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 commitment, so that a failure cannot be undone. This should only be used for writing very primitive parsers - really it is an internal detail of the library.
Apply a parser to an input token sequence.
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.