[Haskell-cafe] What is this function? (RESOLVED)

[John Ky]

Hi Luke,

Thankyou so much.  I need the sequence function.

What I was after was this:

class Scramblable a where
   scramble :: a -> [a]

instance Scramblable [MyType] where
   scramble values = sequence (map scramble values)

instance Scramblable MyType where
   scramble myType = {- blah blah -}

I was basically after exhaustively generating lots ASTs given a template AST
with lots of leaf values changed.

Thanks,

-John

On Thu, Oct 16, 2008 at 8:39 PM, Luke Palmer <lrpalmer at gmail.com> wrote:

> 2008/10/16 John Ky <newhoggy at gmail.com>:
> > Hi,
> >
> > I've written this function here:
> >
> >    scramble [] = []
> >    scramble [x] = [[z] | z <- scramble x]
> >    scramble (x:xs) =
> >       [(y:z)|y <- scramble x, z <- scramble xs]
> >
> > and (I think) it roughly does what I want it to:
> >
> >    *Main> scramble ([]::[Int])
> >    []
> >    *Main> scramble ([1]::[Int])
> >    [[1],[2]]
>
> So, um, this is nonsense.  You've given it only 1, and yet it outputs
> a 2, yet there is no mention of addition or the literal 2 anywhere in
> your function.
>
> This function looks a lot like the more sensible:
>
>  scramble' n xs = sequence (replicate n xs)
>
> >>> scramble' 0 [1,2,3]
> [[]]
> >>> scramble' 1 [1,2,3]
> [[1],[2],[3]]
> >>> scramble' 2 [1,2,3]
> [[1,1],[1,2],[1,3],[2,1],[2,2],[2,3],[3,1],[3,2],[3,3]]
>
> Where your examples all had [1,2] as the argument.
>
> Luke
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20081017/296e57eb/attachment.htm