Good answer, Lukas, thanks! -- jeffrey
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------- Original Message -------
On Saturday, April 30th, 2022 at 4:27 AM, Lukas Atkinson
<[email protected]> wrote:
> Your grammar does not allow the list in that location. Here is the
> relevant excerpt:
>
> ```
> :start ::= language
> #
> # include begin
> language ::= if_statement action_statements action => ifAction
>
> | action_statements action => verbAction
>
> | ('List') variable ('is') comma_list action =>
>
> listAction
> `So you say that a document can EITHER be an if-statement followed by any
> number of action statements, OR any number of action statements, OR a List
> statement. This is why testing a List statement by itself works fine. But in
> your input, you have an if-statement that includes one action statement,
> followed by an unexpected List statement:`
> If documentTitle = A100
> remove Document
> List Author is Dante, Murakami, Ash, Harris
> `Instead, you would likely want to change your grammar so that you can have
> multiple statements of different type, for example:`
> language ::= statement*
>
> statement ::= if_statement | action | list_statement
> `But this might become ambiguous. For example, consider this input:`
> If foo = bar
> remove Doc1
> remove Doc2
> ``Should the `remove Doc2` action be within the if-statement, or should it be
> an unconditional action? In your current grammar, both actions would be part
> of the if-statement. In my suggested grammar, both are allowed and the parse
> is ambiguous. This problem is solved most easily if you can change your
> language to have explicit delimiters, e.g. curly braces:``
> If foo = bar {
> remove Doc1
> }
> remove Doc2
> ``or other compound statement terminators, e.g. an `End` like in Ruby or
> Lua:``
> If foo = bar
> remove Doc1
> End
> remove Doc2
> `Of course, many languages use the indentation level to indicate nesting, for
> example Python or YAML. Marpa has no built-in features for
> indentation-sensitive parsing. If you are trying to parse an existing format
> or really want to use indentation to indicate the scope of the conditional,
> you would have to use Marpa's event system to handle indentation. You could
> use the same trick that Python uses: having special/invisible INDENT and
> DEDENT tokens that mark where the indented block begins and ends. These
> tokens are emitted whenever the indentation changes. In the grammar, these
> tokens can then be used just like curly braces in curly-brace languages. For
> example, you might then define your if-statement like:`
> if_statement ::= ('If') condition (INDENT) action_statements (DEDENT)
> ```
>
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