How should I understand the parameters/argument values in the {} and [] right after a function?

Question

Hi,
How should I understand the parameters/argument values in the {} and [] right after a function?
For instance the {[Name="control"]}[Content] part in the
control=Excel.CurrentWorkbook(){[Name="control"]}[Content]
and also the [Control]{0} part in the periodctrl=control[Control]{0}.
Thanks.
Regards,

Answer 1

The "[]" operator in M is like the "." operator in C-like languages, if that helps. It takes an aggregate value with named components and it selects one of those components. In the case of the record "Foo", "Foo[Bar]"
returns the field named Bar. In the case of the table "Baz", "Baz[Quux]" returns the column named Quux.
Similarly, the "{}" operator in M does subscripting of lists -- like the "[]" operator in C-like languages. If "Foo" is a list, then "Foo{0}" returns the first element of the list, "Foo{1}" returns the second
element of the list, and so on.
In the case of tables, it's also possible to use "{}" with a record argument, in which case it will return the row which matches the record criteria -- provided there is exactly one. So if "Foo" is a table, then "Foo{[Bar = 1]}"
will return the row for which the Bar column has a value of 1.
Many of the data access functions in Power Query return what we call "navigation tables". Sql.Database, for instance, returns a table with columns named Name, Schema, Object, Kind and Data. Each table in the database has a single entry in this
navigation table. When we index into the table with "table{[Schema="dbo", Object="table"]}", we're selecting the single row that matches those criteria. This gives us a record. We then want to pick the "Data" field of
that record, which contains the actual table data. That's why there's typically a "[Data]" at the end of expressions which fetch data from the SQL table.