The type of tokens. The lexical classes are: Int and Float for integer and floating-point numbers; String for string literals, enclosed in double quotes; Char for character literals, enclosed in single quotes; Ident for identifiers (either sequences of letters, digits, underscores and quotes, or sequences of 'operator characters' such as +, *, etc); and Kwd for keywords (either identifiers or single 'special characters' such as (, }, etc).

Construct the lexer function. The first argument is the list of keywords. An identifier s is returned as Kwd s if s belongs to this list, and as Ident s otherwise. A special character s is returned as Kwd s if s belongs to this list, and cause a lexical error (exception Stream.Error with the offending lexeme as its parameter) otherwise. Blanks and newlines are skipped. Comments delimited by (* and *) are skipped as well, and can be nested. A Stream.Failure exception is raised if end of stream is unexpectedly reached.