I was experimenting with using GADTs for subtyping when I found something
interesting. Hopefully someone can satisfy my curiosity.
Here are two equivalent GADTs. My understanding was that GHC would translate
"Foo" and "Bar" into isomorphic data types. However, GHC 6.12.3 generates
better code for 'fooName' than for 'barName'. In 'fooName', there is no
pattern match against 'FooExtra'. In 'barName', there is a pattern match
against 'BarExtra'. What makes these data types different?
data Tagdata TagExtra
--------
data Foo a where Foo :: String -> Foo a FooExtra :: IORef String -> Foo
TagExtra
-- The cmm code for fooName does not match against 'FooExtra'fooName :: Foo Tag
-> StringfooName (Foo s) = s
--------
data Bar a where Bar :: String -> Bar a BarExtra :: a ~ TagExtra => IORef
String -> Bar a
-- The cmm code for barName will try to pattern-match against 'BarExtra'barName
:: Bar Tag -> StringbarName (Bar s) = s
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