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Add a default definition of primitive #228
Description
Almost all PrimMonad instances are monad transformers of other PrimMonad instances. We can give PrimState a default definition and use DefaultSignatures to give primitive one as well. We can use TypeError to produce good error messages when the defaults don't come close to making sense.
class Monad m => PrimMonad m where
-- | State token type
type PrimState m
-- Default for transformed PrimMonad instances
type PrimState m = PrimState (Arg m)
-- | Execute a primitive operation
primitive :: (State# (PrimState m) -> (# State# (PrimState m), a #)) -> m a
default primitive
:: (IsTransformed m, m ~ t n, PrimMonad n, MonadTrans t, PrimState (t n) ~ PrimState n)
=> (State# (PrimState m) -> (# State# (PrimState m), a #)) -> m a
primitive = lift . primitive
{-# INLINE primitive #-}
type family Arg (m :: * -> *) :: * -> * where
Arg (_t n) = n
type family IsTransformed (m :: * -> *) :: Constraint where
IsTransformed (_t (_n :: * -> *)) = ()
IsTransformed m = TypeError
('Text "Cannot use defaults for instance " :$$:
'Text " " :<>: 'ShowType (PrimMonad m) :$$:
'Text "because" :$$:
'Text " " :<>: 'ShowType m :$$:
'Text "is not a transformed monad." )The only error messages that aren't so great are when users write primitive implementations but forget the PrimState ones. I don't know how to fix that without ending up with a bunch of duplicate errors in some cases (which seems like a GHC issue).
Activity
chessai commentedon May 1, 2019
I'm +1 on this, though can it instead be a top-level function, like
defaultPrimitive/primitiveDefault, or something? Default signatures look so ugly on haddocks. Though a default signature does save users from an extra line of typing. The top-level thing is just something to consider, wondering what others' thoughts are.