On Wed, Sep 30, 2009 at 5:47 AM, wren ng thornton <wren@freegeek.org> wrote:

Olex P wrote:

This idea with new level of abstraction is good but in some cases it can
make things overcomplicated / less efficient. Does that mean "leave simple
built-in types as is"?

That's what newtypes are for. A newtype is like a type alias, except that it is type checked. All newtype wrappering/unwrappering is compiled away so the representations are the same. The only performance difference is (== should be) that there can be overhead for strange ways of providing typeclass instances.[1]

[1] By "strange ways" of providing typeclass instances I mean things like using

class Peano p where ...

data Z = Z
newtype S n = S n
instance Peano Z where ...
instance Peano n => Peano (S n) where ...

instead of a straightforward

data ZorS = Z | S Peano
instance Peano ZorS where ...

Because of the newtype, the representation of (S n) is the same as the representation of Z, thus all peano numbers are the same size. However, we still need to keep that info around somewhere, and consequently the size of the (Peano n) dictionary is linear in n (because it needs a pointer to the (Peano (n-1)) dictionary, and so on until (Peano Z)).

On the other hand, with the straightforward version, the size of (n :: ZorS) is linear in n, but the size of the (Peano ZorS) dictionary is constant.

--
Live well,
~wren
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