PUBLIC Hi, I'm trying to compile the following two modules: ``` {-# LANGUAGE NoImplicitPrelude #-} module MiniMonad where class Functor f where fmap :: (a -> b) -> f a -> f b class (Functor f) => Applicative f where pure :: a -> f a class (Applicative m) => Monad m where return :: a -> m a return = pure (>>=) :: m a -> (a -> m b) -> m b infixl 1 >>= ``` ``` {-# LANGUAGE NoImplicitPrelude #-} module SpecMain where import MiniMonad data Identity a = MkIdentity a instance Functor Identity where fmap f (MkIdentity x) = MkIdentity (f x) instance Applicative Identity where pure x = MkIdentity x instance Monad Identity where run :: Identity a -> a run (MkIdentity x) = x foo :: () foo = run (return ()) ``` If I compile these from the CLI, I see three rules firing: ``` ./_build/stage1/bin/ghc -debug -fforce-recomp -dno-typeable-binds input/SpecMain.hs -iinput -O0 -ddump-rule-rewrites -dsuppress-all ``` ``` [1 of 2] Compiling MiniMonad ( input/MiniMonad.hs, input/MiniMonad.o ) [2 of 2] Compiling SpecMain ( input/SpecMain.hs, input/SpecMain.o ) tc_iface_decl/2 Monad input/SpecMain.hs:14:10: warning: [-Wmissing-methods] * No explicit implementation for '>>=' * In the instance declaration for 'Monad Identity' | 14 | instance Monad Identity where | ^^^^^^^^^^^^^^ Rule fired Rule: Class op $p1Monad Module: (BUILTIN) Before: MiniMonad.$p1Monad TyArg SpecMain.Identity ValArg SpecMain.$fMonadIdentity After: SpecMain.$fApplicativeIdentity Cont: StrictArg MiniMonad.pure ApplyToTy a_avS Stop[RhsCtxt(Recursive)] a_avS -> SpecMain.Identity a_avS Rule fired Rule: Class op pure Module: (BUILTIN) Before: MiniMonad.pure TyArg SpecMain.Identity ValArg SpecMain.$fApplicativeIdentity After: $cpure_aw8 Cont: ApplyToTy a_avS Stop[RhsCtxt(Recursive)] a_avS -> SpecMain.Identity a_avS Rule fired Rule: Class op return Module: (BUILTIN) Before: MiniMonad.return TyArg SpecMain.Identity ValArg SpecMain.$fMonadIdentity After: $creturn_avP Cont: ApplyToTy () ApplyToVal nodup hole () -> SpecMain.Identity () GHC.Tuple.Prim.() Select nodup wild_X1 Stop[RhsCtxt(NonRecursive)] () ``` However, if I try this via the API (see attached `main.hs`), then only `Class op return` fires: ``` Processing module MiniMonad Processing module SpecMain input/SpecMain.hs:14:10: warning: [-Wmissing-methods] * No explicit implementation for '>>=' * In the instance declaration for 'Monad Identity' | 14 | instance Monad Identity where | ^^^^^^^^^^^^^^ Rule fired Rule: Class op return Module: (BUILTIN) Before: MiniMonad.return TyArg SpecMain.Identity ValArg SpecMain.$fMonadIdentity After: $creturn_aqm Cont: ApplyToTy () ApplyToVal nodup hole () -> SpecMain.Identity () GHC.Tuple.Prim.() Select nodup wild_X1 Stop[RhsCtxt(NonRecursive)] () All done. ``` This of course leads to much worse code further downstream (and is catastrophic for my use case where I rely on extensive specialisation). Why is that? What is missing from my `main.hs` that only one of the three method rules is correctly used when compiling the second module? Thanks, Gergo This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. 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