[Haskell-cafe] A bug of groupBy implement

[Bulat Ziganshin]

Hello Stephen,

Monday, December 7, 2009, 8:11:01 PM, you wrote:

it's just what goupBy compares with the first element of group rather
than comparing two adjancent elements. look at the trace

it's not a bug, but misunderstanding of specification :)


> Could it not be a bug in

> a) printf
> b) unsafePerformIO
c) >>>
> d) return
> e) <=
> f) show
> g) GHC or GHCi?

> All of the above?

> Best wishes

> Stephen

> 2009/12/7 L.Guo <leaveye.guo at gmail.com>:
>> Hi there:
>>
>> My friend asked me a question, and i suppose he has found a bug of `groupBy'.
>>
>> Here is the code piece:
>>
>>> List.groupBy (\a b -> Foreign.unsafePerformIO (Text.Printf.printf "\t%d <= %d ?: %s\n" a b (show (a<=b)) >> return (a<=b))) [7,3,5,9,6,8,3,5,4]
>>
>> I have tested it in GHC 6.10.4 (Win XP) and GHC 6.8.3 (Linux), both give the wrong result (categaried):
>>
>>        7 <= 3 ?: False
>>        3 <= 5 ?: True
>>        3 <= 9 ?: True
>>        3 <= 6 ?: True
>>        3 <= 8 ?: True
>>        3 <= 3 ?: True
>>        3 <= 5 ?: True
>>        3 <= 4 ?: True
>> [[7],[3,5,9,6,8,3,5,4]]
>>
>>
>>    Regards
>> --------------
>> L.Guo
>> 2009-12-08
>>
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-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com