Documentation

This data type represents parse error messages. There are three kinds of messages:

data Message = Unexpected String
             | Expected   String
             | Message    String

The fine distinction between different kinds of parse errors allows the system to generate quite good error messages for the user.

Extract the message string from an error message.

Test if message string is empty.

The data type ParseError represents parse errors. It provides the source position (SourcePos) of the error and a list of error messages (Message). A ParseError can be returned by the function parse. ParseError is an instance of the Show and Eq type classes.

Extract the source position from ParseError.

Extract the list of error messages from ParseError.

Test whether given ParseError has associated collection of error messages. Return True if it has none and False otherwise.

newErrorMessage m pos creates ParseError with message m and associated position pos. If message m has empty message string, it won't be included.

newErrorUnknown pos creates ParseError without any associated message but with specified position pos.

addErrorMessage m err returns err with message m added. This function makes sure that list of messages is always sorted and doesn't contain duplicates or messages with empty message strings.

setErrorMessage m err returns err with message m added. This function also deletes all existing error messages that were created with the same constructor as m. If message m has empty message string, the function does not add the message to the result (it still deletes all messages of the same type, though).

setErrorPos pos err returns ParseError identical to err, but with position pos.

Merge two error data structures into one joining their collections of messages and preferring longest match. In other words, earlier error message is discarded. This may seem counter-intuitive, but mergeError is only used to merge error messages of alternative branches of parsing and in this case longest match should be preferred.

showMessages ms transforms list of error messages ms into their textual representation.