Absolutely. There’s a whole module that does this: OptCoercion. If you want to see what programs look like without coercion optimisation, try –fno-opt-coercion.
The rules are described in our paper Evidence normalization in System FC: http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/ I hope that’s useful. Simon From: ghc-devs [mailto:[email protected]] On Behalf Of Conal Elliott Sent: 07 May 2014 04:50 To: [email protected] Subject: Simplifying casts and beta-reduction I'm looking for tools to help simplify Core terms that involve casts. In particular, I want to beta-reduce when the function is wrapped with a cast. Do the GHC sources have such utility functions? Here is an example of a lambda expression with a cast. In context, it's applied to two type arguments and two dictionary arguments and returns a function of one more argument. (The function `(-->)` is defined as `(f --> h) g = h . g . f`.) > ((\ (@ a8) (@ b) > ($dEncodable :: Encodable a8) > ($dEncodable1 :: Encodable b) -> > case $dEncodable of _ > D:Encodable _ tpl_B3 -> > case $dEncodable1 of _ > D:Encodable tpl_B2 _ -> > \ (g :: a8 -> b) (eta :: Encode a8) -> tpl_B2 (g (tpl_B3 eta)) > ) > `cast` (forall a8 b. > <Encodable a8>_R > -> <Encodable b>_R > -> <a8 -> b>_R > -> Sub (Sym (TFCo:R:Encode(->)[0] <a8>_N <b>_N)) > :: (forall a8 b. (Encodable a8, Encodable b) => > (a8 -> b) -> Encode a8 -> Encode b) > ~# > (forall a8 b. (Encodable a8, Encodable b) => > (a8 -> b) -> Encode (a8 -> b)))) I can imagine pushing the `cast` down through the type lambdas while stripping off `forall` coercions, then pushing the remaining `cast` through the dictionary arguments, while stripping off the outer `<Encodable t>_R ->` coercion wrappers, and then pushing the cast into the `case` alternatives and the lambda body, leaving something like > (\ (@ a8) (@ b) > ($dEncodable :: Encodable a8) > ($dEncodable1 :: Encodable b) -> > case $dEncodable of _ > D:Encodable _ tpl_B3 -> > case $dEncodable1 of _ > D:Encodable tpl_B2 _ -> > \ (g :: a8 -> b) -> > (\ (eta :: Encode a8) -> tpl_B2 (g (tpl_B3 eta))) > `cast` (Sub (Sym (TFCo:R:Encode(->)[0] <a8>_N <b>_N)) > :: (Encode a8 -> Encode b) > ~# > (Encode (a8 -> b)))) Now, given type, dictionary, and function arguments, I think we could beta-reduce. Before I try implementing these transformations, does something like them already exist in GHC? Thanks, -- Conal
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