[Haskell-cafe] manage effects in a DSL

[Corentin Dupont]

I saw that to write liftQD you decontruct (unwrap) the type and reconstruct
it.
I don't know if I can do that for my Exp (which is a full DSL)...

Anyway, there should be a way to encode the Effect/NoEffect semantic at
type level...
Using Oleg's parametrized monad idea (
http://hackage.haskell.org/package/monad-param-0.0.2/docs/Control-Monad-Parameterized.html),
I tried:

> {-# LANGUAGE KindSignatures, DataKinds, ScopedTypeVariables, GADTs
>   MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
UndecidableInstances #-}

> module DSLEffects where
> import Prelude hiding (return, (>>), (>>=))
> import Control.Monad.Parameterized

This data type will be promoted to kind level (thanks to DataKinds):

> data Eff = Effect | NoEffect

This class allows to specify the semantic on Effects (Effect + NoEffect =
Effect):

> class Effects (m :: Eff) (n::Eff) (r::Eff) | m n -> r
> instance Effects Effect n Effect
> instance Effects NoEffect n n

This is the DSL:

> data Exp :: Eff -> * -> * where
>   ReadAccount  :: Exp NoEffect Int      --ReadAccount has no effect
>   WriteAccount :: Int -> Exp Effect ()  --WriteAccount has effect
>   Const        :: a -> Exp r a
>   Bind         :: Effects m n r => Exp m a -> (a -> Exp n b) -> Exp r b
--Bind comes with a semantic on effects
>   Fmap         :: (a -> b) -> Exp m a -> Exp m b

> instance Functor (Exp r) where
>   fmap = Fmap

> instance Return (Exp r) where
>    returnM = Const

> instance (Effects m n r) => Bind (Exp m) (Exp n) (Exp r) where
>    (>>=) = Bind

> noEff :: Exp NoEffect ()
> noEff = returnM ()

> hasEffect :: Exp Effect ()
> hasEffect = ReadAccount >> (returnM () :: Exp Effect ())

This is working more or less, however I am obliged to put the type
signature on the returnM (last line): why?
Furthermore, I cannot write directly:

> hasEffect :: Exp Effect ()
> hasEffect = ReadAccount


Do you have a better idea?



On Sun, Feb 2, 2014 at 8:55 PM, Lindsey Kuper <lindsey at composition.al>wrote:

> On Sun, Feb 2, 2014 at 2:42 PM, Corentin Dupont
> <corentin.dupont at gmail.com> wrote:
> > you should be able to run an effectless monad in an effectful one.
> > How to encode this semantic?
>
> In LVish we just have a `liftQD` operation that will let you lift a
> deterministic computation to a quasi-deterministic one (recall that
> deterministic computations can perform fewer effects):
>
>   liftQD :: Par Det s a -> Par QuasiDet s a
>
> So, analogously, you could have a `liftEff` and then write `liftEff
> noEff`.  This is also a little bit ugly, but you may find you don't
> have to do it very often (we rarely use `liftQD`).
>
> Lindsey
>
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