[source]

libraries/base/Data/Traversable.hs

Note [Inline default methods]

[note link]

Consider

class … => Traversable t where

… mapM :: Monad m => (a -> m b) -> t a -> m (t b) mapM = traverse – Default method

instance Traversable [] where
    {-# INLINE traverse #-}
    traverse = ...code for traverse on lists ...

This gives rise to a list-instance of mapM looking like this

$fTraversable[]_$ctraverse = ...code for traverse on lists...
     {-# INLINE $fTraversable[]_$ctraverse #-}
$fTraversable[]_$cmapM    = $fTraversable[]_$ctraverse

Now the $ctraverse obediently inlines into the RHS of $cmapM, /but/ that’s all! We get

$fTraversable[]_$cmapM = ...code for traverse on lists...

with NO INLINE pragma! This happens even though ‘traverse’ had an INLINE pragma because the author knew it should be inlined pretty vigorously.

Indeed, it turned out that the rhs of $cmapM was just too big to inline, so all uses of mapM on lists used a terribly inefficient dictionary-passing style, because of its ‘Monad m =>’ type. Disaster!

Solution: add an INLINE pragma on the default method:

class … => Traversable t where

… mapM :: Monad m => (a -> m b) -> t a -> m (t b) {-# INLINE mapM #-} – VERY IMPORTANT! mapM = traverse

instances for Prelude types