[Haskell] Re: About Random Integer without IO

Thomas Davie tatd2 at kent.ac.uk
Thu Nov 11 16:05:57 EST 2004

This method unfortunately depends on having a seed first though.  One 
must use a different value every time the program is started, commonly 
time or the first few bytes from /dev/random.  Any one of these is 
going to require a monadic function to generate (i.e. it must come from 
the environment in some way - it must change every time you run the 

So while this eliminates the IO monad from most of the program, it must 
still be initiated using it.


Tom Davie

Computer Science is as much about computers as astronomy is about 
telescopes -- Edsgar Dijkstra

On 11 Nov 2004, at 20:58, karczma wrote:

> Georg Martius answers the request of:
> Jose Manuel Hernando Cobeña
>>> I need generate random numbers by create polygons with "wxHaskell",
>>> I am searching in the web, but all I only find IO functions like
>>>>> test :: Integer -> IO Integer
> ...
>>> I need this but with types :: Integer -> Integer
>> you need to reallise that Haskell is a pure language. That means a 
>> function has the same result if you call it with the same arguments 
>> (deterministic). A function that produces random numbers are not of 
>> such kind in the first place.
> All depends on how you organize your computations.
> 1. You may in a purely functional, non-monadic way produce an infinite
>  list, a stream of numbers, and use them as you wish, incrementally.
>  gener seed =(aaa*seed + ccc) `mod` mmm     -- or something alike...
>  strm = st seed0 where
>    st x = x : st (gener x)
>  Eventually, you can directly pass the updated seed from one function 
> to
>  another in your program. In two words, write your programs in CPS, or
>  a little 'monadized' style.
>  Of course, you can transform the result from an integer between 0 and
>  mmm-1 to other thing.
> 2. Despite what G. Martius says, you CAN USE a pure, deterministic 
> function
>  to produce something which looks randomly, it suffices to change the
>  argument. Concretely, Ward in Graphics Gems, and other people in other
>  places propose 'chaotic', *ergodic* functions, which vary wildly from
>  arg to arg, even neighbouring.
>  This is a "kind of hashing function".
>  An example can be found in this tutorial
>  http://freespace.virgin.net/hugo.elias/models/m_perlin.htm
>  You take an integer, and mix its bits by taking xors, modulo,
>  polynomials, etc.
>  Then, such a function sampled for j=0,1,2,3, ... N will look like
>  a genuinely random signal, without any need to 'propagate' anything.
> Jerzy Karczmarczuk
> PS. Georg Martius concludes:
>> PS: this question is more appropriate for haskell-cafe.
> Well, why not haskell-bratwurst-mit-pils?  The protocol of making/using
> random stuff within Haskell is a specific problem which is not very 
> well
> developed nor taught, but it is not anecdotical, it may be important.
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