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

14 Apr 2008

      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  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 
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=headblo...
...
...
separate = splitList ","
I am currently thinking about making this module
...
library with some other useful functions, if
into a standalone
people have any opinions
...
on this then please let me know.
Thanks
Neil
On 6/12/06, Sara Kenedy 
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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