[Haskell-cafe] One liner?

2008-10-02 Thread Paul Keir 
Hi all,

There's a common little situation I keep bumping up against. I don't understand 
where I'm going wrong, so I've made a little example. It's to do with binding a 
result to a variable name using -. This code works fine:

--
module Main where

import System.Directory (getDirectoryContents)

main = do dc - getDirectoryContents ./foo/
  mapM_ putStrLn dc
--

But if I try to avoid the use of the bind to dc, I fail:

--
mapM_ putStrLn (getDirectoryContents ./foo/)
--

I've tried using map instead of mapM_, and inserted returns here and there, 
but no luck. Can anyone tell me where and why I'm going wrong? The error 
message is below.

Cheers,
Paul


Couldn't match expected type `[String]'
   against inferred type `IO [FilePath]'
In the second argument of `mapM_', namely
`(getDirectoryContents ./foo/)'
In the expression: mapM_ putStrLn (getDirectoryContents ./foo/)
In the definition of `main':
main = mapM_ putStrLn (getDirectoryContents ./foo/)
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RE: [Haskell-cafe] One liner?

2008-10-02 Thread Mitchell, Neil 
Hi
 
You can translate this step by step.
 
main = do dc - getDirectoryContents ./foo/
  mapM_ putStrLn dc

Translating out the do notation
(http://www.haskell.org/haskellwiki/Keywords#do):

main = getDirectoryContents = \dc -
   mapM_ putStrLn dc

Then we can chop out the dc argument, as its \x -  x, and can be
removed (eta reduction):

main = getDirectoryContents = 
   mapM_ putStrLn

And finally we just remove the newline:

main = getDirectoryContents = mapM_ putStrLn  

Alternatively, we can flip the = for = and write:

main = mapM_ putStrLn = getDirectoryContents

This is now one line, and mirrors how you would write the function if it
was pure using function composition.

Thanks

Neil
 

This material is sales and trading commentary and does not constitute
investment research. Please follow the attached hyperlink to an
important disclaimer 
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From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Paul Keir
Sent: 02 October 2008 4:20 pm
To: haskell-cafe@haskell.org
Subject: [Haskell-cafe] One liner?



Hi all,

There's a common little situation I keep bumping up against. I
don't understand where I'm going wrong, so I've made a little example.
It's to do with binding a result to a variable name using -. This
code works fine:

--
module Main where

import System.Directory (getDirectoryContents)

main = do dc - getDirectoryContents ./foo/
  mapM_ putStrLn dc
--

But if I try to avoid the use of the bind to dc, I fail:

--
mapM_ putStrLn (getDirectoryContents ./foo/)
--

I've tried using map instead of mapM_, and inserted returns
here and there, but no luck. Can anyone tell me where and why I'm going
wrong? The error message is below.

Cheers,
Paul


Couldn't match expected type `[String]'
   against inferred type `IO [FilePath]'
In the second argument of `mapM_', namely
`(getDirectoryContents ./foo/)'
In the expression: mapM_ putStrLn (getDirectoryContents
./foo/)
In the definition of `main':
main = mapM_ putStrLn (getDirectoryContents ./foo/)



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Re: [Haskell-cafe] One liner?

2008-10-02 Thread Ketil Malde 
Paul Keir [EMAIL PROTECTED] writes:

 module Main where

 import System.Directory (getDirectoryContents)

 main = do dc - getDirectoryContents ./foo/
   mapM_ putStrLn dc


 mapM_ putStrLn (getDirectoryContents ./foo/)

 Couldn't match expected type `[String]'

mapM_ putStrLn needs a [String] as an argument, 

against inferred type `IO [FilePath]'

but you try to give it an IO [FilePath] (i.e. IO [String]).

As you probably know, do-notation is syntactic sugar for the monad
operators, and you can rewrite your function thusly:

  main = do dc - getDirectoryContents ./foo/
mapM_ putStrLn dc

=

  main = getDirectoryContents ./foo = \dc - mapM_ putStrLn dc

=

  main = getDirectoryContents ./foo = mapM_ putStrLn

-k
-- 
If I haven't seen further, it is by standing in the footprints of giants
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RE: [Haskell-cafe] One liner?

2008-10-02 Thread Mitchell, Neil 
 main = do dc - getDirectoryContents ./foo/
   mapM_ putStrLn dc
 
 Translating out the do notation
 (http://www.haskell.org/haskellwiki/Keywords#do):
 
 main = getDirectoryContents = \dc -
mapM_ putStrLn dc

Woops, I lost ./foo/ here, but it should be fairly easy to insert
through the remaining translations.

Thanks

Neil

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RE: [Haskell-cafe] One liner?

2008-10-02 Thread Paul Keir 
Thanks, and to Ketil too. I did see past the missing ./foo/. That's certainly 
a solution I'm happy with, and I didn't know the term eta reduction, so thanks 
for that too.
Paul


-Original Message-
From: Mitchell, Neil [mailto:[EMAIL PROTECTED]
Sent: Thu 02/10/2008 16:26
To: Paul Keir; haskell-cafe@haskell.org
Subject: RE: [Haskell-cafe] One liner?
 
Hi
 
You can translate this step by step.
 
main = do dc - getDirectoryContents ./foo/
  mapM_ putStrLn dc

Translating out the do notation
(http://www.haskell.org/haskellwiki/Keywords#do):

main = getDirectoryContents = \dc -
   mapM_ putStrLn dc

Then we can chop out the dc argument, as its \x -  x, and can be
removed (eta reduction):

main = getDirectoryContents = 
   mapM_ putStrLn

And finally we just remove the newline:

main = getDirectoryContents = mapM_ putStrLn  

Alternatively, we can flip the = for = and write:

main = mapM_ putStrLn = getDirectoryContents

This is now one line, and mirrors how you would write the function if it
was pure using function composition.

Thanks

Neil
 

This material is sales and trading commentary and does not constitute
investment research. Please follow the attached hyperlink to an
important disclaimer 
www.credit-suisse.com/emea/legal
outbind://31/www.credit-suisse.com/emea/legal  

 




From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Paul Keir
Sent: 02 October 2008 4:20 pm
To: haskell-cafe@haskell.org
Subject: [Haskell-cafe] One liner?



Hi all,

There's a common little situation I keep bumping up against. I
don't understand where I'm going wrong, so I've made a little example.
It's to do with binding a result to a variable name using -. This
code works fine:

--
module Main where

import System.Directory (getDirectoryContents)

main = do dc - getDirectoryContents ./foo/
  mapM_ putStrLn dc
--

But if I try to avoid the use of the bind to dc, I fail:

--
mapM_ putStrLn (getDirectoryContents ./foo/)
--

I've tried using map instead of mapM_, and inserted returns
here and there, but no luck. Can anyone tell me where and why I'm going
wrong? The error message is below.

Cheers,
Paul


Couldn't match expected type `[String]'
   against inferred type `IO [FilePath]'
In the second argument of `mapM_', namely
`(getDirectoryContents ./foo/)'
In the expression: mapM_ putStrLn (getDirectoryContents
./foo/)
In the definition of `main':
main = mapM_ putStrLn (getDirectoryContents ./foo/)



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Re[2]: [Haskell-cafe] One liner?

2008-10-02 Thread Bulat Ziganshin 
Hello Neil,

Thursday, October 2, 2008, 7:26:23 PM, you wrote:

shortly speaking, getDirectoryContents is an action (having
IO a type) while second mapM_ argument should be a value returned by
this action. by using dc variable or = operator, you can evaluate
action and pass its result to mapM_. of course, after imperative
languages experience, it's hard to see difference. take look at
http://haskell.org/haskellwiki/IO_inside


 Hi
  
 You can translate this step by step.
  
 main = do dc - getDirectoryContents ./foo/
   mapM_ putStrLn dc

 Translating out the do notation
 (http://www.haskell.org/haskellwiki/Keywords#do):

 main = getDirectoryContents = \dc -
mapM_ putStrLn dc

 Then we can chop out the dc argument, as its \x -  x, and can be
 removed (eta reduction):

 main = getDirectoryContents = 
mapM_ putStrLn

 And finally we just remove the newline:

 main = getDirectoryContents = mapM_ putStrLn  

 Alternatively, we can flip the = for = and write:

 main = mapM_ putStrLn = getDirectoryContents

 This is now one line, and mirrors how you would write the function if it
 was pure using function composition.

 Thanks

 Neil
  

 This material is sales and trading commentary and does not constitute
 investment research. Please follow the attached hyperlink to an
 important disclaimer 
 www.credit-suisse.com/emea/legal
 outbind://31/www.credit-suisse.com/emea/legal  

  


 

 From: [EMAIL PROTECTED]
 [mailto:[EMAIL PROTECTED] On Behalf Of Paul Keir
 Sent: 02 October 2008 4:20 pm
 To: haskell-cafe@haskell.org
 Subject: [Haskell-cafe] One liner?
 
 

 Hi all,
 
 There's a common little situation I keep bumping up against. I
 don't understand where I'm going wrong, so I've made a little example.
 It's to do with binding a result to a variable name using -. This
 code works fine:
 
 --
 module Main where
 
 import System.Directory (getDirectoryContents)
 
 main = do dc - getDirectoryContents ./foo/
   mapM_ putStrLn dc
 --
 
 But if I try to avoid the use of the bind to dc, I fail:
 
 --
 mapM_ putStrLn (getDirectoryContents ./foo/)
 --
 
 I've tried using map instead of mapM_, and inserted returns
 here and there, but no luck. Can anyone tell me where and why I'm going
 wrong? The error message is below.
 
 Cheers,
 Paul
 
 
 Couldn't match expected type `[String]'
against inferred type `IO [FilePath]'
 In the second argument of `mapM_', namely
 `(getDirectoryContents ./foo/)'
 In the expression: mapM_ putStrLn (getDirectoryContents
 ./foo/)
 In the definition of `main':
 main = mapM_ putStrLn (getDirectoryContents ./foo/)
 


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 Please access the attached hyperlink for an important electronic 
 communications disclaimer:

 http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html
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-- 
Best regards,
 Bulatmailto:[EMAIL PROTECTED]

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