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Today's Topics:
1. Re: question about show -- RWH chapter 5 (Peter Verswyvelen)
2. Re: question about show -- RWH chapter 5 (Daniel Fischer)
3. Re: question about show -- RWH chapter 5 (7stud)
4. question about styles of recursion (Michael Mossey)
5. Re: question about show -- RWH chapter 5 (7stud)
6. Re: question about show -- RWH chapter 5 (7stud)
7. Re: question about show -- RWH chapter 5
(Brandon S. Allbery KF8NH)
----------------------------------------------------------------------
Message: 1
Date: Thu, 26 Mar 2009 16:47:38 +0100
From: Peter Verswyvelen <[email protected]>
Subject: Re: [Haskell-beginners] question about show -- RWH chapter 5
To: 7stud <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Well, for JSON, I think the rendered string must be enclosed in double
quotes, which is what the show instance for String does.
So
renderJValue (JString s) = show s
is not the same as
renderJValue (JString s) = s
You can easily see this with GHCi. A copy from my Windows session:
C:\>ghci
GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude> let s = "Haskell"
Prelude> s
"Haskell"
Prelude> show s
"\"Haskell\""
Prelude> show (show s)
"\"\\\"Haskell\\\"\""
Prelude>
On Thu, Mar 26, 2009 at 4:41 PM, 7stud <[email protected]> wrote:
> In chapter 5, RWH defines a JValue data type like this:
>
>
> SimpleJSON.hs:
> --------------
>
> module SimpleJSON
> (
> JValue(..)
> ) where
>
> data JValue = JNumber Double
> | JString String
> | JArray [JValue]
> | JObject [(String, JValue)]
> | JBool Bool
> | JNull
> deriving (Eq, Ord, Show)
>
> ------------
>
>
> Then RWH defines some functions like this:
>
> PutJSON.hs:
> ----------
> module PutJSON where
>
> import SimpleJSON
>
> renderJValue::JValue->String
> renderJValue (JNumber f) = show f
> renderJValue (JString s) = show s
> renderJValue (JBool True) = "true"
> renderJValue (JBool False) = "false"
> renderJValue JNull = "null"
> ----------
>
> My question is about the function:
>
> renderJValue (JString s) = show s
>
> A JString value contains a string, so why does the function use
> show to convert s to a string? Why isn't that function defined
> like this:
>
> renderJValue (JString s) = s
>
> Using that modified function seems to work:
>
>
> Main.hs:
> ---------
> module Main () where
>
> import SimpleJSON
> import PutJSON
>
> main = let x = JString "hello"
> in putStrLn (renderJValue x)
>
>
> $ ghc -o simple Main.hs PutJSON.hs SimpleJSON.hs
> /usr/libexec/gcc/i686-apple-darwin8/4.0.1/ld: warning -F: directory name
> (/Users/me/Library/Frameworks) does not exist
>
> $ simple
> hello
>
> Also can anyone tell me why I always get that warning?
>
> Thanks
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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------------------------------
Message: 2
Date: Thu, 26 Mar 2009 16:53:58 +0100
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] question about show -- RWH chapter 5
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Am Donnerstag 26 März 2009 16:41:47 schrieb 7stud:
> In chapter 5, RWH defines a JValue data type like this:
>
>
> SimpleJSON.hs:
> --------------
>
> module SimpleJSON
> (
> JValue(..)
> ) where
>
> data JValue = JNumber Double
>
> | JString String
> | JArray [JValue]
> | JObject [(String, JValue)]
> | JBool Bool
> | JNull
>
> deriving (Eq, Ord, Show)
>
> ------------
>
>
> Then RWH defines some functions like this:
>
> PutJSON.hs:
> ----------
> module PutJSON where
>
> import SimpleJSON
>
> renderJValue::JValue->String
> renderJValue (JNumber f) = show f
> renderJValue (JString s) = show s
> renderJValue (JBool True) = "true"
> renderJValue (JBool False) = "false"
> renderJValue JNull = "null"
> ----------
>
> My question is about the function:
>
> renderJValue (JString s) = show s
>
> A JString value contains a string, so why does the function use
> show to convert s to a string? Why isn't that function defined
> like this:
>
> renderJValue (JString s) = s
The JSON values are rendered so that they can be parsed at the
other end. The parser must have the string enclosed in quotation
marks for that, also some characters need to be escaped.
Consider having
JString "true"
JString "[true, null, false]"
JString "This is a\nmultiline text.\b!"
>
> Using that modified function seems to work:
>
>
> Main.hs:
> ---------
> module Main () where
>
> import SimpleJSON
> import PutJSON
>
> main = let x = JString "hello"
> in putStrLn (renderJValue x)
>
>
> $ ghc -o simple Main.hs PutJSON.hs SimpleJSON.hs
> /usr/libexec/gcc/i686-apple-darwin8/4.0.1/ld: warning -F: directory
name
> (/Users/me/Library/Frameworks) does not exist
>
> $ simple
> hello
>
> Also can anyone tell me why I always get that warning?
Does the directory exist? If it exists, is in in the linker path?
>
> Thanks
------------------------------
Message: 3
Date: Thu, 26 Mar 2009 16:04:19 +0000 (UTC)
From: 7stud <[email protected]>
Subject: [Haskell-beginners] Re: question about show -- RWH chapter 5
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Peter Verswyvelen <bugfact <at> gmail.com> writes:
>
> Well, for JSON, I think the rendered string must be enclosed in double
> quotes,
> which is what the show instance for String does.
>
Ahh. I see. My version yields this output:
main = let x = JString "hello"
y = JNumber 10
rx = renderJValue x
ry = renderJValue y
in putStrLn ("[" ++ rx ++ ", " ++ ry ++ "]")
$ simple
[hello, 10.0]
...and that is not correct JSON format. hello looks like a variable
name--not a string. The books version produces:
["hello", 10.0]
which is what a javascript array that contains a string looks like.
Thanks.
------------------------------
Message: 4
Date: Thu, 26 Mar 2009 09:19:18 -0700
From: Michael Mossey <[email protected]>
Subject: [Haskell-beginners] question about styles of recursion
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
In RWH, in the exercises at the end of the book, I was told to write a
function that averages the integer values in a list. I wanted to do this
using on the tools we had been presented, which did not include
'length'. So I thought of writing a recursive function in which each
case passes an accumulator of the "sum so far" as well as a count of
node "up to the point", and the base case does the actual division. I
was wondering if there is a better way of doing it (using the just ideas
up to chapter 3, so no length, no higher order functions, no foldr/foldl
or map).
myAvg' :: Int -> [Int] -> [ Double ] -> Double
myAvg' sum count [] = sum / fromIntegral count
myAvg' sum count (x:xs) = myAvg' (x + sum) (n + 1) xs
myAvg :: [Double] -> Double
myAvg xs = myAvg' 0 0 xs
Thanks,
Mike
------------------------------
Message: 5
Date: Thu, 26 Mar 2009 16:20:46 +0000 (UTC)
From: 7stud <[email protected]>
Subject: [Haskell-beginners] Re: question about show -- RWH chapter 5
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Daniel Fischer <daniel.is.fischer <at> web.de> writes:
> > $ ghc -o simple Main.hs PutJSON.hs SimpleJSON.hs
> > /usr/libexec/gcc/i686-apple-darwin8/4.0.1/ld: warning -F: directory
> name
> > (/Users/me/Library/Frameworks) does not exist
> >
> > $ simple
> > hello
> >
> > Also can anyone tell me why I always get that warning?
>
> Does the directory exist? If it exists, is in in the linker path?
>
No it doesn't exist. There is a directory:
/Library/Frameworks
This is what I did to install ghc:
1) I downloaded GMP.framework and GNUreadline.framework, which my mac
automatically unzipped and placed on my desktop. I then dragged the
resulting two folders into /Library/Frameworks as per the instructions at:
http://www.haskell.org/ghc/download_ghc_682.html#macosxintel
2) I downloaded ghc-6.8.2
3) I unzipped an untared into /Users/me/my_tar_extractions
4) I cd'ed into the newly created ghc-6.8.2 folder.
5) I read the INSTALL document in the ghc-6.8.2 folder.
6) I ran the command:
$ ./configure
7) Then I ran the command:
$ sudo make install
8) At the end of the install output, I got a message that said:
-------------
Installation of ghc-6.8.2 was successful.
To use, add /usr/local/bin to your PATH.
Warning: this binary distribution does NOT contain documentation!
--------------
9) I appended /usr/local/bin onto the PATH in ~/.bash_profile.
After unzipping and untaring ghc, should I have put the
ghc-6.8.2 folder in /Library/Frameworks like I did with
GMP.framework and GNUreadline.framework? Can I still do that?
------------------------------
Message: 6
Date: Thu, 26 Mar 2009 16:26:06 +0000 (UTC)
From: 7stud <[email protected]>
Subject: [Haskell-beginners] Re: question about show -- RWH chapter 5
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
7stud <bbxx789_05ss <at> yahoo.com> writes:
> 5) I read the INSTALL document in the ghc-6.8.2 folder.
>
This is what INSTALL says:
----
This is the INSTALL instructions for a binary distribution of GHC. For
more details on what on earth this package is up to, please consult
the README and ANNOUNCE.
This distribution can be installed in a location of your choosing.
To set the ball rolling, run the configure script (as usual, run the
script with --help to see what options it supports). eg. to set up
the package for installing in directory <my-dir>, use
./configure --prefix=<my-dir>
The default installation directory is /usr/local.
The configure script will figure out what platform you're running on,
and a couple of other interesting pieces of trivia, which it will then
fill in the Makefile.in template to give you a real Makefile. If
you're of a paranoid persuasion, you might want to take a look at this
Makefile to see if the information is correct.
Now run:
make install
(`make show-install-setup' prints the details of where the different
pieces of the bundle are heading when -- possibly helpful).
For more information, full GHC documentation is available from the
main GHC site:
http://www.haskell.org/ghc
Bug reports/suggestions for improvement to the installation
procedure/setup (as well as other GHC related troubles you're
experiencing, of course), gratefully received. Bug reporting
instructions are here:
http://www.haskell.org/ghc/reportabug
Enjoy,
-- The GHC Team.
---------------
It didn't say anything about putting the ghc folder in /Library/Frameworks.
------------------------------
Message: 7
Date: Thu, 26 Mar 2009 12:43:59 -0400
From: "Brandon S. Allbery KF8NH" <[email protected]>
Subject: Re: [Haskell-beginners] question about show -- RWH chapter 5
To: 7stud <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
On 2009 Mar 26, at 11:41, 7stud wrote:
> $ ghc -o simple Main.hs PutJSON.hs SimpleJSON.hs
> /usr/libexec/gcc/i686-apple-darwin8/4.0.1/ld: warning -F: directory
> name
> (/Users/me/Library/Frameworks) does not exist
My guess is that the binary ghc was compiled to allow for use of
private frameworks, so you get an unnecessary warning when you don't
have any. Just create ~/Library/Frameworks if you want the warning to
go away.
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [email protected]
system administrator [openafs,heimdal,too many hats] [email protected]
electrical and computer engineering, carnegie mellon university KF8NH
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