Custom error messages in case of failure

[MustafaI]

I'd like to use parsley to create custom error messages in the case where runParser returns Failure(..). From what I can see it's currently not possible to retrieve error line, column, expected and unexpected strings and so on without modifying parsley.internal.instructions.Context. Would I be correct in saying this? If so, are there any plans to implement this into Parsley in future?

Thanks.

[j-mie6]

Yes, Parsley 3 will return the ParseError datatype instead of a string, along with a pretty printing typeclass

[MustafaI]

Thanks for the quick response!

If it'll take a while to release then that's fine, I should be able to modify parsley to give me what I need in meantime.

[j-mie6]

Ok! You should find it easy enough: you need to modify the Context so it doesn't call .pretty, then make the ParseError stuff in parsley.internal.instructions.Errors not private [instructions]

[MustafaI]

@j-mie6 A quick follow up question on this, in the case of runParser returning Success(..), would the private fields "line" and "col" in parslet.internal.instructions.Context refer to the start of the text that the parser matched? So if we were parsing for hello in "abcdefhello" would "col" be 6/7 (depending on if col starts at zero)?

[j-mie6]

it would be the current position at the point of success. Your parser should ideally have eof at the end of it to ensure that you read the entire file, so it'll normally just literally be the end of the file's line (and column + 1). On the other hand, the pos combinator can produce an (Int, Int) at any point whilst the parser is running. What do you hope to achieve with the line and column?

[MustafaI]

The pos combinator is exactly what I needed. Thank you.