[Haskell-cafe] Re: Embedding newlines into a string?

[Benjamin L. Russell]

Wow, that's very general.  So you want to divide hanoi
into a main function, a helper function, and a display
function.

I tried it out, and got this far so far:

hanoi :: a -> a -> a -> Int -> [(a, a)]
hanoi a b c n = hanoi_helper a b c n
          
hanoi_helper :: a -> a -> a -> Int -> [(a, a)]
hanoi_helper source using dest n
    | n == 1 = [(source, dest)]
    | otherwise = hanoi_helper source dest using (n-1)

                  ++ hanoi_helper source using dest 1
                         ++ hanoi_helper using source
dest (n-1)

hanoi_shower :: Show a => [(a, a)] -> String
hanoi_shower [(a, b)] = "Move " ++ show a ++ " to " ++
show b ++ "."

However, when I tried to run the code in WinHugs, this
is what I got:

Hugs> :load hanoi_general.hs
Main> putStr (hanoi_shower (hanoi 'a' 'b' 'c' 2))

Program error: pattern match failure: hanoi_shower
[('a','b'),('a','c')] ++ ([] ++ hanoi_helper 'b' 'a'
'c' (2 - 1))

There seems to be a bug in hanoi_shower.

I'm still trying to debug hanoi_shower, but I need to
stop for today and continue on this tomorrow.

Thanks for your help so far!  Perhaps I can get this
general version fully working tomorrow.

Benjamin L. Russell

--- Tillmann Rendel <rendel at daimi.au.dk> wrote:

> Benjamin L. Russell wrote:
> > Ok; much better.  Here's my new type signature and
> > definition:
> > 
> > hanoi :: Int -> IO ()   
> > hanoi_helper :: Char -> Char -> Char -> Int ->
> [String]
> 
> If you want, you can separate the algorithm and the
> output processing 
> even more by providing three functions of these
> types:
> 
> hanoi :: Int -> [(Char, Char)]
> hanoi_helper :: Char -> Char -> Char -> Int ->
> [(Char, Char)]
> hanoi_shower :: [(Char, Char)] -> String
> 
> and at the interpreter level:
> 
>    > putStr (hanoi_shower (hanoi 2))
> 
> added value: you can easily use the output of hanoi
> for automated 
> processing (e.g. testing, controlling a robot,
> producing an animation, 
> counting the number of steps).
> 
> You can go one step further if you consider that
> towers don't have to be 
> named by single characters, but can be named by
> everything:
> 
> hanoi :: a -> a -> a -> Int -> [(a, a)]
> hanoi_helper :: a -> a -> a -> Int -> [(a, a)]
> hanoi_shower :: Show a => [(a, a)] -> String
> 
> now you can use
> 
>    > putStr (hanoi_shower (hanoi 'a' 'b' 'c' 2))
> 
> to get the same result as above, but you are also
> allowed to write
> 
>    > putStr (hanoi_shower (hanoi 1 2 3 2))
> 
> if you want to use numeric tower names.
> 
>    Tillmann
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