[GHC] #14167: GHC often depicts H98 datatypes as GADTs when it shouldn't
#14167: GHC often depicts H98 datatypes as GADTs when it shouldn't -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: GADTs, | Operating System: Unknown/Multiple TemplateHaskell | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- (Spun off from Phab:D3880.) There are at least a couple of points in the GHC codebase where poor decisions are made regarding whether to depict a datatype using H98 syntax or GADT syntax. In particular: 1. When reifying a datatype in Template Haskell, some complicated heuristics are used to determine whether a datatype is reified with `(Rec)GadtC` or not (see `Note [Reifying GADT data constructors]` in `TcSplice` for the horrifying details). But these heuristics are far from perfect, as they will cause some vanilla datatypes to be reified as GADTs (e.g., `data T a = (a ~ Int) => MkT`). 2. When pretty-printing a datatype (for instance, via the `:info` command in GHCi), GHC also chooses to sometimes depict vanilla datatypes as GADTs. For instance: {{{ λ> :set -XExistentialQuantification λ> data Foo a = Show a => Foo λ> :i Foo type role Foo nominal data Foo a where Foo :: Show a => Foo a }}} `Foo` is depicted as a GADT, despite the fact that we didn't use GADT syntax. There might be other places in the code that exhibit these symptoms too—we should take a closer look. I use the phrase "poor decisions" above since GHC really does have the right info it needs to make the correct decisions. That info is in the form of the `isGadtSyntaxTyCon`, which conveniently recalls whether a datatype was declared with GADT syntax or not. With this, we should be able to eliminate the flawed heuristics that GHC is currently using and accurately depict datatypes the way users wrote them. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14167 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14167: GHC often depicts H98 datatypes as GADTs when it shouldn't -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: GADTs, | TemplateHaskell Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
GHC really does have the right info it needs to make the correct decisions.
Yes indeed. Go for it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14167#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14167: GHC often depicts H98 datatypes as GADTs when it shouldn't -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: GADTs, | TemplateHaskell Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3901 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => patch * differential: => Phab:D3901 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14167#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14167: GHC often depicts H98 datatypes as GADTs when it shouldn't
-------------------------------------+-------------------------------------
Reporter: RyanGlScott | Owner: (none)
Type: bug | Status: patch
Priority: normal | Milestone:
Component: Compiler | Version: 8.2.1
Resolution: | Keywords: GADTs,
| TemplateHaskell
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D3901
Wiki Page: |
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Comment (by Ryan Scott
#14167: GHC often depicts H98 datatypes as GADTs when it shouldn't -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1 Resolution: fixed | Keywords: GADTs, | TemplateHaskell Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3901 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: patch => closed * resolution: => fixed * milestone: => 8.4.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14167#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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