[Haskell-cafe] manage effects in a DSL

[adam vogt]

Hi Corentin,

Oleg's January 31 message has: `ReadAccount  :: Exp r Int'. If you use that
version you can define that last `hasEffect :: Exp Effect ()'. Then bind
can just be:

Bind :: Exp m a -> (a -> Exp m b) -> Exp m b

And the `m' from a non-effectful thing (ReadAccount) will be set to Effect
when you bind it with an effectful computation. You can still have
operations that take arguments like `Exp NoEffect a', which will give you a
type error when you pass in a an argument tagged with Effect.

Adam


On Mon, Feb 3, 2014 at 6:44 AM, Corentin Dupont
<corentin.dupont at gmail.com>wrote:

> 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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