Returning a Mutable Variable from a Rule about rust-peg HOT 5 CLOSED

In the general case, mutating a parent rule's variable isn't safe due to backtracking, so it's not supported.

The * operator collects its children into a Vec by default, so your case could be rewritten:

document -> Document
    = children: block*
    { Document { children: children } }

block -> Element
    = ...

where Document.children has type Vec<Element>. It might be possible to make it generic over some Container trait rather than requiring the use of Vec.

from rust-peg.

And if you want to mutate a variable within the same rule, the rule action has ownership, so it can move it into a mutable variable:

document -> Document
    = children: block* {
      let mut children = children;
      children.push(another_element());
      Document { children: children } 
    }

from rust-peg.

Alright, that might work for the basic use case presented in the initial post, but the structure of my program is such that the block rule from above also calls other rules, which, in turn, may return Elements themselves.

If the rules that block calls return a Vec<Element> as well, how would I then add that to the Vec<Element> children?

from rust-peg.

One way would be to have block return Vec<Element> and have the document rule concat_vec the Vec<Vec<Element>> it receives. Yes, it allocates more than a single vector, but matches need to be independent to be able to backtrack.

from rust-peg.

Alright I'll try that. Thanks!

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