Mitchell,
You could pass in another argument that specifies which 's', which might
not be as much of a pain if it was an implicit parameter like here: <
http://lpaste.net/154303>.
Another way to resolve the ambiguity would be to say that foo1 accesses
first State in the list:
type family OuterState xs where
OuterState (State s ': rest) = s
OuterState (x ': xs) = OuterState xs
-- using ScopedTypeVariables
foo1 :: forall r s. (OuterState r ~ s, Member (State s) r, HasInt s) => Eff
r Int
foo1 = getInt <$> (get :: Eff r s)
But I think you probably should just pin down the state type you're
accessing because you can have multiple `State s`and they don't get in each
other’s way at all if they have different types.
get2 :: Eff '[State Int, State Char] (Int,Char)
get2 = do i <- get; c <- get; return (i,c) -- works just fine
Regards,
Adam
On Tue, Mar 8, 2016 at 7:02 PM, Mitchell Rosen
Sorry, in "foo", the body should be "fmap getInt get". Still, same type error.
On Tuesday, March 8, 2016 at 3:59:23 PM UTC-8, Mitchell Rosen wrote:
Hi all,
I'm trying to combine an extensible effects style state with classy lenses. That is, instead of pinning the type of my state down, I'd like to only require the pieces I need.
For example,
{-# language FlexibleContexts #-}
import Control.Monad.Freer import Control.Monad.Freer.State
class HasInt s where getInt :: s -> Int
foo :: (Member (State s) effs, HasInt s) => Eff effs Int foo = get
However, this fails to typecheck:
Overlapping instances for Member (State s0) effs Matching givens (or their superclasses): (Member (State s) effs) bound by the type signature for foo :: (Member (State s) effs, HasInt s) => Eff effs Int at example.hs:9:8-56 Matching instances: instance Data.Open.Union.Member' t r (Data.Open.Union.FindElem t r) => Member t r -- Defined in ‘Data.Open.Union’ (The choice depends on the instantiation of ‘effs, s0’) In the ambiguity check for the type signature for ‘foo’: foo :: forall (effs :: [* -> *]) s. (Member (State s) effs, HasInt s) => Eff effs Int To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature for ‘foo’: foo :: (Member (State s) effs, HasInt s) => Eff effs Int
Is this a weakness of extensible effects, or is there another way to express this function?
Thanks, Mitchell
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