I would like to use lens and extensible-effects for a simple example.
The error messages say that the type is ambiguous due to typeclass with the HasObj x parameter, and GHC cannot figure out where pos HasObj x from, I think.
makeClassy really useful to define abstract data types with common data functions, so this is necessary in my real project.
My questions are: how can I make it work?
Or is there a way to define lenses from data types and use them in Eff.State?
{-# LANGUAGE TemplateHaskell, TypeOperators, DeriveDataTypeable #-} import Control.Eff import Control.Eff.Lift import Control.Eff.State.Strict import Control.Lens hiding (use, (%=), (.=)) import Control.Monad import Data.Typeable -- define data types data Obj = Obj { _pos :: Int } deriving (Show) data Person = Person { _objPerson :: Obj, _greeting :: String } deriving (Show, Typeable) -- make typeclasses and lenses makeClassy ''Obj makeLenses ''Person instance HasObj Person where obj = objPerson -- define lens operators in Eff use k = fmap (^.k) get l %= f = modify (l %~ f) l .= f = modify (l .~ f) -- update :: StateT Person IO () update :: Eff (State Person :> Lift IO :> ()) () update = do lift . putStrLn =<< use greeting pos %= (+10) p <- use pos when (p >= 100) $ do greeting .= "at 100, yeah!" main = do let p = Person (Obj 0) "hi!" print $ p p' <- runLift $ execState p update print $ p'
Error messages:
LensEE.hs:28:3: No instance for (HasObj s0) arising from a use of 'pos' The type variable 's0' is ambiguous Note: there are several potential instances: instance HasObj Obj -- Defined at LensEE.hs:15:1 instance HasObj Person -- Defined at LensEE.hs:18:10 In the first argument of '(%=)', namely 'pos' In a stmt of a 'do' block: pos %= (+ 10) In the expression: do { lift . putStrLn =<< use greeting; pos %= (+ 10); p <- use pos; when (p >= 100) $ do { greeting .= "at 100, yeah!" } } LensEE.hs:28:7: Overlapping instances for Member (State s0) (State Person :> (Lift IO :> ())) arising from a use of '%=' Matching instances: instance [overlap ok] Member tr => Member t (t' :> r) -- Defined in 'Data.OpenUnion1' instance [overlap ok] Member t (t :> r) -- Defined in 'Data.OpenUnion1' (The choice depends on the instantiation of 's0' To pick the first instance above, use IncoherentInstances when compiling the other instance declarations) In a stmt of a 'do' block: pos %= (+ 10) In the expression: do { lift . putStrLn =<< use greeting; pos %= (+ 10); p <- use pos; when (p >= 100) $ do { greeting .= "at 100, yeah!" } } In an equation for 'update': update = do { lift . putStrLn =<< use greeting; pos %= (+ 10); p <- use pos; .... } LensEE.hs:29:8: Overlapping instances for Member (State a0) (State Person :> (Lift IO :> ())) arising from a use of 'use' Matching instances: instance [overlap ok] Member tr => Member t (t' :> r) -- Defined in 'Data.OpenUnion1' instance [overlap ok] Member t (t :> r) -- Defined in 'Data.OpenUnion1' (The choice depends on the instantiation of 'a0' To pick the first instance above, use IncoherentInstances when compiling the other instance declarations) In a stmt of a 'do' block: p <- use pos In the expression: do { lift . putStrLn =<< use greeting; pos %= (+ 10); p <- use pos; when (p >= 100) $ do { greeting .= "at 100, yeah!" } } In an equation for 'update': update = do { lift . putStrLn =<< use greeting; pos %= (+ 10); p <- use pos; .... } LensEE.hs:29:12: No instance for (HasObj a0) arising from a use of 'pos' The type variable 'a0' is ambiguous Note: there are several potential instances: instance HasObj Obj -- Defined at LensEE.hs:15:1 instance HasObj Person -- Defined at LensEE.hs:18:10 In the first argument of 'use', namely 'pos' In a stmt of a 'do' block: p <- use pos In the expression: do { lift . putStrLn =<< use greeting; pos %= (+ 10); p <- use pos; when (p >= 100) $ do { greeting .= "at 100, yeah!" } }
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