Use PartiallyConstructedTypedColumn for functions.col

dataset/src/main/scala/frameless/functions/PartiallyConstructedTypedColumn.scala

@@ -0,0 +1,16 @@
+package frameless
+package functions
+
+import org.apache.spark.sql.functions.{ col => sparkCol }
+import shapeless.Witness
+
+case class PartiallyConstructedTypedColumn[K <: Symbol](column: Witness.Aux[K])
+object PartiallyConstructedTypedColumn {
+  implicit def toTypedColumn[T, K <: Symbol, A](p: PartiallyConstructedTypedColumn[K])(
+    implicit
+    exists: TypedColumn.Exists[T, K, A],
+    encoder: TypedEncoder[A]): TypedColumn[T, A] = {
+    val untypedExpr = sparkCol(p.column.value.name).as[A](TypedExpressionEncoder[A])
+    new TypedColumn[T, A](untypedExpr)
+  }
+}

dataset/src/main/scala/frameless/functions/package.scala

@@ -2,7 +2,6 @@ package frameless
 
 import org.apache.spark.sql.catalyst.ScalaReflection
 import org.apache.spark.sql.catalyst.expressions.Literal
-import org.apache.spark.sql.functions.{ col => sparkCol }
 import shapeless.Witness
 
 package object functions extends Udf with UnaryFunctions {
@@ -20,11 +19,5 @@ package object functions extends Udf with UnaryFunctions {
     }
   }
 
-  def col[T, A](column: Witness.Lt[Symbol])(
-    implicit
-    exists: TypedColumn.Exists[T, column.T, A],
-    encoder: TypedEncoder[A]): TypedColumn[T, A] = {
-    val untypedExpr = sparkCol(column.value.name).as[A](TypedExpressionEncoder[A])
-    new TypedColumn[T, A](untypedExpr)
-  }
+  def col[K <: Symbol](column: Witness.Aux[K]): PartiallyConstructedTypedColumn[K] = PartiallyConstructedTypedColumn(column)
 }

dataset/src/test/scala/frameless/SelectTests.scala

@@ -16,9 +16,10 @@ class SelectTests extends TypedDatasetSuite {
     ): Prop = {
       val dataset = TypedDataset.create(data)
       val A = dataset.col[A]('a)
+      val col = functions.col('a)
 
       val dataset2 = dataset.select(A).collect().run().toVector
-      val symDataset2 = dataset.select(functions.col('a)).collect().run().toVector
+      val symDataset2 = dataset.select(col).collect().run().toVector
       val data2 = data.map { case X4(a, _, _, _) => a }
 
       (dataset2 ?= data2) && (symDataset2 ?= data2)
@@ -398,4 +399,4 @@ class SelectTests extends TypedDatasetSuite {
     val e = TypedDataset.create[(Int, String, Long)]((1, "a", 2L) :: (2, "b", 4L) :: (2, "b", 1L) :: Nil)
     illTyped("""e.select(frameless.functions.aggregate.sum(e('_1)))""")
   }
-}
+}