Allow generation of TTH syntax with TH

In other words allow generation of typed splices and brackets with
Untyped Template Haskell.

That is useful in cases where a library is build with TTH in mind,
but we still want to generate some auxiliary declarations,
where TTH cannot help us, but untyped TH can.
Such example is e.g. `staged-sop` which works with TTH,
but we would like to derive `Generic` declarations with TH.

An alternative approach is to use `unsafeCodeCoerce`, but then the
derived `Generic` instances would be type-checked only at use sites,
i.e. much later. Also `-ddump-splices` output is quite ugly:
user-written instances would use TTH brackets, not `unsafeCodeCoerce`.

This commit doesn't allow generating of untyped template splices
and brackets with untyped TH, as I don't know why one would want to do
that (instead of merging the splices, e.g.)

5 files changed, 15 insertions, 1 deletions

diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib.hs b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
index b52de5b0d3..d6107f9dac 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
@@ -44,7 +44,7 @@ module Language.Haskell.TH.Lib (
         appE, appTypeE, uInfixE, parensE, infixE, infixApp, sectionL, sectionR,
         lamE, lam1E, lamCaseE, lamCasesE, tupE, unboxedTupE, unboxedSumE, condE,
         multiIfE, letE, caseE, appsE, listE, sigE, recConE, recUpdE, stringE,
-        fieldExp, getFieldE, projectionE,
+        fieldExp, getFieldE, projectionE, typedSpliceE, typedBracketE,
     -- **** Ranges
     fromE, fromThenE, fromToE, fromThenToE,
 
diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs b/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
index 8d0cf5adde..eeeff941fa 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
@@ -380,6 +380,12 @@ getFieldE e f = do
 projectionE :: Quote m => NonEmpty String -> m Exp
 projectionE xs = pure (ProjectionE xs)
 
+typedSpliceE :: Quote m => m Exp -> m Exp
+typedSpliceE = fmap TypedSpliceE
+
+typedBracketE :: Quote m => m Exp -> m Exp
+typedBracketE = fmap TypedBracketE
+
 -- ** 'arithSeqE' Shortcuts
 fromE :: Quote m => m Exp -> m Exp
 fromE x = do { a <- x; pure (ArithSeqE (FromR a)) }
diff --git a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
index 034d2687b3..dbe3cb85df 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
@@ -232,6 +232,8 @@ pprExp _ (LabelE s) = text "#" <> text s
 pprExp _ (ImplicitParamVarE n) = text ('?' : n)
 pprExp _ (GetFieldE e f) = pprExp appPrec e <> text ('.': f)
 pprExp _ (ProjectionE xs) = parens $ hcat $ map ((char '.'<>) . text) $ toList xs
+pprExp _ (TypedBracketE e) = text "[||" <> ppr e <> text "||]"
+pprExp _ (TypedSpliceE e) = text "$$" <> pprExp appPrec e
 
 pprFields :: [(Name,Exp)] -> Doc
 pprFields = sep . punctuate comma . map (\(s,e) -> pprName' Applied s <+> equals <+> ppr e)
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index 6d96d414c6..8398bafd53 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -2385,6 +2385,8 @@ data Exp
   | ImplicitParamVarE String           -- ^ @{ ?x }@ ( Implicit parameter )
   | GetFieldE Exp String               -- ^ @{ exp.field }@ ( Overloaded Record Dot )
   | ProjectionE (NonEmpty String)      -- ^ @(.x)@ or @(.x.y)@ (Record projections)
+  | TypedBracketE Exp                  -- ^ @[|| e ||]@
+  | TypedSpliceE Exp                   -- ^ @$$e@
   deriving( Show, Eq, Ord, Data, Generic )
 
 type FieldExp = (Name,Exp)
diff --git a/libraries/template-haskell/changelog.md b/libraries/template-haskell/changelog.md
index f6ed4d6b5f..5a62f6e124 100644
--- a/libraries/template-haskell/changelog.md
+++ b/libraries/template-haskell/changelog.md
@@ -8,6 +8,10 @@
 
     This change enables TemplateHaskell support for `DuplicateRecordFields`.
 
+  * Add support for generating typed splices and brackets in untyped Template Haskell
+    Introduces `typedSpliceE :: Quote m => m Exp -> m Exp` and
+    `typedBracketE :: Quote m => m Exp -> m Exp`
+
 ## 2.20.0.0
 
   * The `Ppr.pprInfixT` function has gained a `Precedence` argument.