Udate hsSyn AST to use Trees that Growwip/new-tree-one-param-2

Summary:
See https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow

This commit prepares the ground for a full extensible AST, by replacing the type
parameter for the hsSyn data types with a set of indices into type families,

    data GhcPs -- ^ Index for GHC parser output
    data GhcRn -- ^ Index for GHC renamer output
    data GhcTc -- ^ Index for GHC typechecker output

These are now used instead of `RdrName`, `Name` and `Id`/`TcId`/`Var`

Where the original name type is required in a polymorphic context, this is
accessible via the IdP type family, defined as

    type family IdP p
    type instance IdP GhcPs = RdrName
    type instance IdP GhcRn = Name
    type instance IdP GhcTc = Id

These types are declared in the new 'hsSyn/HsExtension.hs' module.

To gain a better understanding of the extension mechanism, it has been applied
to `HsLit` only, also replacing the `SourceText` fields in them with extension
types.

To preserve extension generality, a type class is introduced to capture the
`SourceText` interface, which must be honoured by all of the extension points
which originally had a `SourceText`.  The class is defined as

    class HasSourceText a where
      -- Provide setters to mimic existing constructors
      noSourceText  :: a
      sourceText    :: String -> a

      setSourceText :: SourceText -> a
      getSourceText :: a -> SourceText

And the constraint is captured in `SourceTextX`, which is a constraint type
listing all the extension points that make use of the class.

Updating Haddock submodule to match.

Test Plan: ./validate

Reviewers: simonpj, shayan-najd, goldfire, austin, bgamari

Subscribers: rwbarton, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D3609

1 files changed, 34 insertions, 0 deletions

diff --git a/docs/users_guide/8.4.1-notes.rst b/docs/users_guide/8.4.1-notes.rst
index 72d6901d18..5929998e95 100644
--- a/docs/users_guide/8.4.1-notes.rst
+++ b/docs/users_guide/8.4.1-notes.rst
@@ -94,3 +94,37 @@ Template Haskell
 
 ``ghc`` library
 ~~~~~~~~~~~~~~~
+
+- hsSyn Abstract Syntax Tree (AST) is now extensible via the mechanism described in `Trees that Grow <http://www.jucs.org/jucs_23_1/trees_that_grow/jucs_23_01_0042_0062_najd.pdf>`_
+
+  The main change for users of the GHC API is that the AST is no longer indexed
+  by the type used as the identifier, but by a specific index type, ::
+
+      type GhcPs   = GhcPass 'Parsed      -- Old 'RdrName' type param
+      type GhcRn   = GhcPass 'Renamed     -- Old 'Name' type param
+      type GhcTc   = GhcPass 'Typechecked -- Old 'Id' type para,
+      type GhcTcId = GhcTc                -- Old 'TcId' type param
+
+  The simplest way to support the current GHC as well as earlier ones is to define ::
+
+      #if MIN_VERSION_ghc(8,3,0)
+      type ParseI     = GhcPs
+      type RenameI    = GhcRn
+      type TypecheckI = GhcTc
+      #else
+      type ParseI     = RdrName
+      type RenameI    = Name
+      type TypecheckI = Var
+      #endif
+
+  and then replace all hardcoded index types accordingly. For polymorphic types,
+  the constraint ::
+
+      #if MIN_VERSION_ghc(8,3,0)
+      -- |bundle up the constraints required for a trees that grow pass
+      type IsPass pass = (DataId pass, OutputableBndrId pass, SourceTextX pass)
+      else
+      type IsPass pass = (DataId pass, OutputableBndrId pass)
+      #endif
+
+  can be used.