Table.cast prototype, some tests

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

@@ -922,6 +922,11 @@ type Column
     ## UNSTABLE
        Cast the column to a specific type.
 
+       Arguments:
+       - value_type: The `Value_Type` to cast the column to.
+       - on_problems: Specifies how to handle problems if they occur, reporting
+         them as warnings by default.
+
        TODO [RW] this is a prototype needed for debugging, proper implementation
        and testing will come with #6112.
 
@@ -959,11 +964,6 @@ type Column
          If the backend does not support the requested target type, the closest
          supported type is chosen and a `Inexact_Type_Coercion` problem is
          reported.
-
-       Arguments:
-       - value_type: The `Value_Type` to cast the column to.
-       - on_problems: Specifies how to handle problems if they occur, reporting
-         them as warnings by default.
     cast : Value_Type -> Problem_Behavior -> Column ! Illegal_Argument | Inexact_Type_Coercion | Lossy_Conversion
     cast self value_type=self.value_type on_problems=Problem_Behavior.Report_Warning =
         dialect = self.connection.dialect

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

@@ -1391,6 +1391,62 @@ type Table
         msg = "Parsing values is not supported in database tables, the table has to be materialized first with `read`."
         Error.throw (Unsupported_Database_Operation.Error msg)
 
+    ## UNSTABLE
+       Cast the selected columns to a specific type.
+
+       Returns a new table in which the selected columns are replaced with
+       columns having the new types.
+
+       Arguments:
+       - columns: The selection of columns to cast.
+       - value_type: The `Value_Type` to cast the column to.
+       - on_problems: Specifies how to handle problems if they occur, reporting
+         them as warnings by default.
+
+       TODO [RW] this is a prototype needed for debugging, proper implementation
+       and testing will come with #6112.
+
+       In the Database backend, this will boil down to a CAST operation.
+       In the in-memory backend, a conversion will be performed according to
+       the following rules:
+       - Anything can be cast into the `Mixed` type.
+       - Converting to a `Char` type, the elements of the column will be
+         converted to text. If it is fixed length, the texts will be trimmed or
+         padded on the right with the space character to match the desired
+         length.
+       - Conversion between numeric types will replace values exceeding the
+         range of the target type with `Nothing`.
+       - Booleans may also be converted to numbers, with `True` being converted
+         to `1` and `False` to `0`. The reverse is not supported - use `iif`
+         instead.
+       - A `Date_Time` may be converted into a `Date` or `Time` type - the
+         resulting value will be truncated to the desired type.
+       - If a `Date` is to be converted to `Date_Time`, it will be set at
+         midnight of the default system timezone.
+
+       ? Conversion Precision
+
+         In the in-memory backend, if the conversion is lossy, a
+         `Lossy_Conversion` warning will be reported. The only exception is when
+         truncating a column which is already a text column - as then the
+         truncation seems like an intended behaviour, so it is not reported. If
+         truncating needs to occur when converting a non-text column, a warning
+         will still be reported.
+
+         Currently, the warning is not reported for Database backends.
+
+       ? Inexact Target Type
+
+         If the backend does not support the requested target type, the closest
+         supported type is chosen and a `Inexact_Type_Coercion` problem is
+         reported.
+    cast : (Text | Integer | Column_Selector | Vector (Integer | Text | Column_Selector)) -> Value_Type -> Problem_Behavior -> Table ! Illegal_Argument | Inexact_Type_Coercion | Lossy_Conversion
+    cast self columns=[0] value_type=Value_Type.Char on_problems=Problem_Behavior.Report_Warning =
+        selected = self.select_columns columns
+        selected.columns.fold self table-> column_to_cast->
+            new_column = column_to_cast.cast value_type on_problems
+            table.set new_column new_name=column_to_cast.name set_mode=Set_Mode.Update
+
     ## ALIAS dropna
        ALIAS drop_missing_rows
        Remove rows which are all blank or containing blank values.

test/Table_Tests/src/Common_Table_Operations/Cast_Spec.enso

@@ -32,6 +32,12 @@ spec setup =
             c.value_type.is_text . should_be_true
             c.to_vector . should_equal ["true", "false", "true"]
 
+        Test.specify "should allow to cast a text column to fixed-length" pending=(if setup.test_selection.fixed_length_text_columns.not then "Fixed-length Char columns are not supported by this backend.") <|
+            t = table_builder [["X", ["a", "DEF", "a slightly longer text"]]]
+            c = t.at "X" . cast (Value_Type.Char size=3 variable_length=False)
+            c.value_type . should_equal (Value_Type.Char size=3 variable_length=False)
+            c.to_vector . should_equal ["a  ", "DEF", "a s"]
+
         Test.specify "should work if the first row is NULL" <|
             t = table_builder [["X", [Nothing, 1, 2, 3000]], ["Y", [Nothing, True, False, True]]]
 
@@ -69,3 +75,19 @@ spec setup =
             c4 = c2 + 1000
             c4.value_type.is_integer . should_be_true
             c4.to_vector . should_equal [Nothing, 1001, 1000, 1001]
+
+    Test.group prefix+"Table.cast" pending=(if setup.is_database.not then "Cast is not implemented in the in-memory backend yet.") <|
+        Test.specify "should cast the columns in-place and not reorder them" <|
+            t = table_builder [["X", [1, 2, 3000]], ["Y", [4, 5, 6]], ["Z", [7, 8, 9]], ["A", [True, False, True]]]
+            t2 = t.cast ["Z", "Y"] Value_Type.Char
+            t2.column_names . should_equal ["X", "Y", "Z", "A"]
+
+            t2.at "X" . value_type . is_integer . should_be_true
+            t2.at "Y" . value_type . is_text . should_be_true
+            t2.at "Z" . value_type . is_text . should_be_true
+            t2.at "A" . value_type . is_boolean . should_be_true
+
+            t2.at "X" . to_vector . should_equal [1, 2, 3000]
+            t2.at "Y" . to_vector . should_equal ["4", "5", "6"]
+            t2.at "Z" . to_vector . should_equal ["7", "8", "9"]
+            t2.at "A" . to_vector . should_equal [True, False, True]

test/Table_Tests/src/Common_Table_Operations/Main.enso

@@ -89,7 +89,9 @@ type Test_Selection
          each group. Guaranteed in the in-memory backend, but may not be
          supported by all databases.
        - date_time: Specifies if the backend supports date/time operations.
-    Config supports_case_sensitive_columns=True order_by=True natural_ordering=False case_insensitive_ordering=True order_by_unicode_normalization_by_default=False case_insensitive_ascii_only=False take_drop=True allows_mixed_type_comparisons=True supports_unicode_normalization=False is_nan_and_nothing_distinct=True supports_full_join=True distinct_returns_first_row_from_group_if_ordered=True date_time=True
+       - fixed_length_text_columns: Specifies if the backend supports fixed
+         length text columns.
+    Config supports_case_sensitive_columns=True order_by=True natural_ordering=False case_insensitive_ordering=True order_by_unicode_normalization_by_default=False case_insensitive_ascii_only=False take_drop=True allows_mixed_type_comparisons=True supports_unicode_normalization=False is_nan_and_nothing_distinct=True supports_full_join=True distinct_returns_first_row_from_group_if_ordered=True date_time=True fixed_length_text_columns=False
 
 spec setup =
     Core_Spec.spec setup

test/Table_Tests/src/Database/Postgres_Spec.enso

@@ -181,7 +181,7 @@ run_tests connection db_name =
     Common_Spec.spec prefix connection
     postgres_specific_spec connection db_name
 
-    common_selection = Common_Table_Operations.Main.Test_Selection.Config supports_case_sensitive_columns=True order_by_unicode_normalization_by_default=True take_drop=False allows_mixed_type_comparisons=False
+    common_selection = Common_Table_Operations.Main.Test_Selection.Config supports_case_sensitive_columns=True order_by_unicode_normalization_by_default=True take_drop=False allows_mixed_type_comparisons=False fixed_length_text_columns=True
     aggregate_selection = Common_Table_Operations.Aggregate_Spec.Test_Selection.Config first_last_row_order=False aggregation_problems=False
     agg_in_memory_table = (enso_project.data / "data.csv") . read
     agg_table = connection.upload_table (Name_Generator.random_name "Agg1") agg_in_memory_table