Tom's comments.

Author: sfilipi

src/Microsoft.ML.Core/Data/IRowToRowMapper.cs

@@ -16,15 +16,7 @@ namespace Microsoft.ML.Data
     /// so to rebind, the same input column names must be used.
     /// Implementations of this interface are typically created over defined input <see cref="DataViewSchema"/>.
     /// </summary>
-    public interface IRowToRowMapper : IRowToRowMapperBase
-    {
-        /// <summary>
-        /// Given a set of columns, return the input columns that are needed to generate those output columns.
-        /// </summary>
-        IEnumerable<DataViewSchema.Column> GetDependencies(IEnumerable<DataViewSchema.Column> dependingColumns);
-    }
-
-    public interface IRowToRowMapperBase
+    public interface IRowToRowMapper
     {
         /// <summary>
         /// Mappers are defined as accepting inputs with this very specific schema.
@@ -36,6 +28,11 @@ public interface IRowToRowMapperBase
         /// </summary>
         DataViewSchema OutputSchema { get; }
 
+        /// <summary>
+        /// Given a set of columns, return the input columns that are needed to generate those output columns.
+        /// </summary>
+        IEnumerable<DataViewSchema.Column> GetDependencies(IEnumerable<DataViewSchema.Column> dependingColumns);
+
         /// <summary>
         /// Get an <see cref="DataViewRow"/> with the indicated active columns, based on the input <paramref name="input"/>.
         /// The active columns are those for which <paramref name="active"/> returns true. Getting values on inactive

src/Microsoft.ML.Core/Data/ISchemaBindableMapper.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using Microsoft.Data.DataView;
 
@@ -18,7 +19,7 @@ namespace Microsoft.ML.Data
     /// features column. New predictors can implement <see cref="ISchemaBindableMapper"/> directly. Implementing <see cref="ISchemaBindableMapper"/>
     /// includes implementing a corresponding <see cref="ISchemaBoundMapper"/> (or <see cref="ISchemaBoundRowMapper"/>) and a corresponding ISchema
     /// for the output schema of the <see cref="ISchemaBoundMapper"/>. In case the <see cref="ISchemaBoundRowMapper"/> interface is implemented,
-    /// the SimpleRow class can be used in the <see cref="IRowToRowMapperBase.GetRow"/> method.
+    /// the SimpleRow class can be used in the <see cref="IRowToRowMapper.GetRow"/> method.
     /// </summary>
     [BestFriend]
     internal interface ISchemaBindableMapper
@@ -55,20 +56,36 @@ internal interface ISchemaBoundMapper
     }
 
     /// <summary>
-    /// This interface combines <see cref="ISchemaBoundMapper"/> with <see cref="IRowToRowMapper"/>.
+    /// This interface extends <see cref="ISchemaBoundMapper"/>.
     /// </summary>
     [BestFriend]
-    internal interface ISchemaBoundRowMapper : ISchemaBoundMapper, IRowToRowMapperBase
+    internal interface ISchemaBoundRowMapper : ISchemaBoundMapper
     {
         /// <summary>
-        /// There are two schemas from <see cref="ISchemaBoundMapper"/> and <see cref="IRowToRowMapper"/>.
-        /// Since the two parent schema's are identical in all derived classes, we merge them into <see cref="OutputSchema"/>.
+        /// Input schema accepted.
         /// </summary>
-        new DataViewSchema OutputSchema { get; }
+        DataViewSchema InputSchema { get; }
 
         /// <summary>
         /// Given a set of columns, from the newly generated ones, return the input columns that are needed to generate those output columns.
         /// </summary>
         IEnumerable<DataViewSchema.Column> GetDependenciesForNewColumns(IEnumerable<DataViewSchema.Column> dependingColumns);
+
+        /// <summary>
+        /// Get an <see cref="DataViewRow"/> with the indicated active columns, based on the input <paramref name="input"/>.
+        /// The active columns are those for which <paramref name="active"/> returns true. Getting values on inactive
+        /// columns of the returned row will throw. Null predicates are disallowed.
+        ///
+        /// The <see cref="DataViewRow.Schema"/> of <paramref name="input"/> should be the same object as
+        /// <see cref="InputSchema"/>. Implementors of this method should throw if that is not the case. Conversely,
+        /// the returned value must have the same schema as <see cref="ISchemaBoundMapper.OutputSchema"/>.
+        ///
+        /// This method creates a live connection between the input <see cref="DataViewRow"/> and the output <see
+        /// cref="DataViewRow"/>. In particular, when the getters of the output <see cref="DataViewRow"/> are invoked, they invoke the
+        /// getters of the input row and base the output values on the current values of the input <see cref="DataViewRow"/>.
+        /// The output <see cref="DataViewRow"/> values are re-computed when requested through the getters. Also, the returned
+        /// <see cref="DataViewRow"/> will dispose <paramref name="input"/> when it is disposed.
+        /// </summary>
+        DataViewRow GetRow(DataViewRow input, Func<int, bool> active);
     }
 }

src/Microsoft.ML.Data/Transforms/NopTransform.cs

@@ -27,7 +27,7 @@ internal sealed class NopTransform : IDataTransform, IRowToRowMapper
 
         public IDataView Source { get; }
 
-        DataViewSchema IRowToRowMapperBase.InputSchema => Source.Schema;
+        DataViewSchema IRowToRowMapper.InputSchema => Source.Schema;
 
         /// <summary>
         /// Creates a NopTransform if the input is not an IDataTransform.

src/Microsoft.ML.TimeSeries/SequentialTransformerBase.cs

@@ -784,7 +784,7 @@ void ISaveAsPfa.SaveAsPfa(BoundPfaContext ctx)
             return GetActive(predicate);
         }
 
-        DataViewSchema IRowToRowMapperBase.InputSchema => Source.Schema;
+        DataViewSchema IRowToRowMapper.InputSchema => Source.Schema;
 
         public DataViewRow GetRow(DataViewRow input, Func<int, bool> active)
         {