Yes, having the editor insert the closing ], ), } will help.
My project does include a source editor, if it did not I would not bother trying to do error recovery on the parsing. Top-level chunking is enough to restart the parser and allows multiple errors to be reported per file. Imagine for example, if Elm did not do this and only ever gave max 1 error per file, you would not get such a good insight into how many errors you have left to fix, or to choose the order in which you fix them. But for a compiler its probably enough if it can generate one error per chunk at the syntax checking stage.
My source language Salix is a data modelling language, and the compiler can feed it to a configurable set of output code generators. Each code generator can accept or require additional ‘properties’ be added to the source. For example, if I model a 1:1 relationship between 2 records and am generating the DDL for a relational database, I need to decide which table will hold the foreign key. In that case, there might be a sql.fk property to tag one of the record fields with. So beyond parsing, I want feedback from later stages of the compiler to the editor in order to populate auto completion lists with the available or required properties, and so on.
That is my motivation for exploring error recovery deeper within the AST, and not just top-level chunking. I want to succesfully parse the property list, even when it is not syntactically correct.
This illustrates the feedback loop:
Editor --- (Source) ---> Parser --- (AST) --> Later Compiler Stages
^--------------------Context Sensitive Help ------------|
This also leads me to thinking, do I need some kind of additional ‘hole’ in my AST? So where there is an error in the source, I could mark a hole that later stages of the compiler could provide auto completion suggestions for? Perhaps I don’t need to do this, as I am only interested in errors that occur where the cursor currently is, so I can figure out where in the AST the cursor is, and build the auto completion list based on that.
I think where an error occurs, and the parser chomps to recover, I need to capture the chomped string in the error (using getChompedString). I possibly need to trim off the whitespace around that chomped string too. This string and its location will then allow the editor to know what to replace with suggestions from the auto completion list.