Concise way to make a 0+ length list in Marpa grammar?

Question

I'm new to Marpa. I've tried a couple ways to describe a list of 0 or more terms in my grammar, and I want to avoid multiple parse trees. My language will have exactly 1 component followed by 0+ subcomponents: ``` package => component-rule [subcomponent-rule ...] ``` What I tried first was this: ``` { lhs => 'Package', rhs => [qw/component-rule subcomponents/] }, { lhs => 'subcomponents', rhs => [qw/subcomponent-list/] }, { lhs => 'subcomponent-list', rhs => [qw/subcomponent-rule/], action => 'do_subcomponent_list' }, { lhs => 'subcomponent-list', rhs => [qw/subcomponent-list subcomponent-rule/], action => 'do_subcomponent_list' }, { lhs => 'subcomponent-list', rhs => [qw//], action => 'do_subcomponent_empty_list' }, { lhs => 'subcomponent-rule', rhs => [qw/subcomponent subcomponent-name/], action => 'do_subcomponent' }, ``` (Full code at end of post.) Here's my input: ``` $recce->read( 'component', ); $recce->read( 'String', 'MO Factory'); $recce->read( 'subcomponent', ); $recce->rea…

Accepted Answer

Specify a sequence rule with the `min` argument. The value may either be `0` (aka the `*` quantifier in regexes) or `1` (aka the `+` quantifier). You can do this by removing the `subcomponents` and `subcomponent-list` rules. Instead add:

```
{
  lhs => 'subcomponents',
  rhs => ['subcomponent-rule'],
  min => 0,
  action => 'do_subcomponent_list',
}
```

Your grammar then runs without further modifications.

Using sequence rules is preferable: No flattening has to take place, and the grammar should be more efficient.

Note that you are encouraged to use the Scanless Interface. The DSL abstracts nicely over this issue:

```
subcomponents ::= <subcomponent rule>* action => do_subcomponent_list
```

Concise way to make a 0+ length list in Marpa grammar?

Solution

Problem