[Haskell-beginners] Re: permuting a list

[Heinrich Apfelmus]

Patrick LeBoutillier wrote:
>>
>> permute :: [a] -> [[a]]
>> permute xs = [s:ps | (s,ss) <- select xs, ps <- permute ss]
>>
>> select :: [a] -> [(a,[a])]
>> select []     = []
>> select (x:xs) = (x,xs) : [(s,x:ss) | (s,ss) <- select xs]
> 
> When I run this in ghci I always get an empty list:
> 
>   [patrickl at fc9i386 haskell]$ ghci permute.hs
>   GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
>   Loading package base ... linking ... done.
>   [1 of 1] Compiling Main             ( permute.hs, interpreted )
>   Ok, modules loaded: Main.
>   *Main> permute [1,2,3]
>   []
> 
> 
> Am i missing something?

No. The base case

  permute [] = [[]]

was missing.


Regards,
apfelmus

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