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- Scala:
3.X - scala-parser-combinators:
2.1.1
Consider the following motivating example:
importscala.util.parsing.combinator.ParsersenumToken {caseA()caseB()caseC()}objectParserextendsParsers {overridetypeElem=TokendeftokenA:Parser[Token.A]=???deftokenB:Parser[Token.B]=???deftokenC:Parser[Token.C]=???deftokenABC:Parser[Token.A|Token.B|Token.C]= tokenA| tokenB| tokenC// error}
Unfortunately the definitiontokenABC doesn't compile:
Found: Parser[Token]Required: Parser[Token.A | Token.B | Token.C] tokenA | tokenB | tokenCA workaround is to ascribetokenA to the desired union type:
deftokenABC:Parser[Token.A|Token.B|Token.C]= (tokenA:Parser[Token.A|Token.B|Token.C])| tokenB| tokenC
However this looks unnatural and arguably not very intuitive.
If we look at the source, this is how the choice operator is defined:
def| [U>:T](q:=>Parser[U]):Parser[U]
And it is now evident why the code in the above example fails to compile. The obvious change would be to update the signature to:
def| [U](q:=>Parser[U]):Parser[T|U]
Which would definitely break cross compatibility and therefore wouldn't be a viable solution.
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