random gremlin

Conor McBride conor at strictlypositive.org
Wed Oct 6 17:30:54 EDT 2010

Hi Daniel

On 6 Oct 2010, at 21:33, Daniel Fischer wrote:

> On Wednesday 06 October 2010 22:02:57, Conor McBride wrote:
>> tell me
>> where to go.
> http://hackage.haskell.org/trac/ghc/newticket?type=bug
> Component: libraries(other)

OK, I signed up and did that.

> Decide whether you regard it as a documentation bug or a wrong  
> result at
> runtime.
> (Documentation bug seems more reasonable)

Of the options available, yes.

>> Exhibit A, from
>> http://haskell.org/ghc/docs/6.12.2/html/libraries/random-
>> 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.

Never the less, the first sentence makes it clear that my expectations
were in conformance with the documentation, so...

>>   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.

...I beg to differ. The way it's formulated is clear, but not what  

It seems that there's an unexpected tension between the use of Read for
serialization (in which you want *reads* to be compositional) and the
utility of making a StdGen from some sort of identifier.

Perhaps the better fix is to separate the latter functionality.

All the best


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