[Haskell-cafe] manage effects in a DSL

[Corentin Dupont]

Hi Adam,
yes, on second thought, that's really the simpler.
I was just disappointed that the type signature of "ReadAccount", an
operation with no effect, would not be "Exp NoEffect ()".
:)


On Mon, Feb 3, 2014 at 6:47 PM, adam vogt <vogt.adam at gmail.com> wrote:

> 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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>>
>
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