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Today's Topics:
1. Getting a specified number of lines from stdin
(Nicolaas du Preez)
2. Re: Getting a specified number of lines from stdin
(Henk-Jan van Tuyl)
3. Re: Getting a specified number of lines from stdin
(Anton Felix Lorenzen)
4. Re: Getting a specified number of lines from stdin (KwangYul Seo)
----------------------------------------------------------------------
Message: 1
Date: Sun, 6 Mar 2016 12:38:47 +0200
From: Nicolaas du Preez <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Getting a specified number of lines from
stdin
Message-ID: <[email protected]>
Content-Type: text/plain; charset="windows-1252"
Good day All,
Why does
liftM (take 5) $ sequence $ repeat getLine
not stop at reading only 5 lines from stdin but keeps on reading more?
What I?m really trying to achieve is to get input from the user until
an empty line is read with the code below. But this doesn?t work
as I expected, similar to the above.
liftM (takeWhile (/= "")) $ sequence $ repeat getLine
Regards,
Nicolaas du Preez
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Message: 2
Date: Mon, 07 Mar 2016 11:32:36 +0100
From: "Henk-Jan van Tuyl" <[email protected]>
To: [email protected], "Nicolaas du Preez" <[email protected]>
Subject: Re: [Haskell-beginners] Getting a specified number of lines
from stdin
Message-ID: <op.ydx98rlopz0j5l@alquantor>
Content-Type: text/plain; charset=utf-8; format=flowed; delsp=yes
On Sun, 06 Mar 2016 11:38:47 +0100, Nicolaas du Preez
<[email protected]> wrote:
:
> Why does
>
> liftM (take 5) $ sequence $ repeat getLine
>
> not stop at reading only 5 lines from stdin but keeps on reading more?
>
> What I?m really trying to achieve is to get input from the user until
> an empty line is read with the code below. But this doesn?t work
> as I expected, similar to the above.
>
> liftM (takeWhile (/= "")) $ sequence $ repeat getLine
:
It seems that
sequence $ repeat getLine
is too strict for this; to get five lines, you could use:
sequence $ replicate 5 getLine
. To read until the first empty line, I would write something in
do-notation.
Regards,
Henk-Jan van Tuyl
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--
------------------------------
Message: 3
Date: Mon, 7 Mar 2016 12:29:36 +0100
From: Anton Felix Lorenzen <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Getting a specified number of lines
from stdin
Message-ID: <[email protected]>
Content-Type: text/plain; charset=windows-1252; format=flowed
You can define a mixture of takeWhile and sequence as:
takeWhileM :: (a -> Bool) -> [IO a] -> IO [a]
takeWhileM _ [] = return []
takeWhileM p (x:xs) = do
e <- x
if p e
then do xs' <- takeWhileM p xs
return (e : xs')
else return []
and then write:
ghci> takeWhileM (/="") (repeat getLine) >>= print
asdf
asdf
["asdf","asdf"]
Yours,
Anton Felix Lorenzen
------------------------------
Message: 4
Date: Mon, 7 Mar 2016 21:03:12 +0900
From: KwangYul Seo <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Getting a specified number of lines
from stdin
Message-ID:
<caclrxywnwo6p3q7_4el7uqyncftibhtwlgv-phz2trt7j22...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi,
Your code does not work because sequence function can't be lazy due to the
strictness of IO monad's >>= operation.
Here's a more verbose version:
readWhile :: (String -> Bool) -> IO [String]
readWhile p = do
line <- getLine
if p line
then do
lines <- readWhile p
return $ line : lines
else
return []
ghci> readWhile (/="")
foo
bar
["foo","bar"]
On Sun, Mar 6, 2016 at 7:38 PM, Nicolaas du Preez <[email protected]>
wrote:
> Good day All,
>
> Why does
>
> liftM (take 5) $ sequence $ repeat getLine
>
> not stop at reading only 5 lines from stdin but keeps on reading more?
>
> What I?m really trying to achieve is to get input from the user until
> an empty line is read with the code below. But this doesn?t work
> as I expected, similar to the above.
>
> liftM (takeWhile (/= "")) $ sequence $ repeat getLine
>
>
> Regards,
> Nicolaas du Preez
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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