[Haskell-cafe] manage effects in a DSL

[adam vogt]

Hi Corentin,

You need change Effect to NoEffect to reuse the evalNoEffect:

> eval ReadAccount = liftEval $ evalNoEffect ReadAccount
> eval (Return a)     = liftEval $ evalNoEffect (Return a)

Or use a function like `unsafeCoerce :: Nomex Effect a -> Nomex NoEffect a`.

If you rename the types that tag effects to something that describes
exactly what the tags actually represent, maybe the above definition will
be more satisfying:

> data Effects
>  =
HasBeenCombinedWithSomethingThatHasEffectsButICan'tBeSureItActuallyHasEffectsAllByItself
>   | DefinitelyHasNoEffects


Regards,
Adam


On Thu, Feb 6, 2014 at 9:50 AM, Corentin Dupont
<corentin.dupont at gmail.com>wrote:

> Hi guys,
> I'm still exploring some design space for DSLs, following our interesting
> discussion.
>
> I'm trying to write the evaluator for the DSL (see below).
> For the general case, the evaluator looks like:
>
> eval :: Nomex r a -> State Game a
>
> This eval function takes an expression (called Nomex), that can possibly
> have effects.
> It returns a state monad, to allow you to modify the game state.
>
> But for effectless instructions, it would be better to run the evaluator
> in the reader monad:
>
> evalNoEffect :: Nomex NoEffect a -> Reader Game a
>
> So you can have additional guaranties that evaluating your expression will
> not have effects.
> I tried (see below), but it doesn't work for the moment:
>
>
> > {-# LANGUAGE GADTs #-}
>
> > {-# LANGUAGE KindSignatures, DataKinds, ScopedTypeVariables,
> >   MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
> UndecidableInstances #-}
>
> > module DSLEffects where
>
> > import Control.Monad.Error
> > import Control.Monad.State
> > import Control.Monad.Reader
> > import Data.Typeable
>
> This is the DSL:
>
> > data Effects = Effect | NoEffect
>
> > data Nomex :: Effects -> * -> * where
> >   ReadAccount  :: Nomex r Int                --ReadAccount has no
> effect: it can be run in whatever monad
> >   WriteAccount :: Int -> Nomex Effect ()  --WriteAccount has effect
> >   SetVictory   :: Nomex NoEffect Bool -> Nomex Effect () --SetVictory
> don't accept effectful computations
> >   Bind         :: Nomex m a -> (a -> Nomex m b) -> Nomex m b
> >   Return       :: a -> Nomex r a  --wrapping a constant has no effect
>
> > instance Monad (Nomex a) where
> >   return = Return
> >   (>>=) = Bind
>
>
> > noEff :: Nomex NoEffect ()
> > noEff = return ()
>
> > hasEffect :: Nomex Effect ()
> > hasEffect = do
> >    a <- ReadAccount
> >    WriteAccount a
>
> > data Game = Game { victory :: Nomex NoEffect Bool,
> >                    account :: Int}
>
>
> > eval :: Nomex r a -> State Game a
> > eval a at ReadAccount    = liftEval $ evalNoEffect a
> > eval (WriteAccount a) = modify (\g -> g{account = a})
>
> > eval (SetVictory v)   = modify (\g -> g{victory = v})
> > eval a@(Return _)     = liftEval $ evalNoEffect a
> > eval (Bind exp f)     = eval exp >>= eval . f
>
> > evalNoEffect :: Nomex NoEffect a -> Reader Game a
> > evalNoEffect ReadAccount  = asks account
> > evalNoEffect (Return a)   = return a
> > evalNoEffect (Bind exp f) = evalNoEffect exp >>= evalNoEffect . f
>
> > liftEval :: Reader Game a -> State Game a
> > liftEval r = get >>= return . runReader r
>
>
> This is not compiling:
>
> exceptEffect.lhs:60:15:
>     Couldn't match type 'NoEffect with 'Effect
>     Inaccessible code in
>       a pattern with constructor
>         WriteAccount :: Int -> Nomex 'Effect (),
>       in an equation for `evalEffect'
>     In the pattern: WriteAccount a
>     In an equation for `evalEffect':
>         evalEffect (WriteAccount a) = modify (\ g -> g {account = a})
>
> It seems that the type of effectless computations (NoEffect) leaks in the
> type of effectful ones (due to the pattern matching)...
>
> Thanks,
> Corentin
>
>
>
> On Mon, Feb 3, 2014 at 12:44 PM, 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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