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C Rodrigues (red5_2@hotmail.com) says: 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 Tag
data 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 -> String
fooName (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 -> String
barName (Bar s) = s

Trac metadata

Trac field	Value
Version	7.0.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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CC	red5_2@hotmail.com
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