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 [EMAIL PROTECTED] 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 [EMAIL PROTECTED]
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 [EMAIL PROTECTED]
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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