On Aug 23, 2007, at 3:34 , Rich Neswold wrote:
On 8/22/07, Ian Lynagh <[EMAIL PROTECTED]> 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_______________________________________________
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