[Haskell-cafe] System.Random StdGen read fails on some strings?

[Kirsten Chevalier]

[redirecting from haskell-cafe to libraries]

On 3/13/07, Fritz Ruehr <fruehr at willamette.edu> wrote:
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> According to the documentation for System.Random (see <http://haskell.org/ghc/docs/latest/html/libraries/base/System-Random.html>):
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> 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:
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>     * It guarantees to succeed on any string.
>     * ...
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> On the other hand, I get the following on the (admittedly stupid, spur-of-the-moment) String argument "whateva":
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> Hugs> :l System.Random
> System.Random> read "whateva" :: StdGen
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> Program error: Prelude.read: no parse
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> 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
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> Am I missing something here? Or am I just being punished for the stupidity of my particular choice of String? :)
>

You're being punished for using a string with more than six characters
:-) I took a look at the code for System.Random; in particular, the
Read instance for StdGen. If you look at the implementation of
readsPrec in that instance, it clearly fails on any string longer than
six characters that doesn't match [0-9]*[:space:][0-9]* (in that case,
stdFromString is invoked, which consumes only the first six characters
of the string, hence read says "no parse" if the input string has
length greater than six).

I'm not sure whether the code is wrong or whether the comment that
says "It guarantees to succeed on any string." is wrong. (Or both.)
Does anyone know?

Cheers,
Kirsten

-- 
Kirsten Chevalier* chevalier at alum.wellesley.edu *Often in error, never in doubt
"The blues isn't about feeling better, it's about making other people feel
worse, and making a few bucks while you're at it." -- Bleeding Gums Murphy