Problem

The current implementation of 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:

• 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.

• It results in a form of conceptual redundancy: source locations are tree decorations and they belong in the extension points.

These issues are discussed in

• TTG wiki home page, and its sub-pages, especiallly:
• Handling source locations
• 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 the related wiki page.