratosthenes'' can be implemented in Haskell :)] 

''this is of course the famous case of (mislabeled) executable specification, with all the implied pitfalls of inefficiency when (ab)used as if it were an actual code''.