Now it works; viz (in response to Brent Yorgey's
suggestion, I have merged hanoi and hanoi_helper):
hanoi_general_list_comprehension_unwords.hs (based on
Neil Mitchell's suggestion, except for the trailing
'.'):
hanoi :: a -> a -> a -> Int -> [(a, a)]
hanoi source using dest n
| n == 1 = [(source, dest)]
| otherwise = hanoi source dest using (n-1)
++ hanoi source using dest 1
++ hanoi using source dest
(n-1)
hanoi_shower :: Show a => [(a, a)] -> String
hanoi_shower moves = unlines [unwords ["Move", show a,
"to", show b, "."] | (a, b) <- moves]
Then, in WinHugs:
Main> :load
hanoi_general_list_comprehension_unwords.hs
Main> putStr (hanoi_shower (hanoi 'a' 'b' 'c' 2))
Move 'a' to 'b' .
Move 'a' to 'c' .
Move 'b' to 'c' .
Main> putStr (hanoi_shower (hanoi 1 2 3 2))
Move 1 to 2 .
Move 1 to 3 .
Move 2 to 3 .
Notwithstanding Neil's advice on removing the trailing
'.', which I appreciate, I would still prefer to
retain it, because, in the interests of literate
programming, I would like a sequence of English
sentences as commands acceptable even to an English
teacher.
So, to be pedantic and remove the ' ' before the '.'
at each line:
hanoi_shower portion of
hanoi_general_list_comprehension_unlines.hs (based on
Neil Mitchell's suggestion):
hanoi_shower :: Show a => [(a, a)] -> String
hanoi_shower moves = unlines ["Move " ++ show a ++ "
to "++ show b ++ "." | (a, b) <- moves]
Then, in WinHugs:
Main> :load
hanoi_general_list_comprehension_unlines.hs
Main> putStr (hanoi_shower (hanoi 'a' 'b' 'c' 2))
Move 'a' to 'b'.
Move 'a' to 'c'.
Move 'b' to 'c'.
Main> putStr (hanoi_shower (hanoi 1 2 3 2))
Move 1 to 2.
Move 1 to 3.
Move 2 to 3.
Splendid!
Now, just for fun, let's see what other versions also
work:
hanoi_shower portion of hanoi_general_map_unlines.hs
(based on Tillman Rendel's suggestion):
hanoi_shower :: Show a => [(a, a)] -> String
hanoi_shower moves = unlines (map move moves)
where move (a, b) = "Move " ++
show a ++ " to "++ show b ++ "."
Then, in WinHugs:
Main> :load hanoi_general_map_unlines.hs
Main> putStr (hanoi_shower (hanoi 'a' 'b' 'c' 2))
Move 'a' to 'b'.
Move 'a' to 'c'.
Move 'b' to 'c'.
Main> putStr (hanoi_shower (hanoi 1 2 3 2))
Move 1 to 2.
Move 1 to 3.
Move 2 to 3.
Wonderful! Thanks especially to Neil Mitchell and
Tillman Rendel for their constructive suggestions.
Nevertheless, I'm still working on the recursive
version. So far, I've gotten this far:
hanoi_shower portion of hanoi_general_recursive.hs
(based on Tillman Rendel's suggestion):
hanoi_shower :: Show a => [(a, a)] -> String
hanoi_shower ((a, b) : moves)
| null moves = "Move " ++ show a ++ " to "++ show
b ++ "."
| otherwise == "Move " ++ show a ++ " to "++ show
b ++ "." ++ hanoi_shower moves
However, in WinHugs, I get the following error:
Hugs> :load hanoi_general_recursive.hs
ERROR file:hanoi_general_recursive.hs:11 - Syntax
error in declaration (unexpected `}', possibly due to
bad layout)
I haven't used recursion in Haskell much so far; I've
only used it in Scheme, so I'm not used to it.
I need to go to lunch now, so I'll work on this part
later. Perhaps I can get it to work after lunch....
Benjamin L. Russell
--- Neil Mitchell <[EMAIL PROTECTED]> wrote:
> Hi
>
> > hanoi_shower [] = ...
> > hanoi_shower ((a, b) : moves) = ...
> >
> > or (preferably) with map
> >
> > hanoi_shower moves = unlines (map show_move
> moves) where
> > show_move (a, b) = ...
>
> A nice list comprehension works wonders in these
> situations:
>
> hanoi_shower moves = unlines ["Move " ++ show a ++ "
> to " ++ show b ++
> "." | (a,b) <- moves]
>
> I would personally remove the "." from the end, as
> its a list of
> commands, not sentences - but that is personal
> choice. I'd also use
> unwords, as its slightly shorter:
>
> hanoi_shower moves = unlines [unwords ["Move", show
> a, "to", show b] |
> (a,b) <- moves]
>
> Thanks
>
> Neil
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
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