[Haskell-cafe] powerSet = filterM (const [True, False]) ... is this obfuscated haskell?

[Sittampalam, Ganesh]

Sebastian Fischer wrote:
> On Jul 28, 2009, at 11:06 AM, Sittampalam, Ganesh wrote:
> 
>>> perms = sortByM (const [True,False])
>> 
>> and moreover the results will depend on the sorting algorithm chosen.
> 
> Is it only that different sorting algorithms enumerate all
> permutations in different orders or is there a sorting algorithm,
> such that the above definition does not enumerate all permutations?  
> 
[..]
> Hence, perm as defined above can yield a list that contains all
> permutations of the input (at least once) regardless of the sorting
> algorithm.  
> 
> Where is the hitch?

The "at least once" bit - unless your non-determinism is based on set
rather than bag semantics, it's wrong to duplicate results.

Ganesh

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