[Haskell-cafe] Re: wanted: HAppS example combining state and io

[Martin Lütke]

Thomas Hartman <thomas.hartman <at> db.com> writes:

> 
> 
> In the latest happs (darcs pulled, updated
> head is 0.9.1 iirc), I am experimenting with the example file in
src/HAppS/Examples/HTTP1.hs.
> I would like to combine state with io.
> Eventually io will mean stuff like reading from a database, but for now
> I'm just reading a file.
> The example file HTTP1.hs has an example
> that demonstrates state with macid.
> I added an example that allows you to
> execute arbitrary io.
> I tried, but was unable to, add a handler
> that combines state and io. 
>          
>    ,  h ["iohandler"]    GET $ ioReadFileHandler
>          
>    ,  h ["statehandler"] GET $ stateHandler
>          
>    --,  h ["ioandstatehandler"] GET $ ioAndStateHandler
> .....
> -- displays contents of HAPPS.hs in
> current directory
> ioReadFileHandler = iohandler $ readFile
> "./HAppS.hs" 
> -- displays incremented state counter
> stateHandler = ok $ \() () -> 
>          
>          modify (+(1::Int)) >> get >>=
> respond . show
> -- should combine effect of iohandler
> with statehandler
> -- specifically, should display contents
> of HAppS.hs, and under that an incremented state handler
> -- is this possible
> ioAndStateHandler = undefined undefined
> Is mixing state and io possible with
> HAppS? If so, an example showing how to do it would be extremely helpful.
> best, thomas.
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> 
> Attachment (https1-whatsnew): application/octet-stream, 1933 bytes
> Attachment (HTTP1.hs): application/octet-stream, 4794 bytes
> 
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I had no trouble getting this handler to work:

    h ["iohandler"] GET $ \() () -> do
            modify (+1)
            x <- get
            respond $ do
                cnts <- readFile "./sometext.txt"
                sresult 200 (cnts ++ show x)

I believe the trick is that you cant mix io INTO the HAppS ServerPart monad. 
But from the ServerPart monad you can RETURN an io action.