Token Extraction in ErrorBuilder

When vanilla error messages are generated internally to parsley, the unexpected component is usually derived from the raw input, or a name explicitly given to an unexpected combinator. However, that does not necessarily provide the most informative or precise error messages.

Instead, the ErrorBuilder typeclass has an unexpectedToken method that can be used to determine how the token should be formulated in the event that it would have otherwise come raw from the input. Its signature is as follows:

def unexpectedToken(
    cs: Iterable[Char],
    amountOfInputParserWanted: Int,
    lexicalError: Boolean
  ): Token

The first argument, cs, is the input from the point that the bad input was found; the second is the amount of input the parser tried to read when it failed; and lexicalError denotes whether or not the failure happened whilst trying to parse a token from Lexer, or not. The return value, of type Token, is one of the following classes:

case class Named(name: String, span: Int) extends Token
case class Raw(tok: String) extends Token

A Raw token indicates no further processing of the input could occur to get a better token, and some is returned verbatim. Otherwise, a Named token can replace a raw token with something derived from the input -- the span here denotes how wide that token had been determined to be.

The idea is that unexpectedToken should examine the provided arguments and determine if a more specific token can be extracted from the residual input, or, if not, produce a final Raw token of the desired width. In practice, while a user could implement the unexpectedToken method by hand, parsley provides a collection of token extractors that can be mixed-in to an ErrorBuilder to implement it instead.

Basic Extractors

There are three basic extractors available in parsley: SingleChar, MatchParserDemand, and TillNextWhitespace. Each is discussed below. Each of them have special handling for whitespace characters and ones that are unprintable, which are given names.

SingleChar

This extractor simply takes the first codepoint of the input stream cs and returns it. A codepoint is a single unicode character, which may consist of one or two bytes. As an example, the default formatting may be instantiated with this extractor by writing:

import parsley.errors.DefaultErrorBuilder
import parsley.errors.tokenextractors.SingleChar

val builder = new DefaultErrorBuilder with SingleChar

MatchParserDemand

This extractor, as its name suggests, takes more than a single codepoint from the input, instead taking as many as the parser has requested via the amountOfInputParserWanted argument. As an example, the default formatting may be instantiated with this extractor by writing:

import parsley.errors.DefaultErrorBuilder
import parsley.errors.tokenextractors.MatchParserDemand

val builder = new DefaultErrorBuilder with MatchParserDemand

TillNextWhitespace

Unlike the other extractors, this one has additional configuration. It generally aims to take as much input as necessary to find the next the next whitespace character, which can be changed by overriding the isWhitespace method. However, this can be capped as the minimum of the input the parser demanded or until the next whitespace. As an example, the default formatting may be instantiated with this extractor by writing:

import parsley.errors.DefaultErrorBuilder
import parsley.errors.tokenextractors.TillNextWhitespace

val builder = new DefaultErrorBuilder with TillNextWhitespace {
    def trimToParserDemand = true
}

This extractor, with trimToParserDemand = true is the default currented used by parsley for all error messages. By default, isWhitespace matches any character c for which c.isWhitespace is true.

Lexer-backed Extraction

The default strategies outlined above all ignore the lexicalError flag passed to unexpectedToken. To provide a more language-directed token extraction, however, the LexToken extractor is also provided.

It has one compulsory configuration and two more that have defaults:

trait LexToken {
    def tokens: Seq[Parsley[String]]
    def extractItem(cs: Iterable[Char], amountOfInputParserWanted: Int): Token = {
        SingleChar.unexpectedToken(cs)
    }
    def selectToken(matchedToks: List[(String, Int)]): (String, Int) = {
        matchedToks.maxBy(_._2)
    }
}

Here, the tokens are parsers for valid tokens within the language being parsed: each returns the name of that token as it would be displayed in the error message. The extractor will try to parse all of these tokens, and should at least one succeed the non-empty list of parsed tokens will be passed to selectToken for one to be picked to be used in the error: by default, the one which is the widest is chosen. If no tokens could be parsed, or the error occured during the parsing of a token/within the markAsToken combinator (as denoted by lexicalError normally), then extractItem is used instead. This usually should defer to another kind of token extractor, which, for convience, all expose their functionality in their companion objects.

As an example, a language which already has an available lexer built with lexical description desc can implement a LexToken as follows:

import parsley.errors.DefaultErrorBuilder
import parsley.errors.tokenextractors.LexToken

val builder = new DefaultErrorBuilder with LexToken {
    def tokens = Seq(
        lexer.nonlexeme.integer.decimal.map(n => s"integer $n"),
        lexer.nonlexeme.names.identifier.map(v => s"identifier $v")
    ) ++ desc.symbolDesc.hardKeywords.map { k =>
        lexer.nonlexeme.symbol(k).as(s"keyword $k")
    }
}

Obviously, this may not be an exhaustive list of tokens, but is illustrative of how to set things up.