#15495: Handling Source Locations via TTG -------------------------------------+------------------------------------- Reporter: Shayan-Najd | Owner: (none) Type: task | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- == Problem == The current implementation of [https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/TreesThatGro... TTG HsSyn AST] in GHC stores source locations for terms of a datatype `Exp` in a separate wrapper datatype `LExp` which is mutually recursive with `Exp` such that every recursive reference to `Exp` is done **indirectly**, via a reference to the wrapper datatype `LExp` (see the example code below). We refer to this style of storing source locations as the ping-pong style. Besides the indirection and the resulting complications of the ping-pong style, there are two key problems with it: a. It bakes-in the source locations in the base TTG AST, forcing all instances to store source locations, even if they don't need them.For example, TH AST does not carry source locations, or even within GHC, there are generated terms without source locations. b. It results in a form of conceptual redundancy: source locations are tree decorations and they belong in the extension points. (see https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/TreesThatGro... TTG Guidance]) == Solution == We can move the source location decorations to a wrapper constructor and remove the ping-pong style. This can be done smoothly, mechanically, and gradually by using a getter/setter methods for source locations. More details can be found at [https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/HandlingSour... the related wiki page]. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15495 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler