[Haskell-cafe] On the purity of Haskell /Random generators

[Steve Horne]

On 29/12/2011 21:51, Jerzy Karczmarczuk wrote:
> Steve Horne :
>> I only meant that there's random number handling support in the 
>> Haskell library and, and least judging by type signatures, it's pure 
>> functional code with no hint of the IO monad.
> Look well at those functions, please.
Challenge accepted. Some code (intended to be loaded into GHCi and 
played with) that I once wrote when doing the ninety-nine problems thing 
(the one that doesn't have ninety-nine problems - originally based on a 
Prolog tutorial IIRC)...

    --  Randomly select the specified number of items from the list
    --
    --  Usage in GHCi...
    --
    --  import System.Random
    --  randSelect "this is a list" 5 (mkStdGen 9877087)
    --
    --  This will give the same results each time (for the same seed
    given to mkStdGen)
    --
    --  randSelect' does the real work, but needs to know the length of
    the remaining
    --  list and doesn't do error checks (for efficiency reasons).
    module P23 (randSelect) where
       import System.Random

       randSelect' :: RandomGen g => [x] -> Int -> Int -> g -> ([x], g)

       randSelect' [] n l g = ([], g)  --  n and l should be == 0, but
    no need for run-time check

       --  optimisation cases - no choice left
       randSelect' xs n l g | (n == l) = (xs, g)
                            | (n == 0) = ([], g)

       randSelect' (x:xs) n l g = let xsLen      = (l - 1)
                                      (rnd, g')  = randomR (0, xsLen) g
                                      (keep, n') = if (rnd < n) then
    (True, (n-1)) else (False, n)
                                      (xs', g'') = randSelect' xs n'
    xsLen g'
                                  in ((if keep then (x:xs') else xs'), g'')

       randSelect :: RandomGen g => [x] -> Int -> g -> ([x], g)

       randSelect xs n g = let len = (length xs)
                           in if (n > len) then error "Not enough items
    in the list!"
                                           else randSelect' xs n len g

I see no IO monad anywhere in there. Of course I'm cheating - providing 
a constant seed at runtime. It's a bit like the classic "chosen by a 
throw of a fair die" joke in a way. But the functions I'm using are pure.

I don't claim to know every Haskell library function, of course. If 
there's further functions for that, all well and good - but there's 
still a perfectly adequate pure functional subset.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20111229/7e18d96f/attachment.htm>