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

[Abhay Parvate]

Yes, they are. That's what perhaps Neil Mitchell means by

mapM_ putStrLn == putStr . unlines

And whether the trailing newline is to be called the last blank line depends
upon the convention; The string that is output in both the cases contains a
single newline character. Are you calling that a blank line at end?

And I actually meant to reply to haskell-cafe as well; omitted haskell-cafe
by mistake. Anyway, you got the same answer from two people! This time I am
including haskell-cafe in the recipients.

Regards,
Abhay

On Mon, Apr 14, 2008 at 4:05 PM, Benjamin L. Russell <dekudekuplex at yahoo.com>
wrote:

> Abhay Parvate,
>
> Thank you; that answered my question.
>
> Then, the following two lines of code should be
> equivalent:
>
> In hanoi.hs:
> hanoi n = mapM_ putStrLn (hanoi_helper 'a' 'b' 'c' n)
>
> In hanoi_unlines.hs:
> hanoi n = putStr (unlines(hanoi_helper 'a' 'b' 'c' n))
>
> I tested them both out on WinHugs (Version Sep 2006);
> they both generated one blank line at the end.
>
> Benjamin L. Russell
>
> --- Abhay Parvate <abhay.parvate at gmail.com> wrote:
>
> > unlines puts newline after each string; putStrLn
> > puts newline after the
> > given string. As a result, the output contains two
> > newlines in the end. You
> > can use putStr instead, since the resultant string
> > from 'unlines' will have
> > a newline at the end.
> >
> > On Mon, Apr 14, 2008 at 2:12 PM, Benjamin L. Russell
> > <dekudekuplex at yahoo.com>
> > wrote:
> >
> > > Ok; much better.  Here's my new type signature and
> > > definition:
> > >
> > > hanoi.hs:
> > > hanoi :: Int -> IO ()
> > > hanoi n = mapM_ putStrLn (hanoi_helper 'a' 'b' 'c'
> > n)
> > >
> > > hanoi_helper :: Char -> Char -> Char -> Int ->
> > > [String]
> > > hanoi_helper source using dest n
> > >    | n == 1 = ["Move " ++ show source ++ " to " ++
> > > show dest ++ "."]
> > >    | otherwise = hanoi_helper source dest using
> > (n-1)
> > >
> > >                  ++ hanoi_helper source using dest
> > 1
> > >                         ++ hanoi_helper using
> > source
> > > dest (n-1)
> > >
> > > Then in WinHugs (Version Sep 2006):
> > >
> > > Hugs> :load hanoi.hs
> > > Main> hanoi 2
> > > Move 'a' to 'b'.
> > > Move 'a' to 'c'.
> > > Move 'b' to 'c'.
> > >
> > > Great!
> > >
> > > One minor question:  I tried out both of your
> > > following suggestions:
> > >
> > > >    > mapM_ putStrLn (hanoi 2) -- outputs each
> > move
> > > > in a new line
> > > >    > putStrLn (unlines (hanoi 2)) -- same as
> > > > previous line
> > >
> > > and discovered that putStrLn with unlines (the
> > lower
> > > option) in fact generates one extra blank line at
> > the
> > > end.  Just curious as to why....
> > >
> > > Benjamin L. Russell
> > >
> > > --- Tillmann Rendel <rendel at daimi.au.dk> wrote:
> > >
> > > > 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
> > > > between
> > > >
> > > >    > '\n'
> > > >
> > > > and
> > > >
> > > >    > 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 ...)
> > > >
> > > > and
> > > >
> > > >    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
> > > >
> > > > Tillmann
> > > > _______________________________________________
> > > > Haskell-Cafe mailing list
> > > > Haskell-Cafe at haskell.org
> > > >
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> > > >
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> >
>
>
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