Using "ghc -O2" while tuning a class instance for performance, I
obtained a 13% speedup by applying the transformation
instance (Ord a, Num b) ⇒ Sum PSum a b where
empty = empty
insert = insert
union = union
unions = unions
extractMin = extractMin
fromList = fromList
toList = toList
map = map
mapMaybe = mapMaybe
and defining the instance functions outside the instance declaration,
rather than inside the instance declaration.
Conceptually, I understand this as follows: After this transformation,
none of the recursive calls have to go through the class dictionary.
Is this a transformation that ghc could automatically apply while
optimizing? It is clear at compile time that the recursive calls are
from this instance. Every 13% helps.
(My example is adapting a pairing heap to sums of terms, e.g. to
define an algebra from a small category. So far, I can't beat
Data.Map, but I'm not done tuning. Other examples might not show the
same performance increase from this transformation.)
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