[Haskell-cafe] Help using CGIT
bringert at cs.chalmers.se
Fri Aug 24 03:38:15 EDT 2007
On Aug 23, 2007, at 3:34 , Rich Neswold wrote:
> On 8/22/07, Ian Lynagh <igloo at earth.li> 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:
> 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
I haven't added it to the list of tutorials yet, since it's still
The section on using CGIT is just a stub, perhaps you would like to
contribute to it? See
More information about the Haskell-Cafe