[Pitch] `buildPartialBlock` for result builders

Evolution Pitches
10

Just wanted to share our experience taking this for a spin in swift-parsing. This feature is great!

A few fun observations:

  • We were able to delete over 21K lines of generated code.
  • We were able to improve the arity of concatenating parsers from 6 total per block to 10+, where 10 is the number of non-Void parsers, and + is an unlimited number of Void parsers throughout.
    Expand for example…

    Previously, we were limited to 6 parsers total in a builder block:

    let x = Parse {
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
  Int.parser()
}

    Because Void outputs are automatically discarded, we could nest to get things further along, but it makes things much less readable:

    let x = Parse {
  Parse {
    "hello"
    "world"
    Int.parser()
    "hello"
    "world"
  }
  Parse {
    Int.parser()
    "hello"
    "world"
  }
  Parse {
    Int.parser()
    "hello"
    "world"
  }
  Parse {
    Int.parser()
    "hello"
    "world"
  }
  Parse {
    Int.parser()
    "hello"
    "world"
  }
  Parse {
    Int.parser()
    "hello"
    "world"
  }
}

    Using buildPartialBlock, we can now do this:

    let x = Parse {
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
  Int.parser()
  "hello"
  "world"
   Int.parser()
  "hello"
  "world"
}
  • We were able to improve the arity of alternating parsers from 10 to unlimited.
  • Compile times also improved significantly (from 20 seconds to <2 seconds in debug mode).
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