I'm trying to parse C style comments using FParsec. Not sure why this is failing:
My parser code:
let openComment : Parser<_,unit> = pstring "/*"
let closeComment : Parser<_,unit> = pstring "*/"
let comment = pstring "//" >>. restOfLine true
<|> openComment >>. (charsTillString "*/" true System.Int32.MaxValue) |>> Comment
//<|> openComment >>. manyCharsTill anyChar closeComment |>> Comment
let spaceComments = many ((spaces1 |>> IgnoreU) <|> comment)
let str s = spaceComments >>. pstring s .>> spaceComments
Test Harness:
let testStr = @"
// test comment
/* a block comment
*/
x // another comment
"
match run (str "x") testStr with
| Success(result, _, _) -> printfn "Success: %A" result
| Failure(errorMsg, _, _) -> assert false
()
Error messager. It is the same for both charsTillString and manyCharsTill
Error in Ln: 6 Col: 4
^
Note: The error occurred at the end of the input stream.
Could not find the string '*/'.
Comment and IgnoreU are both a discrimated type of string
The problem is that the combinators in your comment
parser don't have the precedence/associativity that you want. You can fix this by grouping with parens:
let comment = (pstring "//" >>. restOfLine true)
<|> (openComment >>. (charsTillString "*/" true System.Int32.MaxValue)) |>> Comment
I find that choice
is often easier to read than <|>
for complex parsers:
let comment =
choice [
pstring "//" >>. restOfLine true
openComment >>. (charsTillString "*/" true System.Int32.MaxValue)
] |>> Comment