diff options
| -rw-r--r-- | compiler/GHC/Core/Unify.hs | 2 | ||||
| -rw-r--r-- | compiler/GHC/Utils/Monad.hs | 19 |
2 files changed, 21 insertions, 0 deletions
diff --git a/compiler/GHC/Core/Unify.hs b/compiler/GHC/Core/Unify.hs index fdf4d05068..bd54ecee39 100644 --- a/compiler/GHC/Core/Unify.hs +++ b/compiler/GHC/Core/Unify.hs @@ -1393,6 +1393,8 @@ instance Applicative UM where (<*>) = ap instance Monad UM where + {-# INLINE (>>=) #-} + -- See Note [INLINE pragmas and (>>)] in GHC.Utils.Monad m >>= k = UM (\state -> do { (state', v) <- unUM m state ; unUM (k v) state' }) diff --git a/compiler/GHC/Utils/Monad.hs b/compiler/GHC/Utils/Monad.hs index a65947e59e..056651bdde 100644 --- a/compiler/GHC/Utils/Monad.hs +++ b/compiler/GHC/Utils/Monad.hs @@ -355,6 +355,25 @@ function might be required. For example in FCode we use: where FCode m = FCode' $ oneShot (\cgInfoDown -> oneShot (\state ->m cgInfoDown state)) +INLINE pragmas and (>>) +~~~~~~~~~~~~~~~~~~~~~~~ +A nasty gotcha is described in #20008. In brief, be careful if you get (>>) via +its default method: + + instance Applicative M where + pure a = MkM (\s -> (s, a)) + (<*>) = ap + + instance Monad UM where + {-# INLINE (>>=) #-} + m >>= k = MkM (\s -> blah) + +Here we define (>>), via its default method, in terms of (>>=). If you do this, +be sure to put an INLINE pragma on (>>=), as above. That tells it to inline +(>>=) in the RHS of (>>), even when it is applied to only two arguments, which +in turn conveys the one-shot info from (>>=) to (>>). Lacking the INLINE, GHC +may eta-expand (>>), and with a non-one-shot lambda. #20008 has more discussion. + Derived instances ~~~~~~~~~~~~~~~~~ One caveat of both approaches is that derived instances don't use the smart |