[Haskell-cafe] what exactly does deriving (Functor, Monad, MonadIO) do?

2007-05-01 Thread Thomas Hartman 

I was trying to follow the reasoning in Don's article on using haskell
for shell scripting

 http://cgi.cse.unsw.edu.au/~dons/blog/2007/03/10

In the source listing at the end we is

 newtype Shell a = Shell { runShell :: ErrorT String IO a }
   deriving (Functor, Monad, MonadIO)

and I don't understand it what deriving is doing here, nor have I
been able to find documentation on it.

 http://en.wikibooks.org/wiki/Haskell/Class_declarations

claims:

You can only use deriving with a limited set of built-in classes. They are:

Eq  Ord Enum Bounded  Show  Read 

But, here we are deriving classes not in that list. So, is this a
recently added feature? Or something that came in from

 {-# OPTIONS -fglasgow-exts #-} ?

I would just like to understand this, and I can't figure out how to begin.

Thanks for any help!

thomas.
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Re: [Haskell-cafe] what exactly does deriving (Functor, Monad, MonadIO) do?

2007-05-01 Thread Donald Bruce Stewart 
tphyahoo:
 I was trying to follow the reasoning in Don's article on using haskell
 for shell scripting
 
  http://cgi.cse.unsw.edu.au/~dons/blog/2007/03/10
 
 In the source listing at the end we is
 
  newtype Shell a = Shell { runShell :: ErrorT String IO a }
deriving (Functor, Monad, MonadIO)
 
 and I don't understand it what deriving is doing here, nor have I
 been able to find documentation on it.

That's 'cunning newtype deriving, my new favourite ghc language
extension.


http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#newtype-deriving

We also use it in xmonad,

newtype X a = X (ReaderT XConf (StateT XState IO) a)
deriving (Functor, Monad, MonadIO, MonadState XState, MonadReader XConf)

:-)

-- Don
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Re: [Haskell-cafe] what exactly does deriving (Functor, Monad, MonadIO) do?

2007-05-01 Thread Thomas Hartman 

Thanks Dons.

There's also a short and sweet explanation here.

http://hackage.haskell.org/trac/haskell-prime/wiki/NewtypeDeriving

I am going to try and wrap my head around this, as I am very
interested in solutions for haskell / shell interaction.

Are there are any good examples of code written without this
extension, alongside code condensed by using this extension. That
would be helpful for understanding what's going on.

Thomas.



2007/5/1, Donald Bruce Stewart [EMAIL PROTECTED]:

tphyahoo:
 I was trying to follow the reasoning in Don's article on using haskell
 for shell scripting

  http://cgi.cse.unsw.edu.au/~dons/blog/2007/03/10

 In the source listing at the end we is

  newtype Shell a = Shell { runShell :: ErrorT String IO a }
deriving (Functor, Monad, MonadIO)

 and I don't understand it what deriving is doing here, nor have I
 been able to find documentation on it.

That's 'cunning newtype deriving, my new favourite ghc language
extension.


http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#newtype-deriving

We also use it in xmonad,

newtype X a = X (ReaderT XConf (StateT XState IO) a)
deriving (Functor, Monad, MonadIO, MonadState XState, MonadReader XConf)

:-)

-- Don


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Re: [Haskell-cafe] what exactly does deriving (Functor, Monad, MonadIO) do?

2007-05-01 Thread Brandon S. Allbery KF8NH 


On May 1, 2007, at 6:05 , Thomas Hartman wrote:


Are there are any good examples of code written without this
extension, alongside code condensed by using this extension. That
would be helpful for understanding what's going on.


I think all this does is save you from having to write a bunch of  
wrappers that unwrap the contained value, do something to it, and  
rewrap the result.


--
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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Re: [Haskell-cafe] what exactly does deriving (Functor, Monad, MonadIO) do?

2007-05-01 Thread David House 

On 01/05/07, Brandon S. Allbery KF8NH [EMAIL PROTECTED] wrote:

I think all this does is save you from having to write a bunch of
wrappers that unwrap the contained value, do something to it, and
rewrap the result.


Exactly. Basically what newtype deriving does is if you have a
declaration like the following:

 newtype T = TConstructor M

And M instantiates some class (like Monad, Functor etc), you can
derive that class for T. For example, here's how the Functor instance
would look for the following newtype:

 newtype MyMaybe a = MM (Maybe a) deriving (Functor)

 -- The instance looks like this:
 instance Functor MyMaybe where
   fmap f (MM a) = MM (fmap f a)

The instance just unwraps and rewraps the newtype constructor.

--
-David House, [EMAIL PROTECTED]
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