Use of liftM over fmap?

streaming-bytestring has been undergoing some updates lately, and we’re wondering about the deeper difference between liftM and fmap (for IO, in this case).

Here are the relevant definitions:

instance Functor IO where
   fmap f x = x >>= (pure . f)

instance Monad IO  where
    {-# INLINE (>>)   #-}
    {-# INLINE (>>=)  #-}
    (>>)  = (*>)
    (>>=) = bindIO

liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r
liftM f m1 = do { x1 <- m1; return (f x1) }

hlint unilaterally suggests that liftM be replaced with fmap:

Char8.hs:203:27-31: Warning: Use fmap

But we’re curious: would there ever be a reason not to?

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It may be worth noting, that while IO is the most typical Monad for streaming bytestrings, the code is polymorphic in the monad. So the question could be stated for general Monads. Should one use:

  • fmap f m
  • liftM f m
  • m >>= return . f
  • m >>= \x -> return $! f x

  • Assuming that the result will ultimately be wanted strictly, and the code is doing low-level I/O where cycles per byte tend to matter.

Partly answering my own question, the first three variants generate identical optimised code (with GHC 8.8 or 8.10 and -O2). The last ($! annotated) variant does indeed produce slightly different code:

                     \r [s]
                         case m s of {
                         (#,#) ipv [Occ=Once] ipv1 [Occ=Once] ->
-                        let {
-                          sat [Occ=Once]
-                            :: Streaming.Internal.Stream
-                                 (Data.ByteString.Streaming.Internal.ByteString GHC.Types.IO)
-                                 GHC.Types.IO
-                                 ()
-                          [LclId] =
-                              \u [] Main.main ipv1;
-                        } in  (#,#) [ipv sat];
+                        case Main.main ipv1 of vx [Occ=Once] {
+                        _ -> (#,#) [ipv vx];
+                        };
               } in  Streaming.Internal.Effect [sat];

but running it produced no obvious performance advantages either in memory allocations or CPU, indeed when “grouping” the bytes in a bytestream, the bang annotated version ran ~1% slower. So I’m inclined to conclude that fmap vs. liftM or just infix <$> seems just a matter of style not substance, and the hlint suggestion to use fmap has no obvious downside.

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We decided to trust hlint and stick with fmap after all.

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