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Refactor the treatment of nested Template Haskell splices

Matthew Pickering requested to merge wip/refactor-th-splices into master

  • The difference between a normal splice, a quasiquoter and implicit splice caused by lifting is stored in the AST after renaming.

  • Information that the renamer learns about splices is stored in the relevant splice extension points (XUntypedSpliceExpr, XQuasiQuote).

  • Normal splices and quasi quotes record the flavour of splice (exp/pat/dec etc)

  • Implicit lifting stores information about why the lift was attempted, so if it fails, that can be reported to the user.

  • After renaming, the decision taken to attempt to implicitly lift a variable is stored in the XXUntypedSplice extension field in the HsImplicitLiftSplice constructor.

  • Since all the information is stored in the AST, in HsUntypedSplice, the type of PendingRnSplice now just stores a HsUntypedSplice.

  • Error messages since the original program can be easily printed, this is noticeable in the case of implicit lifting.

  • The user-written syntax is directly type-checked. Before, some desugaring took place in the

  • Fixes .hie files to work better with nested splices (nested splices are not indexed)

  • The location of the quoter in a quasiquote is now located, so error messages will precisely point to it (and again, it is indexed by hie files)

In the future, the typechecked AST should also retain information about the splices and the specific desugaring being left to the desugarer.

Also, runRnSplice should call tcUntypedSplice, otherwise the typechecking logic is duplicated (see the QQError and QQTopError tests for a difference caused by this).

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