On Tue, 13 Mar 2007 14:44:46 +0100, Zara wrote:

On Tue, 13 Mar 2007 00:37:46 -0700, Fritz Ruehr wrote:

...

According to the documentation for System.Random (see ):

...

In addition, read may be used to map an arbitrary string (not necessarily one produced by show) onto a value of type StdGen. In general, the read instance of StdGen has the following properties:

* It guarantees to succeed on any string. * ...

On the other hand, I get the following on the (admittedly stupid, spur-of-the-moment) String argument "whateva":

...

Hugs> :l System.Random System.Random> read "whateva" :: StdGen

Program error: Prelude.read: no parse <..>

There seems to be a limit on 6 characters. Any string under 6 characters works nice, any over that limit will fail.

test program:

\begin{code} module Test where

import Random

rd :: String -> StdGen rd = read

testit :: String -> [Int] testit [] = [] testit xt@(_:xs) = (testit xs) ++ [fst (randomR (1::Int,50) (rd xt))]

\end{code}

testit "thing" wotks nice,

testit "morethanthing" stops after the sixth number in the list.

Both in HUGS and GHC, last versions

The problem seems to lie in base/System/Random.hs: \begin{code} {- If we cannot unravel the StdGen from a string, create one based on the string given. -} stdFromString :: String -> (StdGen, String) stdFromString s = (mkStdGen num, rest) where (cs, rest) = splitAt 6 s num = foldl (\a x -> x + 3 * a) 1 (map ord cs) \end{code} If we change the number on splitAt, the string limit to give an error changes. I think that when we feed an arbitrary string to readsPrec of Random.StdGen, we should return nothing, that is, we should consume the whole string. So, I propose th change that function to be: \begin{code} {- If we cannot unravel the StdGen from a string, create one based on the string given. We consume it all. -} stdFromString :: String -> (StdGen, String) stdFromString s = (mkStdGenBis num, "") where num = foldl (\a x -> x + 3 * a) 1 (map ord s) \end{code} This solution solves *our* problem, but I would like to receive comments to it. Where may I find someone responsible for this library, to see if the change is adequate, or why not? Best regards, Zara

On 3/19/07, Simon Peyton-Jones wrote:

Zara

Good point. It's a bit stupid that 'read' fails utterly on strings shorter than 6.

I don't thin StdRandom has an "owner" at the moment. There's a process for proposing library changes, described under "guidelines for developers" here

http://haskell.org/haskellwiki/Libraries_and_tools

That's the way to get your change adopted.

There's already a thread about this on the libraries list: http://www.haskell.org/pipermail/libraries/2007-March/007052.html Ian commented with a proposal for fixing it, and no one has followed up yet. That would probably be a good place to direct any more discussion. Cheers, Kirsten