feat: Typed derive should add new columns to the end of the schema.

Author: mchav

src/DataFrame/Typed/Operations.hs

@@ -215,7 +215,7 @@ derive ::
     , Columnable a
     , AssertAbsent name cols
     ) =>
-    TExpr cols a -> TypedDataFrame cols -> TypedDataFrame (T.Column name a ': cols)
+    TExpr cols a -> TypedDataFrame cols -> TypedDataFrame (Snoc cols (T.Column name a))
 derive (TExpr expr) (TDF df) = unsafeFreeze (D.derive colName expr df)
   where
     colName = T.pack (symbolVal (Proxy @name))

src/DataFrame/Typed/Schema.hs

@@ -23,6 +23,7 @@ module DataFrame.Typed.Schema (
     RenameInSchema,
     RenameManyInSchema,
     Append,
+    Snoc,
     Reverse,
     ColumnNames,
     AssertAbsent,
@@ -76,6 +77,11 @@ type family Lookup (name :: Symbol) (cols :: [Type]) :: Type where
         TypeError
             ('Text "Column '" ':<>: 'Text name ':<>: 'Text "' not found in schema")
 
+-- | Add type to the end of a list.
+type family Snoc (xs :: [k]) (x :: k) :: [k] where
+    Snoc '[]       x = '[x]
+    Snoc (y ': ys) x = y ': Snoc ys x
+
 -- | Check whether a column name exists in a schema (type-level Bool).
 type family HasName (name :: Symbol) (cols :: [Type]) :: Bool where
     HasName name (Column name _ ': _) = 'True