Embedding newlines into a string? [Was: Re: [Haskell-cafe] Separate a string into a list of strings]

[Benjamin L. Russell]

A friend and I were working on a Haskell version of
Towers of Hanoi yesterday, and I tried writing out the
program today, but got stuck on outputting newlines as
part of the string; viz:

hanoi :: Int -> String
hanoi n = hanoi_helper 'a' 'b' 'c' n
          
hanoi_helper :: Char -> Char -> Char -> Int -> String
hanoi_helper source using dest n
    | n == 1 = putStrLn "Move " ++ show source ++ " to
" ++ show dest ++ "." ++ show '\n'
    | otherwise = hanoi_helper source dest using (n-1)

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

The problem is that the newlines ('\n') get embedded
as escaped newlines into the output string, instead of
as newlines.

E.g., 

Hugs> :load hanoi.hs
Main> hanoi 2
"Move 'a' to 'b'.'\\n'Move 'a' to 'c'.'\\n'Move 'b' to
'c'.'\\n'"

Instead, what I want is the following:

Hugs> :load hanoi.hs
Main> hanoi 2
"Move 'a' to 'b'.
Move 'a' to 'c'.
Move 'b' to 'c'.
"

However, when I try to use putStrLn to avoid this
problem, as follows:

    | n == 1 = putStrLn "Move " ++ show source ++ " to
" ++ show dest ++ "." ++ show '\n'

the compiler generates the following error:

ERROR file:hanoi.hs:6 - Type error in application
*** Expression     : putStrLn "Move " ++ show source
++ " to " ++ show dest ++ "." ++ show '\n'
*** Term           : putStrLn "Move "
*** Type           : IO ()
*** Does not match : [Char]

Simply changing the type signature does not solve this
problem.

I searched through the past messages on this list, and
came up with the message below, but simply quoting the
newlines as '\n' doesn't seem to help.

Does anybody know a way to embed a newline into a
string as output of type String of a function so that
the newline characters are not escaped?

Benjamin L. Russell

--- Jared Updike <jupdike at gmail.com> wrote:

> Funny. I have a module called Useful.hs with some of
> these same sorts
> of functions. (coming from Python where I used
> .split(',') and
> .replace('\r', '') and such a lot):
> 
> ------------------
> module Useful where
> 
> import List ( intersperse, tails )
> import Numeric ( readHex )
> 
> hex2num :: (Num a) => String -> a
> hex2num s = let (result, _):_ = readHex s in result
> 
> toEnv s = map tuple (split ';' s)
> 
> tuple :: String -> (String, String)
> tuple line = case split '=' line of
>    a:b:_ -> (a,b)
>    a:_   -> (a,"")
>    _     -> ("","") -- not good, probably won't
> happen for my typical usage...
> 
> split       :: Char -> String -> [String]
> split _ ""  =  []
> split c s   =  let (l, s') = break (== c) s
>                  in  l : case s' of
>                            []      -> []
>                            (_:s'') -> split c s''
> 
> beginsWith []       []     = True
> beginsWith _        []     = True
> beginsWith []       _      = False
> beginsWith (a:aa)   (b:bb)
>     | a == b               = aa `beginsWith` bb
>     | otherwise            = False
> 
> dropping []     []     = []
> dropping []     _      = []
> dropping x      []     = x
> dropping s@(a:aa) (b:bb) | a == b    = dropping aa
> bb
>                          | otherwise = s
> 
> -- replace all occurrences of 'this' with 'that' in
> the string 'str'
> -- like Python replace
> replace _    _    []  = []
> replace this that str
>     | str `beginsWith` this = let after = (str
> `dropping` this)
>                                in  that ++ replace
> this that after
>     | otherwise             =
>         let x:xs = str
>           in x : replace this that xs
> 
> eat s = replace s ""
> 
> -- sometimes newlines get out of hand on the end of
> form POST submissions,
> -- so trim all the end newlines and add a single
> newline
> fixEndingNewlines = reverse . ('\n':) . dropWhile
> (=='\n') . reverse .
> filter (/= '\r')
> 
> endsWith a b = beginsWith (reverse a) (reverse b)
> 
> a `contains` b = any (`beginsWith` b) $ tails a
> ------------------
> 
>   Jared.
> 
> On 6/12/06, Neil Mitchell <ndmitchell at gmail.com>
> wrote:
> > Hi,
> >
> > I tend to use the module TextUtil (or Util.Text)
> from Yhc for these
> > kind of string manipulations:
> >
> >
>
http://www-users.cs.york.ac.uk/~malcolm/cgi-bin/darcsweb.cgi?r=yhc;a=headblob;f=/src/compiler98/Util/Text.hs
> >
> > separate = splitList ","
> >
> > I am currently thinking about making this module
> into a standalone
> > library with some other useful functions, if
> people have any opinions
> > on this then please let me know.
> >
> > Thanks
> >
> > Neil
> >
> >
> > On 6/12/06, Sara Kenedy <sarakenedy at gmail.com>
> wrote:
> > > Hi all,
> > >
> > > I want to write a function to separate a string
> into a list of strings
> > > separated by commas.
> > >
> > > Example:
> > > separate :: String -> [String]
> > >
> > > separate "Haskell, Haskell, and Haskell" =
> ["Haskell", "Haskell", "and Haskell"]
> > >
> > > If anyone has some ideas, please share with me.
> Thanks.
> > >
> > > S.
> > > _______________________________________________
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> > >
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> > >
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> >
> 
> 
> -- 
> http://www.updike.org/~jared/
> reverse ")-:"
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