Remove List/concat

This can be implemented in terms of `List/build` and `List/fold`, and those
permit build/fold fusion, unlike `List/concat`

src/Dhall/Core.hs

@@ -246,8 +246,6 @@ data Expr a
     | ListIndexed
     -- | > ListReverse                              ~  List/reverse
     | ListReverse
-    -- | > ListConcat                               ~  List/concat
-    | ListConcat
     -- | > Maybe                                    ~  Maybe
     | Maybe
     -- | > MaybeLit t [e]                           ~  [e] : Maybe t
@@ -321,7 +319,6 @@ instance Monad Expr where
     ListLast          >>= _ = ListLast
     ListIndexed       >>= _ = ListIndexed
     ListReverse       >>= _ = ListReverse
-    ListConcat        >>= _ = ListConcat
     Maybe             >>= _ = Maybe
     MaybeLit t es     >>= k = MaybeLit (t >>= k) (fmap (>>= k) es)
     MaybeFold         >>= _ = MaybeFold
@@ -515,8 +512,6 @@ buildExpr6 ListIndexed =
     "List/indexed"
 buildExpr6 ListReverse =
     "List/reverse"
-buildExpr6 ListConcat =
-    "List/concat"
 buildExpr6 List =
     "List"
 buildExpr6 Maybe =
@@ -1796,9 +1791,6 @@ typeWith _      ListIndexed       = do
                 (App List (Record (Data.Map.fromList kts))) ) )
 typeWith _      ListReverse       = do
     return (Pi "a" (Const Type) (Pi "_" (App List "a") (App List "a")))
-typeWith _      ListConcat        = do
-    return
-        (Pi "a" (Const Type) (Pi "_" (App List (App List "a")) (App List "a")))
 typeWith _      Maybe             = do
     return (Pi "_" (Const Type) (Const Type))
 typeWith ctx e@(MaybeLit t xs   ) = do
@@ -1957,17 +1949,6 @@ normalize e = case e of
                           ]
             App (App ListReverse _) (ListLit t xs) ->
                 normalize (ListLit t (Data.Vector.reverse xs))
-            App (App ListConcat _) (ListLit (App List t) xs)
-                | Data.Vector.all isList xs ->
-                    case traverse extract xs of
-                        Just ys -> normalize (ListLit t (Control.Monad.join ys))
-                        _       -> error "normalize: Malformed `List/concat`"
-              where
-                isList (ListLit _ _) = True
-                isList  _            = False
-
-                extract (ListLit _ y) = Just y
-                extract  _            = Nothing
             App (App (App (App (App MaybeFold _) (MaybeLit _ xs)) _) just) nothing ->
                 normalize (maybe nothing just' (toMaybe xs))
               where

src/Dhall/Lexer.x

@@ -106,7 +106,6 @@ tokens :-
     "List/last"                         { emit ListLast         }
     "List/indexed"                      { emit ListIndexed      }
     "List/reverse"                      { emit ListReverse      }
-    "List/concat"                       { emit ListConcat       }
     "Maybe"                             { emit Maybe            }
     "Maybe/fold"                        { emit MaybeFold        }
     "absurd"                            { emit Absurd           }
@@ -274,7 +273,6 @@ data Token
     | ListLast
     | ListIndexed
     | ListReverse
-    | ListConcat
     | Maybe
     | MaybeFold
     | TextLit Builder
@@ -391,8 +389,6 @@ instance Buildable Token where
         = "List/indexed"
     build  ListReverse
         = "List/reverse"
-    build  ListConcat
-        = "List/concat"
     build  ListFold
         = "List/fold"
     build  Maybe

src/Dhall/Parser.y

@@ -91,7 +91,6 @@ import qualified NeatInterpolation
     'List/last'       { Dhall.Lexer.ListLast       }
     'List/indexed'    { Dhall.Lexer.ListIndexed    }
     'List/reverse'    { Dhall.Lexer.ListReverse    }
-    'List/concat'     { Dhall.Lexer.ListConcat     }
     'Maybe'           { Dhall.Lexer.Maybe          }
     'Maybe/fold'      { Dhall.Lexer.MaybeFold      }
     'absurd'          { Dhall.Lexer.Absurd         }
@@ -218,8 +217,6 @@ Expr6
         { ListIndexed }
     | 'List/reverse'
         { ListReverse }
-    | 'List/concat'
-        { ListConcat }
     | 'Maybe'
         { Maybe }
     | 'Maybe/fold'