How to run the Rest parser when a OneOf parser fails

[SwiftDevJournal]

I have a OneOf parser in my code. When all the parsers in the OneOf parser fail, I want to consume the remaining text in the input. The Rest parser consumes the remaining text. I see there's a .replaceError modifier that lets you return a string when the OneOf parser fails.

How do I run the Rest parser on the remaining text in the input and supply the results to the .replaceError modifier when the OneOf parser fails?

[mbrandonw]

Hi @SwiftDevJournal, I think you will need to provide some code (as well as the text format you are trying to parse) in order for me to better understand your situation. I would think something like OneOf { …; Rest() } would do what you want given your description, but I'm not sure.

[SwiftDevJournal]

I'm trying to parse the commit message of a commit in the Jujutsu version control system. The details of a commit consist of the following:

• Information about the commit, such as the ID, the timestamp and the person who made the commit
• The commit message. In Jujutsu terminology it's called a change description. The commit message is indented.
• What changed in the commit. Some commits can have nothing that changed.

A typical commit looks like the following:

Commit ID: 73d31d1766b0bc211d22b87f2f911429f3846a61
Change ID: lyrzonnxolkpxytwwykkqpvvlqmvlxqm
Bookmarks: trunk@origin
Author   : Swift Dev Journal <mark@swiftdevjournal.com> (2025-06-23 18:04:09)
Committer: Swift Dev Journal <mark@swiftdevjournal.com> (2025-06-23 18:04:49)

    Another change

Modified regular file GitHub.md:
    ...
   4    4: 
   5    5: ## Working with Existing GitHub Repo
   6    6: 
   7    7: In most cases you have an existing GitHub repo. This is what you do.
        8: 
        9: You have to set a bookmark and call `jj git push`.

To get the commit message, I skip everything through the first blank line and any whitespace after the blank line. I read everything until there's a blank line and the start of the details of what changed, such as the text "Modified regular file". Here's the code I have.

func parseDescription(output: String) {
  let modifiedFile = Parse {
    PrefixUpTo("\n\nModified regular file")
  }

  let addedFile = Parse {
    PrefixUpTo("\n\nAdded regular file")
  }

  let removedFile = Parse { 
    PrefixUpTo("\n\nRemoved regular file")
  }

  let descriptionInfo = Parse {
     // Skip the text before the description starts
    Skip {
      PrefixThrough("\n\n")
        Whitespace()
    }

    // Read everything to the start of what changed.
    // I want to run the Rest() parser if all three parsers fail.
    OneOf {
      modifiedFile
      addedFile
      removedFile
    }
    .map(String.init)
    // Have to go through the whole string to get the parser to match anything.
    Skip {
      Rest()
    }
  }
}

If a commit doesn't have one of the markers for the start of what changed, I want to run the Rest parser, read everything to the end of the input, and return that text as the commit message.

[SwiftDevJournal]

I tried the following code:

OneOf {
  modifiedFile
  addedFile
  removedFile
  Rest()
}

When I have a commit in my app that causes the first three parsers to fail, there is no longer an error message in Xcode's console about unexpected input. However, when I run a unit test for the parsing code, the commit message the parsing code returns is a blank string, causing the test to fail.