A parser is a template metafunction class that takes the following arguments:

The function parses a prefix of the input string. When the parsing is successful, it returns an accept value. When there is a parsing error, the parser returns a reject value. The is_error metafunction can be used to determine about the result of a parser if it succeeded or failed.

The documentation refers often to the result of a parser. This means that the parser accepts the input and refers to what get_result returns for the value returned by the parser.