[Haskell-cafe] Help using CGIT

Rich Neswold rich.neswold at gmail.com
Wed Aug 22 14:27:00 EDT 2007


I've been having a tough time trying to use the CGI monad transformer
(CGIT). Hopefully someone can show me my misstep(s).

I have a CGI script that's evolving. Early on, it needed a single
database access. Now it's doing two accesses (and it looks like I'll
be adding more.) Rather than making a connection for each access, the
script needs to connect once. To do this, I want to combine the CGI
monad with the Reader monad.

This version compiles cleanly:

> module AppMonad (App (..), runApp)
>     where
> import Control.Exception (bracket)
> import Control.Monad.Reader
> import Network.CGI.Monad
> import Network.CGI.Protocol
> import System.IO (stdin, stdout)
> import Database.HSQL.PostgreSQL
> newtype App a = App (ReaderT Connection (CGIT IO) a)
>    deriving (Monad, MonadIO, MonadReader Connection)
> runApp :: App CGIResult -> IO ()
> runApp (App a) =
>     bracket (connect "host" "dbname" "user" "password")
>             disconnect
>             (\c -> do { env <- getCGIVars
>                       ; hRunCGI env stdin stdout (runCGIT (runReaderT a c))
>                       ; return () } )

Unfortunately, when another module tries to actually use the monad, I
get warnings about "No instance for (MonadCGI App)". I tried making an

> instance MonadCGI App where
>     cgiAddHeader = ?
>     cgiGet = ?

But I don't know how to define these functions. I tried various
'lift'ing combinations, but couldn't come up with a solution that
would compile. I'm also disappointed that I had to break apart
'runCGI' (by cut-and-pasting its source) because I couldn't make it
believe my monad looked enough like MonadCGI.

My previous experiment with monad transformers was successful. It
didn't use CGIT, however, so the 'run*' functions were simpler.

Does anyone have an example of using CGIT (I didn't find any from
Google)? Shouldn't I be able to use 'runCGI' with my monad? CGIT users
shouldn't be required to re-implement 'runCGI", right?

Any help or ideas is appreciated!


JID: rich at neswold.homeunix.net
AIM: rnezzy

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