Get nested field for struct column

dataset/src/main/scala/frameless/RecordEncoder.scala

@@ -69,7 +69,7 @@ object NewInstanceExprs {
         Literal.fromObject(()) +: tail.from(exprs)
     }
 
-  implicit def deriveNonUnit[K <: Symbol, V , T <: HList]
+  implicit def deriveNonUnit[K <: Symbol, V, T <: HList]
     (implicit
       notUnit: V =:!= Unit,
       tail: NewInstanceExprs[T]

dataset/src/main/scala/frameless/TypedColumn.scala

@@ -30,6 +30,7 @@ sealed class TypedColumn[T, U](expr: Expression)(
   }
 
   override def typed[W, U1: TypedEncoder](c: Column): TypedColumn[W, U1] = c.typedColumn
+
   override def lit[U1: TypedEncoder](c: U1): TypedColumn[T,U1] = flit(c)
 }
 
@@ -828,6 +829,18 @@ abstract class AbstractTypedColumn[T, U]
       w1: With.Aux[TT2, W1, W2]
     ): ThisType[W2, Boolean] =
       typed(self.untyped.between(lowerBound.untyped, upperBound.untyped))
+
+  /**
+    * Returns a nested column matching the field `symbol`.
+    * 
+    * @param V the type of the nested field
+    */
+  def field[V](symbol: Witness.Lt[Symbol])(implicit
+      i0: TypedColumn.Exists[U, symbol.T, V],
+      i1: TypedEncoder[V]
+    ): ThisType[T, V] = 
+    typed(self.untyped.getField(symbol.value.name))
+
 }
 
 

dataset/src/test/scala/frameless/ColumnTests.scala

@@ -425,4 +425,17 @@ class ColumnTests extends TypedDatasetSuite with Matchers {
     "ds.select(ds('_1).opt.map(x => x))" shouldNot typeCheck
     "ds.select(ds('_2).opt.map(x => x))" shouldNot typeCheck
   }
+
+  test("field") {
+    val ds = TypedDataset.create((1, (2.3F, "a")) :: Nil)
+    val rs = ds.select(ds('_2).field('_2)).collect().run()
+
+    rs shouldEqual Seq("a")
+  }
+
+  test("field compiles only for valid field") {
+    val ds = TypedDataset.create((1, (2.3F, "a")) :: Nil)
+
+    "ds.select(ds('_2).field('_3))" shouldNot typeCheck
+  }
 }