[Haskell-cafe] Re: System.Random StdGen read fails on some strings? [also continues: Random/StdGen/read: there is something unclear (or misunderstood!)]

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

System.Random> map read $ words "this is a test of the System dot Random StdGen read, which seems to fail only on ... whateva" :: [StdGen] [4580 1,440 1,101 1,4584 1,445 1,1485 1,35261 1,1377 1,32825 1,34047 1,13422 1,14037 1,13637 1,469 1,4132 1,4514 1,453 1,626 1, Program error: Prelude.read: no parse

Am I missing something here? Or am I just being punished for the stupidity of my particular choice of String? :)

-- Fritz

PS: this happens with both Hugs Version: September 2006 and GHC Interactive, version 6.6, running on Mac/Tiger, for what that's worth.

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 Best regards, Zara