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Syntax

rascal-0.34.0

Synopsis​

The textual syntax of Lisp.

Description​

The first step in defining Lisp's textual format, is to define a grammar for it:

module demo::lang::Lisra::Syntax

layout Whitespace      = [\t-\n\r\ ]*; 
lexical IntegerLiteral = [0-9]+ !>> [0-9];
lexical AtomExp        = (![0-9()\t-\n\r\ ])+ !>>  ![0-9()\t-\n\r\ ];

start syntax LispExp            // TODO: remove constructor names (needed for compiler)
      = int_lit: IntegerLiteral
      | atom_exp: AtomExp
      | par_exp: "(" LispExp* ")"
      ;

Whitespace defines the characters that can be ignored between tokens.

IntegerLiteral defines integer constants. In a first approximation [0-9] is enough. However, to ensure that the longest possible sequence of digits is used, the !>> [0-9] part ensures that an integer cannot be followed by another digit.

AtomExp defines a Lisp symbol that may contain a wide range of characters (except layout and digits).

The main syntactic concept is a LispExp that may be an IntegerLiteral, AtomExp or a list of LispExps surrouned by parentheses.

Examples​

This grammar is demonstrated in Parse.