Decent Parser Error Messages

[Matthewar]

Currently parser messages are not implemented (aside from GenericParserError, which provides no helpful information).

A first stage solution for this is to implement a fail-on-error system using the current monadic parser.

• Within the parser, the monad is accessible from the parser productions using {% ... } (type Alex a)
• Perhaps even better, the lookhead token can be passed into the production with {%^ ... } (type Token -> Alex a)
• Implement useful error messages by displaying what was expected, and what was actually obtained

[Matthewar]

Example test code:

• Parser

  import Parser.Alex.Functions (alexError)
  import Parser.Alex.BaseTypes (AlexPosn(..))
  import Parser.ErrorTypes
  ...
  design_file :: { DesignFile }
          : design_unit_list {DesignFile $1}
          | error              {% alexError $ PosnWrapper { getPos = AlexPn 0 1 0, unPos = TestParseError }}

• With added type in Parser.ErrorTypes
  • TestParserError