Lennart Augustsson wrote:
The values Z, S Z, and S (S Z) all have the same runtime
representation and there is no linear increase in size when you add a
extra S.
BUT, if you make something overloaded and there is a dictionary
associated with the type (Z, S Z, or S (S Z)) then the dictionary
takes up space, and that space is linear in the number of S
constructors.
Ah yes, that makes more sense. Since your instance would look like:
instance Foo a => Foo (S a) where
foo :: a -> Int
a dictionary for Foo (S(S Z)) would have entries for foo@(S(S Z)) and
also the dictionary for Foo (S Z) which has foo@(S Z) and a dictionary
for Foo Z which has...
It's still something to watch out for if you're really worrying about
performance. I wonder if this is documented on the wiki's section about
performance anywhere, the overhead for inductive type class instances I
mean.
--
Live well,
~wren
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