[Haskell-beginners] question about show -- RWH chapter 5

[Peter Verswyvelen]

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 <bbxx789_05ss at yahoo.com> 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
>
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>
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