Template Haskell bikeshedding: UnboundVarE or not?

[Richard Eisenberg]

Hi devs,

Jan Stolarek has been hard at work allowing Template Haskell to deal with holes in expressions. Thanks, Janek. In this patch, any unbound variable is treated like a hole, which is exactly the way that GHC normally treats unbound variables these days. This is great -- it allows TH quotes to work with unbound names.

The question is this: suppose a user writes [| x |], where x is unbound. Should that produce a (VarE (mkName "x")) or an (UnboundVarE (mkName "x")), where UnboundVarE is a new constructor for Exp?

Reasons for UnboundVarE:
- It communicates information GHC has to clients of TH.

Reasons against UnboundVarE:
- UnboundVarE and VarE are treated identically in *splices*. The only point of UnboundVarE is in the output of *quotes*. This may be confusing to users.
- The information communicated by GHC to TH can be gotten by other means. Specifically, if you try to `reify` an unbound name, you'll get an error (which can be caught gracefully). Bound names `reify` correctly. So a TH client can figure out the boundedness of a variable, albeit awkwardly and in a monad. This counts as a reason against UnboundVarE because the distinction between UnboundVarE and VarE is technically redundant. (You could also probably learn this information by looking to see if a Name has a Unique attached to it. But that's a bit dirty. At least it's pure, though.)

So, what do you think? To UnboundVarE or not to UnboundVarE, that is the question.

Ticket: https://ghc.haskell.org/trac/ghc/ticket/10267
Diff: https://phabricator.haskell.org/D835

Thanks!
Richard