Table with no columns is not valid, No_Output_Columns is always an error (#4073)

Implements https://www.pivotaltracker.com/story/show/184226020

distribution/lib/Standard/Base/0.0.0-dev/src/Data/Vector.enso

@@ -1061,7 +1061,7 @@ type Builder
     unsafe_append : Any -> Nothing
     unsafe_append self item = self.java_builder.add item
 
-    ## Gets an element from the vector at a specified index (0-based).
+    ## Gets an element from the vector builder at a specified index (0-based).
 
        Arguments:
        - index: The location in the vector to get the element from. The index is

distribution/lib/Standard/Base/0.0.0-dev/src/Warning.enso

@@ -58,7 +58,7 @@ type Warning
          attached warnings are unaffected.
 
              mapper warning = case warning of
-                 Matching.No_Matches_Found criteria -> Maybe.Some (Missing_Input_Columns criteria)
+                 Matching.No_Matches_Found criteria -> Maybe.Some (Missing_Input_Columns.Error criteria)
                  _ -> Nothing
              Warning.map_attached_warnings mapper value
     map_attached_warnings : (Any -> Maybe Any) -> Any -> Any
@@ -80,7 +80,7 @@ type Warning
          other attached warnings or errors are unaffected.
 
              mapper error = case error of
-                 Matching.No_Matches_Found criteria -> Maybe.Some (Missing_Input_Columns criteria)
+                 Matching.No_Matches_Found criteria -> Maybe.Some (Missing_Input_Columns.Error criteria)
                  _ -> Nothing
              Warning.map_warnings_and_errors mapper value
     map_warnings_and_errors : (Any -> Maybe Any) -> Any -> Any

distribution/lib/Standard/Database/0.0.0-dev/src/Data/Table.enso

@@ -141,7 +141,9 @@ type Table
          - If two distinct indices would refer to the same column, a
            `Input_Indices_Already_Matched`, indicating that the additional
            indices will not introduce additional columns.
-         - If there are no columns in the output table, a `No_Output_Columns`.
+         - If there are no columns in the output table, a `No_Output_Columns` is
+           raised as an error regardless of the problem behavior, because it is
+           not possible to create a table without any columns.
 
        > Example
          Select columns by name.
@@ -193,7 +195,9 @@ type Table
          - If two distinct indices would refer to the same column, a
            `Input_Indices_Already_Matched`, indicating that the additional
            indices will not introduce additional columns.
-         - If there are no columns in the output table, a `No_Output_Columns`.
+         - If there are no columns in the output table, a `No_Output_Columns` is
+           raised as an error regardless of the problem behavior, because it is
+           not possible to create a table without any columns.
 
        > Example
          Remove columns with given names.
@@ -670,17 +674,14 @@ type Table
            `Floating_Point_Grouping` warning.
     distinct : Vector Text | Column_Selector -> Case_Sensitivity -> Problem_Behavior -> Table
     distinct self (columns = Column_Selector.By_Name (self.columns.map .name)) case_sensitivity=Case_Sensitivity.Sensitive on_problems=Report_Warning =
-        problem_builder = Problem_Builder.new
+        key_columns = self.columns_helper.select_columns selector=columns reorder=True on_problems=Problem_Behavior.Report_Error . catch No_Output_Columns _->
-        warning_mapper error = case error of
+            Error.throw No_Input_Columns_Selected
-            No_Output_Columns -> Maybe.Some No_Input_Columns_Selected
-            _ -> Nothing
-        key_columns = Warning.map_warnings_and_errors warning_mapper <|
-            self.columns_helper.select_columns selector=columns reorder=True on_problems=on_problems
         text_case_insensitive = case case_sensitivity of
             Case_Sensitivity.Sensitive -> False
             Case_Sensitivity.Insensitive locale ->
                 Helpers.assume_default_locale locale <|
                     True
+        problem_builder = Problem_Builder.new
         new_table = self.connection.dialect.prepare_distinct self key_columns text_case_insensitive problem_builder
         problem_builder.attach_problems_before on_problems new_table
 
@@ -982,7 +983,9 @@ type Table
          The following problems can occur:
          - If a column name is not in the input table, a `Missing_Input_Columns`.
          - If a column index is out of range, a `Column_Indexes_Out_Of_Range`.
-         - If there are no valid columns in the output table, a `No_Output_Columns`.
+         - If there are no columns in the output table, a `No_Output_Columns` is
+           raised as an error regardless of the problem behavior, because it is
+           not possible to create a table without any columns.
          - If there are invalid column names in the output table, a `Invalid_Output_Column_Names`.
          - If there are duplicate column names in the output table, a `Duplicate_Output_Column_Names`.
          - If grouping on or computing the `Mode` on a floating point number, a `Floating_Point_Grouping`.
@@ -1343,8 +1346,9 @@ type Table
 # make_table : Connection -> Text -> Vector [Text, SQL_Type] -> Context -> Table
 make_table : Connection -> Text -> Vector -> Context -> Table
 make_table connection table_name columns ctx =
-    cols = columns.map (p -> Internal_Column.Value p.first p.second (SQL_Expression.Column table_name p.first))
+    if columns.is_empty then Error.throw (Illegal_State.Error "Unexpectedly attempting to create a Database Table with no columns. This is a bug in the Database library.") else
-    Table.Value table_name connection cols ctx
+        cols = columns.map (p -> Internal_Column.Value p.first p.second (SQL_Expression.Column table_name p.first))
+        Table.Value table_name connection cols ctx
 
 ## PRIVATE
 

distribution/lib/Standard/Table/0.0.0-dev/src/Data/Table.enso

@@ -79,11 +79,10 @@ type Table
             case c of
                 _ : Vector -> Column.from_vector (c.at 0) (c.at 1) . java_column
                 Column.Value java_col -> java_col
-        # TODO enable this once we stop returning tables without columns
+        if cols.is_empty then Error.throw (Illegal_Argument.Error "Cannot create a table with no columns.") else
-        # if cols.is_empty then Error.throw (Illegal_Argument.Error "Cannot create a table with no columns.") else
+            if (cols.all c-> c.getSize == cols.first.getSize).not then Error.throw (Illegal_Argument.Error "All columns must have the same row count.") else
-        if (cols.all c-> c.getSize == cols.first.getSize).not then Error.throw (Illegal_Argument.Error "All columns must have the same row count.") else
+                if cols.distinct .getName . length != cols.length then Error.throw (Illegal_Argument.Error "Column names must be distinct.") else
-            if cols.distinct .getName . length != cols.length then Error.throw (Illegal_Argument.Error "Column names must be distinct.") else
+                    Table.Value (Java_Table.new cols.to_array)
-                Table.Value (Java_Table.new cols.to_array)
 
     ## Creates a new table from a vector of column names and a vector of vectors
        specifying row contents.
@@ -256,7 +255,9 @@ type Table
          - If two distinct indices refer to the same column, an
            `Input_Indices_Already_Matched`, with the column included the first
            time it is matched.
-         - If there are no columns in the output table, a `No_Output_Columns`.
+         - If there are no columns in the output table, a `No_Output_Columns` is
+           raised as an error regardless of the problem behavior, because it is
+           not possible to create a table without any columns.
 
        > Example
          Select columns by name.
@@ -308,7 +309,9 @@ type Table
          - If two distinct indices refer to the same column, an
            `Input_Indices_Already_Matched`, with the column included the first
            time it is matched.
-         - If there are no columns in the output table, a `No_Output_Columns`.
+         - If there are no columns in the output table, a `No_Output_Columns` is
+           raised as an error regardless of the problem behavior, because it is
+           not possible to create a table without any columns.
 
        > Example
          Remove columns with given names.
@@ -515,7 +518,9 @@ type Table
          The following problems can occur:
          - If a column name is not in the input table, a `Missing_Input_Columns`.
          - If a column index is out of range, a `Column_Indexes_Out_Of_Range`.
-         - If there are no valid columns in the output table, a `No_Output_Columns`.
+         - If there are no columns in the output table, a `No_Output_Columns` is
+           raised as an error regardless of the problem behavior, because it is
+           not possible to create a table without any columns.
          - If there are invalid column names in the output table, a `Invalid_Output_Column_Names`.
          - If there are duplicate column names in the output table, a `Duplicate_Output_Column_Names`.
          - If grouping on or computing the `Mode` on a floating point number, a `Floating_Point_Grouping`.
@@ -676,11 +681,8 @@ type Table
            `Floating_Point_Grouping` warning.
     distinct : Vector Text | Column_Selector -> Case_Sensitivity -> Problem_Behavior -> Table
     distinct self (columns = Column_Selector.By_Name (self.columns.map .name)) case_sensitivity=Case_Sensitivity.Sensitive on_problems=Report_Warning =
-        warning_mapper error = case error of
+        key_columns = self.columns_helper.select_columns selector=columns reorder=True on_problems=Problem_Behavior.Report_Error . catch No_Output_Columns _->
-            No_Output_Columns -> Maybe.Some No_Input_Columns_Selected
+            Error.throw No_Input_Columns_Selected
-            _ -> Nothing
-        key_columns = Warning.map_warnings_and_errors warning_mapper <|
-            self.columns_helper.select_columns selector=columns reorder=True on_problems=on_problems
         java_columns = key_columns.map .java_column
         text_folding_strategy = Case_Sensitivity.folding_strategy case_sensitivity
         java_table = Illegal_Argument.handle_java_exception <|

distribution/lib/Standard/Table/0.0.0-dev/src/Delimited/Delimited_Format.enso

@@ -12,9 +12,12 @@ import project.Delimited.Quote_Style.Quote_Style
 type Delimited_Format
     ## Read delimited files such as CSVs into a Table.
 
-       If a row does not match the first row's column count, the function raises
+       When reading this format, the following problems may occur:
-       an `Invalid_Row`. If a quote is opened and never closed, a
+       - If a row does not match the first row's column count, the function
-       `Mismatched_Quote` warning occurs.
+         raises an `Invalid_Row`.
+       - If a quote is opened and never closed, a `Mismatched_Quote` problem is
+         reported.
+       - If an empty file is passed, an `Empty_File_Error` is thrown.
 
        Arguments:
        - delimiter: The delimiter character to split the file into columns. An

distribution/lib/Standard/Table/0.0.0-dev/src/Delimited/Delimited_Reader.enso

@@ -8,7 +8,7 @@ import project.Data.Data_Formatter.Data_Formatter
 import project.Delimited.Delimited_Format.Delimited_Format
 import project.Delimited.Quote_Style.Quote_Style
 import project.Internal.Java_Problems
-from project.Errors import Additional_Warnings, Parser_Error
+from project.Errors import Additional_Warnings, Parser_Error, Empty_File_Error
 
 polyglot java import org.enso.base.encoding.NewlineDetector
 polyglot java import org.enso.table.read.DelimitedReader
@@ -85,7 +85,7 @@ read_stream format stream on_problems max_columns=default_max_columns related_fi
      integer.
 read_from_reader : Delimited_Format -> Reader -> Problem_Behavior -> Integer -> Any
 read_from_reader format java_reader on_problems max_columns=4096 =
-    Illegal_Argument.handle_java_exception <| handle_parsing_failure <| handle_parsing_exception <|
+    Illegal_Argument.handle_java_exception <| handle_parsing_failure <| handle_parsing_exception <| Empty_File_Error.handle_java_exception <|
         reader = prepare_reader java_reader format max_columns on_problems
         result_with_problems = reader.read
         parsing_problems = Vector.from_polyglot_array (result_with_problems.problems) . map Java_Problems.translate_problem

distribution/lib/Standard/Table/0.0.0-dev/src/Errors.enso

@@ -2,6 +2,9 @@ from Standard.Base import all
 
 polyglot java import org.enso.table.error.ColumnCountMismatchException
 polyglot java import org.enso.table.error.ColumnNameMismatchException
+polyglot java import org.enso.table.error.EmptyFileException
+polyglot java import org.enso.table.error.EmptySheetException
+
 
 type Missing_Input_Columns
     ## PRIVATE
@@ -215,6 +218,26 @@ type Leading_Zeros
     to_display_text : Text
     to_display_text self = "Leading zeros in column "+self.column+" with datatype "+self.datatype.to_text+"."
 
+## Indicates that an empty file was encountered, so no data could be loaded.
+type Empty_File_Error
+
+    to_display_text : Text
+    to_display_text = "It is not allowed to create a Table with no columns, so an empty file could not have been loaded."
+
+    ## PRIVATE
+    handle_java_exception =
+        Panic.catch EmptyFileException handler=(_ -> Error.throw Empty_File_Error)
+
+## Indicates that an empty sheet was encountered, so no data could be loaded.
+type Empty_Sheet_Error
+
+    to_display_text : Text
+    to_display_text = "It is not allowed to create a Table with no columns, so an empty sheet could not have been loaded."
+
+    ## PRIVATE
+    handle_java_exception =
+        Panic.catch EmptySheetException handler=(_ -> Error.throw Empty_Sheet_Error)
+
 ## Indicates that multiple `Column_Type_Selector` match the same column.
 
    If all matching selectors indicate the same type, the warning is reported but

distribution/lib/Standard/Table/0.0.0-dev/src/Excel/Excel_Reader.enso

@@ -6,7 +6,7 @@ import project.Data.Table.Table
 import project.Excel.Excel_Range.Excel_Range
 import project.Excel.Excel_Section.Excel_Section
 
-from project.Errors import Invalid_Location, Duplicate_Output_Column_Names, Invalid_Output_Column_Names
+from project.Errors import Invalid_Location, Duplicate_Output_Column_Names, Invalid_Output_Column_Names, Empty_Sheet_Error
 
 polyglot java import org.enso.table.excel.ExcelHeaders
 polyglot java import org.enso.table.read.ExcelReader
@@ -45,7 +45,7 @@ handle_reader file reader =
     bad_argument caught_panic = Error.throw (Invalid_Location.Error caught_panic.payload.getCause)
     handle_bad_argument = Panic.catch InvalidLocationException handler=bad_argument
 
-    File_Error.handle_java_exceptions file <| handle_bad_argument <| handle_bad_format <|
+    File_Error.handle_java_exceptions file <| handle_bad_argument <| handle_bad_format <| Empty_Sheet_Error.handle_java_exception <|
         file.with_input_stream [File_Access.Read] stream->
             stream.with_java_stream reader
 

distribution/lib/Standard/Table/0.0.0-dev/src/Internal/Aggregate_Column_Helper.enso

@@ -70,11 +70,11 @@ prepare_aggregate_columns aggregates table =
         Pair.new new_name agg
 
     # Build Problems Output
-    if renamed_columns.is_empty then
+    case renamed_columns.is_empty of
-        problem_builder.report_other_warning No_Output_Columns
+        True -> Error.throw No_Output_Columns
-    problem_builder.report_unique_name_strategy unique
+        False ->
-
+            problem_builder.report_unique_name_strategy unique
-    Validated_Aggregate_Columns.Value unique_key_columns renamed_columns problem_builder.build_problemset
+            Validated_Aggregate_Columns.Value unique_key_columns renamed_columns problem_builder.build_problemset
 
 ## PRIVATE
    Defines the default name of an `Aggregate_Column`.

distribution/lib/Standard/Table/0.0.0-dev/src/Internal/Join_Helpers.enso

@@ -2,13 +2,11 @@ from Standard.Base import all
 import Standard.Base.Error.Common.Index_Out_Of_Bounds
 import Standard.Base.Error.Illegal_State.Illegal_State
 
-from project.Errors import Invalid_Value_Type
+from project.Errors import Invalid_Value_Type, No_Such_Column, Missing_Input_Columns, Column_Indexes_Out_Of_Range
 import project.Data.Join_Condition.Join_Condition
 import project.Data.Value_Type.Value_Type
 import project.Internal.Problem_Builder.Problem_Builder
 
-import project.Errors.No_Such_Column
-
 type Join_Condition_Resolver
     ## PRIVATE
     Value left_at right_at make_equals make_equals_ignore_case make_between
@@ -25,7 +23,7 @@ type Join_Condition_Resolver
     resolve : Join_Condition | Text | Vector (Join_Condition | Text) -> Problem_Behavior -> Join_Condition_Resolution
     resolve self conditions on_problems =
         redundant_names = Vector.new_builder
-        problem_builder = Problem_Builder.new
+        problem_builder = Problem_Builder.new types_to_always_throw=[Missing_Input_Columns, Column_Indexes_Out_Of_Range]
 
         resolve_selector resolver selector =
             r_1 = resolver selector
@@ -66,10 +64,9 @@ type Join_Condition_Resolver
                 right_upper = resolve_right right_upper_selector
                 if left.is_nothing || right_lower.is_nothing || right_upper.is_nothing then Nothing else
                     self.make_between problem_builder left right_lower right_upper
-        problem_builder.throw_unmatched_columns_as_error <|
+        problem_builder.attach_problems_before on_problems <|
             if converted.contains Nothing then Panic.throw (Illegal_State.Error "Impossible: unresolved columns remaining in the join resolution. This should have raised a dataflow error. This is a bug in the Table library.") else
-                problem_builder.attach_problems_after on_problems <|
+                Join_Condition_Resolution.Result converted redundant_names.to_vector
-                    Join_Condition_Resolution.Result converted redundant_names.to_vector
 
 type Join_Condition_Resolution
     Result (conditions : Vector Any) (redundant_column_names : Vector Text)

distribution/lib/Standard/Table/0.0.0-dev/src/Internal/Problem_Builder.enso

@@ -6,7 +6,7 @@ import project.Internal.Vector_Builder.Vector_Builder
 from project.Errors import Missing_Input_Columns, Column_Indexes_Out_Of_Range, Duplicate_Column_Selectors, Input_Indices_Already_Matched, Column_Matched_By_Multiple_Selectors, Duplicate_Output_Column_Names, Invalid_Output_Column_Names
 
 type Problem_Builder
-    Value oob_indices duplicate_column_selectors input_indices_already_matched missing_input_columns other
+    Value types_to_always_throw oob_indices duplicate_column_selectors input_indices_already_matched missing_input_columns other
 
     report_oob_indices self indices =
         append_to_ref self.oob_indices indices
@@ -41,48 +41,55 @@ type Problem_Builder
             if vec.not_empty then
                 problems.append (problem_creator vec)
 
+        build_vector_and_append self.missing_input_columns Missing_Input_Columns.Error
         build_vector_and_append self.oob_indices Column_Indexes_Out_Of_Range.Error
         build_vector_and_append self.duplicate_column_selectors Duplicate_Column_Selectors.Error
         build_vector_and_append self.input_indices_already_matched Input_Indices_Already_Matched.Error
-        build_vector_and_append self.missing_input_columns Missing_Input_Columns.Error
         self.other.to_vector.each problems.append
 
         problems.to_vector
 
-    ## If there are any missing columns or out of bound indices, the
-       corresponding error is raised, otherwise, the provided action is executed
-       and its result is returned.
-    throw_unmatched_columns_as_error : Any -> Any
-    throw_unmatched_columns_as_error self ~action =
-         case self.missing_input_columns.get.not_empty of
-            True ->
-                err = Missing_Input_Columns.Error self.missing_input_columns.get.build
-                Error.throw err
-            False -> case self.oob_indices.get.not_empty of
-                True ->
-                    err = Column_Indexes_Out_Of_Range.Error self.oob_indices.get.build
-                    Error.throw err
-                False -> action
-
     ## Attaches gathered warnings to the result.
        Any errors from the `result` take precedence over the ones owned by this
        builder.
     attach_problems_after : Problem_Behavior -> Any -> Any
     attach_problems_after self problem_behavior result =
-        problem_behavior.attach_problems_after result self.build_problemset
+        case result of
+            _ -> case self.get_problemset_throwing_distinguished_errors of
+                problems ->
+                    problem_behavior.attach_problems_after result problems
 
     ## Attaches gathered warnings to the result of the provided computation.
        If in `Report_Error` mode and there are any problems gathered, the first
        one will be returned as error without even running the computation.
     attach_problems_before : Problem_Behavior -> Any -> Any
     attach_problems_before self problem_behavior ~computation =
-        problem_behavior.attach_problems_before self.build_problemset computation
+        case self.get_problemset_throwing_distinguished_errors of
+            problems ->
+                problem_behavior.attach_problems_before problems computation
+
+    ## PRIVATE
+       Returns the generated problem set if no errors force throwing or throws
+       the first error that is meant to be thrown regardless of problem behavior.
+    get_problemset_throwing_distinguished_errors : Vector
+    get_problemset_throwing_distinguished_errors self =
+        problems = self.build_problemset
+        distinguished_problem = problems.find if_missing=Nothing problem->
+            self.types_to_always_throw.any tpe->
+                problem.is_a tpe
+        case distinguished_problem of
+            Nothing -> problems
+            problem -> Error.throw problem
 
     ## PRIVATE
        Creates a new helper object for aggregating problems to report.
-    new : Problem_Builder
+
-    new =
+       An optional list of types of can be provided. Problems matching these
-        Problem_Builder.Value (Ref.new Vector_Builder.empty) (Ref.new Vector_Builder.empty) (Ref.new Vector_Builder.empty) (Ref.new Vector_Builder.empty) other=Vector.new_builder
+       types will be raised as an error by the `attach_` methods regardless of
+       the `Problem_Behavior` used.
+    new : Vector -> Problem_Builder
+    new types_to_always_throw=[] =
+        Problem_Builder.Value types_to_always_throw (Ref.new Vector_Builder.empty) (Ref.new Vector_Builder.empty) (Ref.new Vector_Builder.empty) (Ref.new Vector_Builder.empty) other=Vector.new_builder
 
 ## PRIVATE
    Appends a `Vector` to a `Vector_Builder` stored in a `Ref`.

distribution/lib/Standard/Table/0.0.0-dev/src/Internal/Table_Helpers.enso

@@ -61,9 +61,8 @@ type Table_Column_Helper
     select_columns self selector reorder on_problems =
         problem_builder = Problem_Builder.new
         result = self.select_columns_helper selector reorder problem_builder
-        if result.is_empty then
+        problem_builder.attach_problems_before on_problems <|
-            problem_builder.report_other_warning No_Output_Columns
+            if result.is_empty then Error.throw No_Output_Columns else result
-        problem_builder.attach_problems_after on_problems result
 
     ## PRIVATE
        A helper function encapsulating shared code for `remove_columns`
@@ -88,9 +87,8 @@ type Table_Column_Helper
         result = self.internal_columns.filter column->
             should_be_removed = selected_names.get column.name False
             should_be_removed.not
-        if result.is_empty then
+        if result.is_empty then Error.throw No_Output_Columns else
-            problem_builder.report_other_warning No_Output_Columns
+            problem_builder.attach_problems_after on_problems result
-        problem_builder.attach_problems_after on_problems result
 
     ## PRIVATE
        A helper function encapsulating shared code for `reorder_columns`
@@ -168,14 +166,29 @@ type Table_Column_Helper
             ## We cannot just use a custom_column in the aggregate because of
                how the column selector works. We may need to revisit this. For
                now we need to use tricks like that:
-            modified_table = blanks.fold (self.table.select_columns [] on_problems=Problem_Behavior.Ignore) table-> blanks_col->
+
+               To be backend agnostic, we cannot create a new table with the
+               columns above just out of thin air (actually we may want to allow
+               this in the future if all columns come from the same context, but
+               currently it's not possible). Instead, we add our blank columns
+               to the current table and then remove any other columns we are not
+               interested in. Note that we do not have to care about potential
+               name conflicts, as we are dropping any other columns anyway, and
+               adding a new column with a clashing name will not affect any
+               other columns computed from the old column with that name.
+            table_with_blank_indicators = blanks.fold self.table table-> blanks_col->
                 table.set blanks_col.name blanks_col
+            just_indicators = table_with_blank_indicators.select_columns (blanks.map .name) on_problems=Problem_Behavior.Report_Error
             # Maximum is equivalent to Exists and Minimum is equivalent to Forall.
             col_aggregate = if when_any then Maximum _ else Minimum _
             aggregates = blanks.map blanks_col-> col_aggregate blanks_col.name
-            result = self.materialize <| Panic.rethrow <|
+
-                modified_table.aggregate aggregates on_problems=Problem_Behavior.Report_Error
+            aggregate_result = just_indicators.aggregate aggregates on_problems=Problem_Behavior.Report_Error
-            counts = result.rows.first
+            materialized_result = self.materialize <| aggregate_result.catch Any error->
+                msg = "Unexpected dataflow error has been thrown in an `select_columns_helper`. This is a bug in the Table library. The unexpected error was: "+error.to_display_text
+                Panic.throw (Illegal_State.Error message=msg cause=error)
+
+            counts = materialized_result.rows.first
 
             ## The `reorder` argument has no meaning for Blank_Columns selector
                - either way all blank columns are selected in the order that

std-bits/base/src/main/java/org/enso/base/Array_Builder.java

@@ -5,13 +5,13 @@ import java.util.List;
 
 public class Array_Builder<T> {
   private static final Object[] EMPTY_ARRAY = new Object[0];
-  private final int capacity;
+  private final int initialCapacity;
   private int size;
   private Object primitiveArray;
   private Object[] objectArray;
 
-  private Array_Builder(int capacity) {
+  private Array_Builder(int initialCapacity) {
-    this.capacity = Math.max(1, capacity);
+    this.initialCapacity = Math.max(1, initialCapacity);
   }
 
   /**
@@ -79,15 +79,15 @@ public class Array_Builder<T> {
         assert primitiveArray == null;
         assert size == 0;
         if (e instanceof Long l) {
-          var arr = new long[capacity];
+          var arr = new long[initialCapacity];
           arr[0] = l;
           primitiveArray = arr;
         } else if (e instanceof Double d) {
-          var arr = new double[capacity];
+          var arr = new double[initialCapacity];
           arr[0] = d;
           primitiveArray = arr;
         } else {
-          var arr = new Object[capacity];
+          var arr = new Object[initialCapacity];
           arr[0] = e;
           objectArray = arr;
         }

std-bits/table/src/main/java/org/enso/table/data/table/Table.java

@@ -39,18 +39,17 @@ public class Table {
   }
 
   public Table(Column[] columns, AggregatedProblems problems) {
+    if (columns.length == 0) {
+      throw new IllegalArgumentException("A Table must have at least one column.");
+    }
+
     this.columns = columns;
     this.problems = problems;
   }
 
   /** @return the number of rows in this table */
   public int rowCount() {
-    // TODO I think we can make this check obsolete once we start requiring >=1 column in tables.
+    return columns[0].getSize();
-    if (columns.length == 0) {
-      return 0;
-    } else {
-      return columns[0].getSize();
-    }
   }
 
   /** @return the columns of this table */

std-bits/table/src/main/java/org/enso/table/error/EmptyFileException.java

@@ -0,0 +1,7 @@
+package org.enso.table.error;
+
+public class EmptyFileException extends RuntimeException {
+  public EmptyFileException() {
+    super("Cannot parse an empty file.");
+  }
+}

std-bits/table/src/main/java/org/enso/table/error/EmptySheetException.java

@@ -0,0 +1,7 @@
+package org.enso.table.error;
+
+public class EmptySheetException extends RuntimeException {
+  public EmptySheetException() {
+    super("Cannot parse an empty sheet.");
+  }
+}

std-bits/table/src/main/java/org/enso/table/read/DelimitedReader.java

@@ -7,6 +7,7 @@ import org.enso.table.data.column.builder.string.StringStorageBuilder;
 import org.enso.table.data.column.storage.Storage;
 import org.enso.table.data.table.Column;
 import org.enso.table.data.table.Table;
+import org.enso.table.error.EmptyFileException;
 import org.enso.table.parsing.DatatypeParser;
 import org.enso.table.parsing.TypeInferringParser;
 import org.enso.table.parsing.problems.AdditionalInvalidRows;
@@ -438,7 +439,12 @@ public class DelimitedReader {
   /** Reads the input stream and returns a Table. */
   public WithProblems<Table> read() {
     ensureHeadersDetected();
-    initBuilders(getColumnCount());
+    int columnCount = getColumnCount();
+    if  (columnCount == 0) {
+      throw new EmptyFileException();
+    }
+
+    initBuilders(columnCount);
     while (canFitMoreRows()) {
       var currentRow = readNextRow();
       if (currentRow == null) break;

std-bits/table/src/main/java/org/enso/table/read/ExcelReader.java

@@ -10,6 +10,7 @@ import org.enso.table.data.column.builder.object.InferredBuilder;
 import org.enso.table.data.column.storage.ObjectStorage;
 import org.enso.table.data.table.Column;
 import org.enso.table.data.table.Table;
+import org.enso.table.error.EmptySheetException;
 import org.enso.table.error.InvalidLocationException;
 import org.enso.table.excel.ExcelHeaders;
 import org.enso.table.excel.ExcelRange;
@@ -312,6 +313,10 @@ public class ExcelReader {
             .mapToObj(idx -> new Column(excelHeaders.get(idx + startCol), builders.get(idx).seal()))
             .toArray(Column[]::new);
 
+    if (columns.length == 0) {
+      throw new EmptySheetException();
+    }
+
     return new WithProblems<>(new Table(columns), excelHeaders.getProblems());
   }
 

test/Table_Tests/src/Common_Table_Operations/Aggregate_Spec.enso

@@ -1192,10 +1192,9 @@ spec setup =
             table_builder [col1, col2]
 
         Test.specify "should raise a warning when there are no output columns" <|
-            action = table.aggregate [] on_problems=_
+            [Problem_Behavior.Ignore, Problem_Behavior.Report_Warning, Problem_Behavior.Report_Error].each pb->
-            problems = [No_Output_Columns]
+                t = table.aggregate [] on_problems=pb
-            tester = expect_column_names []
+                t.should_fail_with No_Output_Columns
-            Problems.test_problem_handling action problems tester
 
         Test.specify "should raise a warning when can't find a column by name" <|
             action = table.aggregate [Group_By "Missing", Group_By "Index"] on_problems=_
@@ -1209,10 +1208,10 @@ spec setup =
             tester = expect_column_names ["Index"]
             Problems.test_problem_handling action problems tester
 
-        Test.specify "should raise warnings when an invalid column index and no valid output" <|
+        Test.specify "should raise warnings when grouped by an invalid column index" <|
-            action = table.aggregate [Group_By -3] on_problems=_
+            action = table.aggregate [Group_By -3, Count] on_problems=_
-            problems = [Column_Indexes_Out_Of_Range.Error [-3], No_Output_Columns]
+            problems = [Column_Indexes_Out_Of_Range.Error [-3]]
-            tester = expect_column_names []
+            tester = expect_column_names ["Count"]
             Problems.test_problem_handling action problems tester
 
         Test.specify "should raise a warning when an invalid output name" <|
@@ -1240,9 +1239,9 @@ spec setup =
             Problems.test_problem_handling action problems tester
 
         Test.specify "should ignore Count_Distinct if no columns matched" <|
-            action = table.aggregate [Count_Distinct (Column_Selector.By_Index [-100])] on_problems=_
+            action = table.aggregate [Count_Distinct (Column_Selector.By_Index [-100]), Count] on_problems=_
-            problems = [Column_Indexes_Out_Of_Range.Error [-100], No_Output_Columns]
+            problems = [Column_Indexes_Out_Of_Range.Error [-100]]
-            tester = expect_column_names []
+            tester = expect_column_names ["Count"]
             Problems.test_problem_handling action problems tester
 
     Test.group prefix+"Table.aggregate should raise warnings when there are issues computing aggregation" pending=(resolve_pending test_selection.aggregation_problems) <|

test/Table_Tests/src/Common_Table_Operations/Distinct_Spec.enso

@@ -1,7 +1,7 @@
 from Standard.Base import all
 
 from Standard.Table import Column_Selector, Sort_Column, Sort_Column_Selector
-from Standard.Table.Errors import Floating_Point_Grouping
+from Standard.Table.Errors import all
 
 from Standard.Test import Test, Problems
 import Standard.Test.Extensions
@@ -91,3 +91,21 @@ spec setup =
 
             va.at 2 . should_equal "b"
             [3, 5, 8].contains (vb.at 2) . should_be_true
+
+        Test.specify "should report missing input columns as errors regardless of on_problems" <|
+            t1 = table_builder [["X", [1, 2, 3]]]
+            [Problem_Behavior.Ignore, Problem_Behavior.Report_Warning, Problem_Behavior.Report_Error].each pb->
+                t2 = t1.distinct ["Y", "Z"] on_problems=pb
+                t2.should_fail_with Missing_Input_Columns
+                t2.catch . should_equal (Missing_Input_Columns.Error ["Y", "Z"])
+
+                t3 = t1.distinct (Column_Selector.By_Name ["X", "Y"]) on_problems=pb
+                t3.should_fail_with Missing_Input_Columns
+                t3.catch . should_equal (Missing_Input_Columns.Error ["Y"])
+
+                t4 = t1.distinct (Column_Selector.By_Index [0, 42]) on_problems=pb
+                t4.should_fail_with Column_Indexes_Out_Of_Range
+                t4.catch . should_equal (Column_Indexes_Out_Of_Range.Error [42])
+
+                t5 = t1.distinct [] on_problems=pb
+                t5.should_fail_with No_Input_Columns_Selected

test/Table_Tests/src/Common_Table_Operations/Missing_Values_Spec.enso

@@ -52,17 +52,6 @@ spec setup =
             t4.row_count . should_equal 0
             t4.at "X" . to_vector . should_equal []
 
-            empty = table_builder [["X", [1, 2, 3]]] . select_columns []
-            empty.row_count . should_equal 0
-            empty.columns . should_equal []
-
-            t5 = empty.filter_blank_rows when_any=True
-            t5.row_count . should_equal 0
-            t5.columns . should_equal []
-            t6 = empty.filter_blank_rows when_any=False
-            t6.row_count . should_equal 0
-            t6.columns . should_equal []
-
         Test.specify "should allow to remove blank columns" <|
             r1 = t1.remove_columns (Column_Selector.Blank_Columns when_any=False)
             r1.columns.map .name . should_equal ["a", "b", "c", "d", "e"]
@@ -137,12 +126,9 @@ spec setup =
 
         Test.specify "Blank_Columns selector should deal with edge cases" <|
             t = table_builder [["X", [1, 2, 3, 4]]]
-            no_cols = t.select_columns []
             no_rows = t.filter "X" (Filter_Condition.Equal to=0)
-            no_cols.columns . should_equal []
             no_rows.row_count . should_equal 0
             no_rows.at "X" . to_vector . should_equal []
 
-            no_cols.select_columns Column_Selector.Blank_Columns . columns . map .name . should_equal []
             no_rows.select_columns Column_Selector.Blank_Columns . columns . map .name . should_equal ["X"]
-            no_rows.remove_columns Column_Selector.Blank_Columns . columns . map .name . should_equal []
+            no_rows.remove_columns Column_Selector.Blank_Columns . columns . map .name . should_fail_with No_Output_Columns

test/Table_Tests/src/Common_Table_Operations/Select_Columns_Spec.enso

@@ -143,23 +143,17 @@ spec setup =
             Problems.test_problem_handling action problems tester
 
         Test.specify "should correctly handle problems: no columns in the output" <|
-            selector = By_Name []
+            [Problem_Behavior.Ignore, Problem_Behavior.Report_Warning, Problem_Behavior.Report_Error].each pb->
-            action = table.select_columns selector on_problems=_
+                t = table.select_columns (By_Index []) on_problems=pb
-            tester = expect_column_names []
+                t.should_fail_with No_Output_Columns
-            problems = [No_Output_Columns]
-            Problems.test_problem_handling action problems tester
 
         Test.specify "should correctly handle multiple problems" <|
-            selector = By_Name ["hmmm"]
+            table.select_columns (By_Name ["hmmm"]) . should_fail_with No_Output_Columns
-            action = table.select_columns selector on_problems=_
-            tester = expect_column_names []
-            problems = [Missing_Input_Columns.Error ["hmmm"], No_Output_Columns]
-            Problems.test_problem_handling action problems tester
 
-            action_2 = table.select_columns (By_Index [0, -7, 0, 100]) on_problems=_
+            action = table.select_columns (By_Index [0, -7, 0, 100]) on_problems=_
-            problems_2 = [Column_Indexes_Out_Of_Range.Error [100], Duplicate_Column_Selectors.Error [0], Input_Indices_Already_Matched.Error [-7]]
+            problems = [Column_Indexes_Out_Of_Range.Error [100], Duplicate_Column_Selectors.Error [0], Input_Indices_Already_Matched.Error [-7]]
-            tester_2 = expect_column_names ["foo"]
+            tester = expect_column_names ["foo"]
-            Problems.test_problem_handling action_2 problems_2 tester_2
+            Problems.test_problem_handling action problems tester
 
     Test.group prefix+"Table.remove_columns" <|
         Test.specify "should work as shown in the doc examples" <|
@@ -267,18 +261,15 @@ spec setup =
             Problems.test_problem_handling action problems tester
 
         Test.specify "should correctly handle problems: no columns in the output" <|
-            selector = By_Name [".*"] (Regex_Matcher.Value case_sensitivity=Case_Sensitivity.Sensitive)
+            [Problem_Behavior.Ignore, Problem_Behavior.Report_Warning, Problem_Behavior.Report_Error].each pb->
-            action = table.remove_columns selector on_problems=_
+                selector = By_Name [".*"] (Regex_Matcher.Value case_sensitivity=Case_Sensitivity.Sensitive)
-            tester = expect_column_names []
+                t = table.remove_columns selector on_problems=pb
-            problems = [No_Output_Columns]
+                t.should_fail_with No_Output_Columns
-            Problems.test_problem_handling action problems tester
 
         Test.specify "should correctly handle multiple problems" <|
             selector = By_Name [".*", "hmmm"] (Regex_Matcher.Value case_sensitivity=Case_Sensitivity.Sensitive)
-            action = table.remove_columns selector on_problems=_
+            t1 = table.remove_columns selector
-            tester = expect_column_names []
+            t1.should_fail_with No_Output_Columns
-            problems = [Missing_Input_Columns.Error ["hmmm"], No_Output_Columns]
-            Problems.test_problem_handling action problems tester
 
             action_2 = table.remove_columns (By_Index [0, -7, 0, 100]) on_problems=_
             problems_2 = [Column_Indexes_Out_Of_Range.Error [100], Duplicate_Column_Selectors.Error [0], Input_Indices_Already_Matched.Error [-7]]

test/Table_Tests/src/Database/Codegen_Spec.enso

@@ -61,13 +61,6 @@ spec =
             t2 = t1.limit 5
             t2.to_sql.prepare . should_equal ['SELECT "T1"."A" AS "A", "T1"."B" AS "B", "T1"."C" AS "C" FROM "T1" AS "T1" LIMIT 5', []]
 
-        Test.specify "should work correctly when there are no columns" <|
-            empty = t1.select_columns (Column_Selector.By_Name [])
-            json = JS_Object.from_pairs [["query", Nothing], ["message", "The table has no columns so a query cannot be generated."]] . to_text
-            empty.to_json . should_equal json
-            empty.column_count . should_equal 0
-            empty.to_sql . should_fail_with Unsupported_Database_Operation.Error
-
     Test.group "[Codegen] Building Expressions" <|
         Test.specify "should allow building expressions from columns and constants" <|
             a = t1.at "A"

test/Table_Tests/src/IO/Delimited_Read_Spec.enso

@@ -87,10 +87,9 @@ spec =
             t2.row_count .  should_equal 0
             t2.at "x" . to_vector . should_equal []
 
-        Test.specify "should be able to load even an empty file" <|
+        Test.specify "should raise an informative error when loading an empty file" <|
-            table = Data.read (enso_project.data / "empty.txt") (Delimited "," headers=True value_formatter=Nothing)
+            t = Data.read (enso_project.data / "empty.txt") (Delimited "," headers=True value_formatter=Nothing)
-            table.columns.map .name . should_equal []
+            t.should_fail_with Empty_File_Error
-            table.row_count . should_equal 0
 
         Test.specify "should correctly handle file opening issues" <|
             nonexistent_file = enso_project.data / "a_filename_that_does_not_exist.foobar"

test/Table_Tests/src/IO/Delimited_Write_Spec.enso

@@ -50,15 +50,6 @@ spec =
                 text.should_equal (lines.join separator suffix=separator)
                 file.delete
 
-        Test.specify "should be able to write an empty table" <|
-            table = Table.new []
-            file = (enso_project.data / "transient" / "empty.csv")
-            file.delete_if_exists
-            table.write file on_problems=Report_Error . should_succeed
-            text = Data.read_text file
-            text.should_equal ''
-            file.delete
-
         Test.specify 'should quote values that contain the delimiter or quotes, in the [,""] variant' <|
             data_formatter = Data_Formatter.Value decimal_point=","
             table = Table.new [['The Column "Name"', ["foo","'bar'",'"baz"', 'one, two, three']], ["Hello, Column?", [1.0, 1000000.5, 2.2, -1.5]]]

test/Table_Tests/src/IO/Excel_Spec.enso

@@ -4,7 +4,7 @@ import Standard.Base.Error.Illegal_Argument.Illegal_Argument
 
 from Standard.Table import Table, Match_Columns, Column_Name_Mapping, Excel, Excel_Range, Data_Formatter, Sheet_Names, Range_Names, Worksheet, Cell_Range, Delimited, Column_Selector
 
-from Standard.Table.Errors import Invalid_Output_Column_Names, Duplicate_Output_Column_Names, Invalid_Location, Range_Exceeded, Existing_Data, Column_Count_Mismatch, Column_Name_Mismatch
+from Standard.Table.Errors import Invalid_Output_Column_Names, Duplicate_Output_Column_Names, Invalid_Location, Range_Exceeded, Existing_Data, Column_Count_Mismatch, Column_Name_Mismatch, Empty_Sheet_Error
 
 from Standard.Test import Test, Test_Suite, Problems
 import Standard.Test.Extensions
@@ -36,9 +36,9 @@ spec_fmt header file read_method =
             t.columns.map .name . should_equal ['Student Name', 'Enrolment Date']
             t.at 'Enrolment Date' . map .day . to_vector . should_equal [2, 26, 4, 24, 31, 7]
 
-        Test.specify "should read an empty table" <|
+        Test.specify "should give an informative error when reading an empty table" <|
             t = read_method file (Excel (Worksheet "Empty"))
-            t.column_count.should_equal 0
+            t.should_fail_with Empty_Sheet_Error
 
         Test.specify "should gracefully handle duplicate column names and formulas" <|
             t = read_method file (Excel (Worksheet "Duplicate Columns"))

test/Table_Tests/src/In_Memory/Table_Spec.enso

@@ -77,13 +77,12 @@ spec =
         Test.specify "should handle error scenarios gracefully" <|
             Table.new [["X", [1,2,3]], ["Y", [4]]] . should_fail_with Illegal_Argument.Error
             Table.new [["X", [1]], ["X", [2]]] . should_fail_with Illegal_Argument.Error
+            Table.new [] . should_fail_with Illegal_Argument.Error
 
             Table.from_rows ["X", "X"] [] . should_fail_with Illegal_Argument.Error
             Table.from_rows ["X", "Y"] [[1,2], [1]] . should_fail_with Index_Out_Of_Bounds.Error
-
-        Test.specify "should handle error scenarios gracefully (merge with above once enabled)" pending="To be enabled after errors refactor where instead of returning a table with no columns we throw an error." <|
-            Table.new [] . should_fail_with Illegal_Argument.Error
             Table.from_rows [] [] . should_fail_with Illegal_Argument.Error
+            Table.from_rows [] [[]] . should_fail_with Illegal_Argument.Error
 
         Test.specify "should correctly infer storage types" <|
             varied_type_table.at "strs" . storage_type . should_equal Storage.Text
@@ -729,18 +728,6 @@ spec =
             problems2 = [Floating_Point_Grouping.Error "X"]
             Problems.test_problem_handling action2 problems2 tester2
 
-        Test.specify "should report a warning and report the whole table if no columns were selected" <|
-            t = Table.new [["A", [1, 2, 1, 1]]]
-            test table = table.should_equal t
-
-            action1 = t.distinct (Column_Selector.By_Name []) on_problems=_
-            problems1 = [No_Input_Columns_Selected]
-            Problems.test_problem_handling action1 problems1 test
-
-            action2 = t.distinct (Column_Selector.By_Name ["mismatched"]) on_problems=_
-            problems2 = [Missing_Input_Columns.Error ["mismatched"], No_Input_Columns_Selected]
-            Problems.test_problem_handling action2 problems2 test
-
         Test.specify "until hashing is supported, should throw an error when trying to aggregate a custom object" <|
             t = Table.new [["X", [My.Data 1 2, My.Data 3 4, My.Data 1 2]]]
             t.distinct . should_fail_with Illegal_Argument.Error

test/Visualization_Tests/src/Geo_Map_Spec.enso

@@ -14,10 +14,6 @@ spec =
         Json.parse result . should_equal <| Json.parse expected_json_text
 
     Test.group "Geo_Map" <|
-        Test.specify "works with empty table" <|
-            table  = Table.from_rows [] []
-            expect table '{}'
-
         Test.specify "skips unrecognized columns" <|
             header = ['α' , 'β' , 'ω']
             row_1  = [11  , 10  , 09 ]

test/Visualization_Tests/src/Histogram_Spec.enso

@@ -23,11 +23,6 @@ spec =
         json.should_equal expected_json
 
     Test.group "Histogram Visualization" <|
-
-        Test.specify "deals with an empty table" <|
-            table  = Table.from_rows [] []
-            expect table Nothing []
-
         Test.specify "plots first column if none recognized" <|
             header = ['α', 'ω']
             row_1  = [11 , 10 ]

test/Visualization_Tests/src/Scatter_Plot_Spec.enso

@@ -29,11 +29,6 @@ spec =
     no_labels  = 'null'
 
     Test.group "Scatter Plot Visualization" <|
-
-        Test.specify "deals with an empty table" <|
-            table  = Table.from_rows [] []
-            expect table 'null' '[]'
-
         Test.specify "plots first column if none recognized" <|
             header = ['α', 'ω']
             row_1  = [11 , 10 ]