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Summary:
This patch implements primitive unboxed sum types, as described in
https://ghc.haskell.org/trac/ghc/wiki/UnpackedSumTypes.
Main changes are:
- Add new syntax for unboxed sums types, terms and patterns. Hidden
behind `-XUnboxedSums`.
- Add unlifted unboxed sum type constructors and data constructors,
extend type and pattern checkers and desugarer.
- Add new RuntimeRep for unboxed sums.
- Extend unarise pass to translate unboxed sums to unboxed tuples right
before code generation.
- Add `StgRubbishArg` to `StgArg`, and a new type `CmmArg` for better
code generation when sum values are involved.
- Add user manual section for unboxed sums.
Some other changes:
- Generalize `UbxTupleRep` to `MultiRep` and `UbxTupAlt` to
`MultiValAlt` to be able to use those with both sums and tuples.
- Don't use `tyConPrimRep` in `isVoidTy`: `tyConPrimRep` is really
wrong, given an `Any` `TyCon`, there's no way to tell what its kind
is, but `kindPrimRep` and in turn `tyConPrimRep` returns `PtrRep`.
- Fix some bugs on the way: #12375.
Not included in this patch:
- Update Haddock for new the new unboxed sum syntax.
- `TemplateHaskell` support is left as future work.
For reviewers:
- Front-end code is mostly trivial and adapted from unboxed tuple code
for type checking, pattern checking, renaming, desugaring etc.
- Main translation routines are in `RepType` and `UnariseStg`.
Documentation in `UnariseStg` should be enough for understanding
what's going on.
Credits:
- Johan Tibell wrote the initial front-end and interface file
extensions.
- Simon Peyton Jones reviewed this patch many times, wrote some code,
and helped with debugging.
Reviewers: bgamari, alanz, goldfire, RyanGlScott, simonpj, austin,
simonmar, hvr, erikd
Reviewed By: simonpj
Subscribers: Iceland_jack, ggreif, ezyang, RyanGlScott, goldfire,
thomie, mpickering
Differential Revision: https://phabricator.haskell.org/D2259
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