Improving inlining by reducing function arity?

[sjakobi]

Many functions are currently defined with all their arguments on the LHS, e.g.

unordered-containers/Data/HashMap/Internal.hs

Lines 587 to 594 in 42a25db

	lookup :: (Eq k, Hashable k) => k -> HashMap k v -> Maybe v
	-- GHC does not yet perform a worker-wrapper transformation on
	-- unboxed sums automatically. That seems likely to happen at some
	-- point (possibly as early as GHC 8.6) but for now we do it manually.
	lookup k m = case lookup# k m of
	(# (# #) | #) -> Nothing
	(# | a #) -> Just a
	{-# INLINE lookup #-}

This means that these functions can only be inlined when they are applied to all their arguments too! For example lookup will not be inlined in this code

foo = lookup k . bar

Therefore it may be useful to reduce the arity of lookup and similar functions at least by one, e.g.

lookup k = \m -> case lookup# k m of …

[sjakobi]

See e.g. haskell/bytestring#345 for prior work in bytestring.