Detect start of line

[Krever]

I'm trying to parse a format where different parts are separated by <start_of_line>#### fragment and so, I would like to be able to detect the <start_of_line>.

IMHO the logic should be similar to P.start | <prev_char = '\n'>.

I'm not sure if that matters, but I'm trying to parse Intellij HTTP client file format with an explicit requirement of supporting ### in the first line, so for example

###
// A basic request
http://example.com/a/

###

// A second request using the GET method
http://example.com:8080/api/html/get?id=123&value=content

[johnynek]

I would make a parser for a separator:

val sep = (P.start | P.char('\n')).soft *> P.string("###")

Then I would define your white space to be something like (!sep) *> P.charIn(" \t\r\n").rep

now. you can use repSep or something.

[Krever]

Yup, I'm using something like this right now, but the problem with such defined separator is that it consumes the \n char. I thought that not doing so would be a cleaner solution, so it works regardless of what is "before" it.

[Krever]

I implement few utils for my use case, but the problem is I had to do part of it in cats-parse itself - I couldn't find a way to access relevant internals from outside.

    case class PeekAt(n: Int) extends Parser0[Char] {
      if (n < 0) throw new IllegalArgumentException(s"required offset > 0, found $n")

      override def parseMut(state: State): Char = {
        if(n < state.str.length){
          state.str.charAt(n)
        } else {
          state.error = Chain.one(Expectation.FailWith(n, "Offset out of range"))
          '\u0000'
        }
      }
    }

  def prevChar(c: Char): Parser0[Char] = P.index
    .flatMap(idx => {
      if (idx == 0) P.failWith("Lookback at start of input") else P.peekAt(idx - 1)
    }).flatMap(cx => if (c == cx) P.pure(c) else P.failWith("Lookback failed"))

[johnynek]

I'm definitely open to having some mechanism to look back. The most general would be to access the whole String. But a more constrained would be to match the previous N characters with a given parser.

I tend to think accessing the String and then a convenience method to implement what you need (hopefully without flatMap which is generally more costly).

[johnynek]

Note, we added Parser.caret in #301 which allows you to access the line, column and total offset.

You can use that to implement line start with Parser.caret.filter(_.col == 0)