How to parse a little language in Prolog

cratylus (crat) — Sun, 22 Apr 2007 01:44:35 GMT

How to parse a token list into functors (structured terms) in Prolog.
The book "Logic in Prolog" by Gibbins has some good example code
See http://www.ddj.com/184404172, listing 9 for the little language



%

% This is a simple (nay,trivial!)  "dialect" 

% with only two commands.

 

% Obviously only scratches the surface - 

% Written as a learning exercise!

% First define a grammar using prolog's Definite 

% Clause Grammar (DCG) notation

% DCG is a bit like a macro system - 

% the grammar rules are expanded into

% ordinary prolog clauses  before execution:

% Thanks to prolog unification, the Cmd variable

% will end up being instantiated to a functor like

% sell(abc,10,5) or buy(xyz,55):



cmd(Cmd) --> sell,!,amount(Amount),of,stock(Stock),at,price(Price), 

             { Cmd = sell(Stock,Amount,Price) }.

cmd(Cmd) --> buy,!,amount(Amount),of,stock(Stock), 

             {Cmd = buy(Stock,Amount)}.

sell --> [sell].

of --> [of].

at --> [at].

buy --> [buy].

amount(Amount) --> [Amount].

stock(Stock)   --> [Stock].

price(Price)   --> [Price].



% mini-evaluator:



eval(sell(Stock,Amount,Price)) :- 

 format('Sold ~d ~a shares at $~d.~n',[Amount,Stock,Price]).

eval(buy(Stock,Amount)) :- 

 format('Bought ~d ~a shares.~n',[Amount,Stock]).



% parse a statement, if it's a command, 

% evaluate it, otherwise write an error

% (NB.  ";" is prolog's % "or".)



interp(Statement) :- cmd(Cmd,Statement,[]),

             eval(Cmd);write('Unrecognised command!').



% Examples: 

% ( First two match , the last fails.)



test :-

    interp([sell,100,of,xyx,at,50]),

    interp([buy,45,of,abc]),

    interp([not,accepted]).

DZone Snippets: dsl code

How to parse a little language in Prolog