Introduce unboxedSum{Data,Type}Name to template-haskell

Summary:
In D2448 (which introduced Template Haskell support for unboxed
sums), I neglected to add `unboxedSumDataName` and `unboxedSumTypeName`
functions, since there wasn't any way you could write unboxed sum data or type
constructors in prefix form to begin with (see #12514). But even if you can't
write these `Name`s directly in source code, it would still be nice to be able
to use these `Name`s in Template Haskell (for instance, to be able to treat
unboxed sum type constructors like any other type constructors).

Along the way, this uncovered a minor bug in `isBuiltInOcc_maybe` in
`TysWiredIn`, which was calculating the arity of unboxed sum data constructors
incorrectly.

Test Plan: make test TEST=T12478_5

Reviewers: osa1, goldfire, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2854

GHC Trac Issues: #12478, #12514

2 files changed, 45 insertions, 0 deletions

diff --git a/libraries/template-haskell/Language/Haskell/TH.hs b/libraries/template-haskell/Language/Haskell/TH.hs
index 5a497936ae..fd5c06f2f1 100644
--- a/libraries/template-haskell/Language/Haskell/TH.hs
+++ b/libraries/template-haskell/Language/Haskell/TH.hs
@@ -60,6 +60,8 @@ module Language.Haskell.TH(
         -- ** Built-in names
         tupleTypeName, tupleDataName,   -- Int -> Name
         unboxedTupleTypeName, unboxedTupleDataName, -- :: Int -> Name
+        unboxedSumTypeName, -- :: SumArity -> Name
+        unboxedSumDataName, -- :: SumAlt -> SumArity -> Name
 
     -- * The algebraic data types
     -- | The lowercase versions (/syntax operators/) of these constructors are
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index c9bccf665f..9de531ab9e 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -1199,6 +1199,49 @@ mk_unboxed_tup_name n space
     n_commas = n - 1
     tup_mod  = mkModName "GHC.Tuple"
 
+-- Unboxed sum data and type constructors
+-- | Unboxed sum data constructor
+unboxedSumDataName :: SumAlt -> SumArity -> Name
+-- | Unboxed sum type constructor
+unboxedSumTypeName :: SumArity -> Name
+
+unboxedSumDataName alt arity
+  | alt > arity
+  = error $ prefix ++ "Index out of bounds." ++ debug_info
+
+  | alt <= 0
+  = error $ prefix ++ "Alt must be > 0." ++ debug_info
+
+  | arity < 2
+  = error $ prefix ++ "Arity must be >= 2." ++ debug_info
+
+  | otherwise
+  = Name (mkOccName sum_occ)
+         (NameG DataName (mkPkgName "ghc-prim") (mkModName "GHC.Prim"))
+
+  where
+    prefix     = "unboxedSumDataName: "
+    debug_info = " (alt: " ++ show alt ++ ", arity: " ++ show arity ++ ")"
+
+    -- Synced with the definition of mkSumDataConOcc in TysWiredIn
+    sum_occ = '(' : '#' : bars nbars_before ++ '_' : bars nbars_after ++ "#)"
+    bars i = replicate i '|'
+    nbars_before = alt - 1
+    nbars_after  = arity - alt
+
+unboxedSumTypeName arity
+  | arity < 2
+  = error $ "unboxedSumTypeName: Arity must be >= 2."
+         ++ " (arity: " ++ show arity ++ ")"
+
+  | otherwise
+  = Name (mkOccName sum_occ)
+         (NameG TcClsName (mkPkgName "ghc-prim") (mkModName "GHC.Prim"))
+
+  where
+    -- Synced with the definition of mkSumTyConOcc in TysWiredIn
+    sum_occ = '(' : '#' : replicate (arity - 1) '|' ++ "#)"
+
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