The Parser datatype
Parser datatype is a fairly generic parsing monad with error
reporting. It can be used for arbitrary token types, not just
String input. (If you require a running state, use module PolyState
A return type like Either, that distinguishes not only between right and wrong answers, but also has gradations of wrongness. This should only be used for writing very primitive parsers - really it is an internal detail of the library.
Push some tokens back onto the front of the input stream and reparse. This is useful e.g. for recursively expanding macros. When the user-parser recognises a macro use, it can lookup the macro expansion from the parse state, lex it, and then stuff the lexed expansion back down into the parser.