Add RowToList class (#11)

* Add RowToList class

* Add RowToList and ListToRow

Author: LiamGoodacre

src/Type/Prelude.purs

@@ -12,5 +12,5 @@ import Type.Data.Ordering (kind Ordering, LT, EQ, GT, OProxy(..), class IsOrderi
 import Type.Proxy (Proxy(..))
 import Type.Data.Symbol (SProxy(..), class IsSymbol, reflectSymbol, reifySymbol, class CompareSymbol, compareSymbol, class AppendSymbol, appendSymbol)
 import Type.Equality (class TypeEquals, from, to)
-import Type.Row (class RowLacks)
+import Type.Row (class RowLacks, class RowToList, class ListToRow)
 

src/Type/Row.purs

@@ -1,6 +1,11 @@
 module Type.Row
   ( class RowLacks
   , class RowLacking
+  , kind RowList
+  , Nil
+  , Cons
+  , class RowToList
+  , class ListToRow
   ) where
 
 -- Must not be exported
@@ -32,3 +37,30 @@ instance rowLacks
      , RowLacking Entry key typ row )
   => RowLacks key row
 
+
+-- | A type-level list representation of a row
+foreign import kind RowList
+foreign import data Nil :: RowList
+foreign import data Cons :: Symbol -> Type -> RowList -> RowList
+
+-- | Extract the collection of entries in a closed row of types.
+-- | The list of entries is sorted by label and preserves duplicates.
+-- | The inverse of this operation is `ListToRow`.
+-- | Solved by the compiler.
+class RowToList (row :: # Type)
+                (list :: RowList) |
+                row -> list
+
+-- | Convert a RowList to a row of types.
+-- | The inverse of this operation is `RowToList`.
+class ListToRow (list :: RowList)
+                (row :: # Type) |
+                list -> row
+
+instance listToRowNil
+  :: ListToRow Nil ()
+
+instance listToRowCons
+  :: ( ListToRow tail tailRow
+     , RowCons label ty tailRow row )
+  => ListToRow (Cons label ty tail) row