I'm curious about global constant propagation in GHC. It's a fairly basic optimization in the CFG-based compiler domain, and it's similar to constructor specialization, but it doesn't seem to be in GHC's repertoire. Perhaps it's usually subsumed by other optimizations or it's more complicated than I am thinking. Is this optimization worth implementing? This optimization can help when a case expression returns a product, some fields of which are the same in all branches. The following program is a minimal example of an optimizable situation that GHC doesn't exploit. {-# OPTIONS_GHC -O3 -funbox-strict-fields #-} data D = D !Int !Int foo n = if n > 0 then D 0 0 else D 0 n main = case foo $ read "7" of D x y -> if x == 0 then return () else print y >> putStrLn "A" After inlining and case-of-case transformation, GHC produces main = let n = read "7" k x y = case x of {0 -> return (); _ -> print y >> putStrLn "A"} in if n > 0 then k 0 0 else k 0 n If the simplifier could discover that x is always 0, it could eliminate one parameter of 'k' and the case expression.