Table of contents |
2 Implementation in functional languages 3 References |
Consider the following grammar in BNF:
procedure parse_F()
begin
if ch = 'A' then read_ch('A'); read_ch('N'); read_ch('D'); parse_E(); end-if;
if ch = 'O' then read_ch('O'); read_ch('R'); parse_E(); end-if;
end
To determine which rule should be applied in case of a certain terminal the same algorithm can be used as the one for constructing the parsing table of an LL parser.
Example
We define a procedure parse_A() for every nonterminal A that parses a string in the language of A. In this procedure it is first determined with the current symbol in the input stream which rule for the nonterminal will be used. Then it simply calls the procedures read_ch(a) and parse_A() for every terminal a and nonterminal A in the right-hand side for the rule. procedure parse_E()
begin
if ch = 'N' then read_ch('N'); read_ch('O'); read_ch('T'); parse_E(); end-if;
if ch = '(' then read_ch('('); parse_E(); parse_F(); read_ch(')'); end-if;
if ch = 'T' then read_ch('T'); read_ch('R'); read_ch('U'); read_ch('E'); end-if;
if ch = 'F' then read_ch('F'); read_ch('A'); read_ch('L'); read_ch('S'); read_ch('E'); end-if;
end
These procedures use a global variable ch that contains the current first character in the input stream. The procedure read_ch(a) reports an error if ch is not equal to a and otherwise reads the next character from the input stream into ch.Implementation in functional languages
Recursive descent parsers are particularly easy to implement in functional languages such as Haskell. See Functional Pearls: Monadic Parsing in Haskell (pdf format).References
This article (or an earlier version of it) contains material from FOLDOC, used with permission.