[Haskell-cafe] Design of protocol implementation?

[amindfv at gmail.com]

El Mar 5, 2015, a las 5:58, Christopher Done <chrisdone at gmail.com> escribió:

> Right, what you have there is the simplest implementation and it seems
> pretty common to me, for RPC-style services.
> 
> The only adjustments I like to make is to ensure input and return are
> matched up.
> 
> Another way I've taken to is e.g. in Fay:
> 
> data Returns a = Returns
> data Command =
>  Procedure Int Char (Returns (Text,Maybe [Int]))
> 
> call :: (Returns a -> Command) -> IO a
> call = …
> 
> produce :: Returns a -> IO a -> IO a
> produce _ m = m
> 
> handle :: Command -> IO ()
> handle (Procedure i c r) = produce r (procedure i c)
> 
> Another way I've taken was in e.g. ghc-server was to use a GADT:
> 
>    data Command a where
>      LoadTarget :: Text -> Command (Producer Msg (SuccessFlag,Integer))
>      Eval :: Text -> Command (Duplex Text Text EvalResult)
>      Ping :: Integer -> Command (Returns Integer)
>      TypeOf :: Text -> Command (Returns Text)
>      LocationAt :: FilePath -> Text -> Int -> Int -> Int -> Int ->
> Command (Returns SrcSpan)
>      TypeAt :: FilePath -> Text -> Int -> Int -> Int -> Int ->
> Command (Returns Text)
>      UsesAt :: FilePath -> Text -> Int -> Int -> Int -> Int ->
> Command (Returns Text)
>      KindOf :: Text -> Command (Returns Text)
>      InfoOf :: Text -> Command (Returns [Text])
>      Set :: Text -> Command (Returns ())
>      PackageConf :: FilePath -> Command (Returns ())
>      SetCurrentDir :: FilePath -> Command (Returns ())
> 
> My type looks like a pipe or a conduit, but was more involved because
> it was a duplex:
> 
>    type Duplex i o r = DuplexT IO i o r
>    type Producer o r = Duplex () o r
>    type Returns r = Duplex () () r
>    type Unit = Duplex () () ()
> 
> But in essence the commands could return a result and/or accept
> incoming input and produce output. E.g. evaluating a line of Haskell
> lets you send/receive stdin/stdout and eventually return a
> success/fail result.
> 
> I wrote a few lines of TH to do the dispatching code.
> 
> I've also seen an approach using type-level literals to put strings in
> the types to use dispatching, but I can't remember who wrote it.
> 
> You can also use e.g. closed type families to express a finite state
> machine that this-must-happen-before-that-state etc. if you really
> want to ensure every state change is valid.
> 
> It's hard to find links to any of these things I've seen, not sure
> what the keywords are.



The search term for this is "session types" -- iirc there's a nice example in spj's paper "fun with type families"

Tom





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