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

Tillmann Rendel rendel at daimi.au.dk
Mon Apr 14 03:41:39 EDT 2008

Benjamin L. Russell wrote:
> but got stuck on outputting newlines as part of the string;

quoting is done by the show function in Haskell, so you have to take 
care to avoid calling show. your code calls show at two positions:
(1) when you insert the newline into the string
(2) when you output the string

with respect to (1):

you use (show '\n') to create a newline-only string, which produces a 
machine-readable (!) textual representation of '\n'. try the difference 

   > '\n'


   > show '\n'

to see what I mean. instead of using (show '\n'), you should simply use 
"\n" to encode the string of length 1 containing a newline character.

with respect to (2):

the type of your top-level expression is String, which is automatically 
print'ed by the interpreter. but print x = putStrLn (show x), so there 
is another call to show at this point. to avoid this call, write an IO 
action yourself. try the difference between

   putStrLn (hanoi ...)


   print (hanoi ...)

to see what I mean.

Last, but not least, I would like to point out a different aproach to 
multiline output which is often used by Haskell programmers: The worker 
functions in this aproach produces a list of strings, which is joined 
together with newlines by the unlines function. In your case:

   hanoi_helper :: ... -> [String]
     | ... = ["Move " ++ ...]
     | otherwise = hanoi_helper ... ++ hanoi_helper ...

   hanoi n = hanoi_helper 'a' 'b' 'c' n

and in the interpreter one of these:

   > hanoi 2 -- outputs a list
   > mapM_ putStrLn (hanoi 2) -- outputs each move in a new line
   > putStrLn (unlines (hanoi 2)) -- same as previous line


More information about the Haskell-Cafe mailing list