random gremlin

[Daniel Fischer]

On Wednesday 06 October 2010 22:02:57, Conor McBride wrote:
> M'luds,
>
> There's probably a place to report this properly, but I thought if
> I mentioned it here, you could throw cabbages at me and tell me
> where to go.

http://hackage.haskell.org/trac/ghc/newticket?type=bug

Component: libraries(other)

Decide whether you regard it as a documentation bug or a wrong result at 
runtime.

(Documentation bug seems more reasonable)

>
> Exhibit A, from
> http://haskell.org/ghc/docs/6.12.2/html/libraries/random-1.0.0.2/System-
>Random.html
>
>    "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
                            ^^^^
Note that that should be uppercase, Read.

>    following properties:
>
>      * It guarantees to succeed on any string."

It does in the sense that

reads foo :: [(StdGen,[Char])] /= []

for all foo. The way it's formulated invites wrong interpretations, though.

>
> Exhibit B, from ghci
>
>    Prelude System.Random> read "00-00-00-00-00-00" :: StdGen
>    Loading package old-locale-1.0.0.2 ... linking ... done.
>    Loading package time-1.1.4 ... linking ... done.
>    Loading package random-1.0.0.2 ... linking ... done.
>    *** Exception: Prelude.read: no parse
>
> The prosecution rests.
>
> Conor