Jaro Reinders pushed to branch wip/reduce-type-in-stg at Glasgow Haskell Compiler / GHC Commits: ad58ea40 by Jaro Reinders at 2026-06-06T20:09:49+02:00 Add note - - - - - 1 changed file: - compiler/GHC/Stg/Syntax.hs Changes: ===================================== compiler/GHC/Stg/Syntax.hs ===================================== @@ -108,8 +108,55 @@ import GHC.Builtin.Types.Prim smallArrayPrimTyCon, smallMutableArrayPrimTyCon ) +-- This is a newtype because it is important to distinguish it from Type which +-- Kind is otherwise equal to. +-- See Note [Kinds in STG] newtype StgKind = MkStgKind { getStgKind :: Kind } +{- +Note [Kinds in STG] +~~~~~~~~~~~~~~~~~~~ + +Whereas Core is type-annotated, STG is kind-annotated. + +Just as many different values may have a single type, so many different +types may have a single kind. So kinds are a "coarser approximation" to the +values being manipulated; and that is what we want in STG. + +There are two reasons for this: + +(1) It is easier for third party projects to compile to STG. The type system of + another language might not be compatible with GHC's type system. In such a + case the kind system is often still compatible because it is so much coarser. + Example projects are Jaro Reinders' agda2stg and Csaba Hruska's external-stg. + +(2) It allows for more aggressive optimizations. In STG we may do + type-incorrect things that are kind-correct. For example consider + the following function: + + f :: Either a b -> Either a c + f = \x -> case x of r + Left x -> Left x + Right _ -> error "urk" + + We would want to avoid reallocating the 'Left' constructor: + + f :: Either a b -> Either a c + f = \x -> case x of r + Left _ -> r <------------- NB + Right _ -> error "urk" + + This is not type-safe in Core, but it is kind-safe in STG. So, using + the coarser notion of kinds in STG allows us to do more aggressive + optimizations. Note, however, that we do not implement any such + optimizations yet. + +Note that the kinds do not always accurately reflect the final runtime +representation. For example, on the JS backend the kind 'TYPE Int64Rep' +might eventually be rewritten to 'TYPE (TupleRep [Int32Rep,Int32Rep])' +because there is no 64 bit integer type in JS. +-} + {- ************************************************************************ * * View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/ad58ea406717b320641d644e39dc5aea... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/ad58ea406717b320641d644e39dc5aea... You're receiving this email because of your account on gitlab.haskell.org.