On 10/19/10 2:12 PM, Christopher Done wrote:
Questions (I'm talking about GHC when I refer to compilation):
(1) Are fromString and fromIntegral ran at compile time? I don't think that this is the case. I think they are just translated to fromString "Hello, World!" and fromIntegral 2 verbatim. (2) Regardless of this, the implementation of fromString and fromIntegral is essentially a no-op, it's just fromString = Foo, fromIntegral = Bar, which is in turn essentially fromString = id, fromIntegral = id, as far as I understand it.
Foo and unFoo are /essentially/ id, but they're not actually id. In particular, they stick around as System Fc coersions in the core language, whereas id can be compiled away entirely. Unfortunately this means that rewrite rules involving id won't fire, which is why I often add things like: {-# RULES "map Foo" map Foo = unsafeCoerce "fmap Foo" fmap Foo = unsafeCoerce "liftA Foo" liftA Foo = unsafeCoerce "liftM Foo" liftM Foo = unsafeCoerce "map unFoo" map unFoo = unsafeCoerce "fmap unFoo" fmap unFoo = unsafeCoerce "liftA unFoo" liftA unFoo = unsafeCoerce "liftM unFoo" liftM unFoo = unsafeCoerce #-} if I want to ensure them. Unfortunately, last I heard the use of unsafeCoerce can interfere with other rewrite rules, too, since it's also /essentially/ but not exactly id. I'd love to get an up-to-date story on how exactly newtypes and things like fromString, fromInteger, fromEnum, and fromRational are handled re how they get optimized in GHC 7. -- Live well, ~wren