Re: [Haskell-cafe] Help using CGIT

On Aug 23, 2007, at 3:34 , Rich Neswold wrote:

On 8/22/07, Ian Lynagh wrote: On Wed, Aug 22, 2007 at 01:27:00PM -0500, Rich Neswold wrote:

...

...

newtype App a = App (ReaderT Connection (CGIT IO) a) deriving (Monad, MonadIO, MonadReader Connection)

Unfortunately, when another module tries to actually use the

monad, I

...

get warnings about "No instance for (MonadCGI App)". I tried making an instance:

...

instance MonadCGI App where cgiAddHeader = ? cgiGet = ?

You have three choices:

1:

2:

3: Provide a single instance for App that does the whole thing: instance MonadCGI App where cgiAddHeader n v = App $ lift $ cgiAddHeader n v cgiGet x = App $ lift $ cgiGet x This one you would obviously have to change if you added a StateT.

Bingo! Method #3 works beautifully! I missed the using-lift-with- the-constructor permutation.

Thanks for your help!

I started writing a tutorial for Haskell web programming with the cgi package a while back, but haven't worked on it for a while, see http://www.haskell.org/haskellwiki/Practical_web_programming_in_Haskell I haven't added it to the list of tutorials yet, since it's still rather incomplete. The section on using CGIT is just a stub, perhaps you would like to contribute to it? See http://www.haskell.org/haskellwiki/ Practical_web_programming_in_Haskell#Extending_the_CGI_monad_with_monad_ transformers /Björn