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Prolog download - swi windows.
Grammar rules form a comfortable interface to difference lists . They are designed both to support writing parsers that build a parse tree from a list of characters or tokens and for generating a flat list from a term.
Grammar rules look like ordinary clauses using --> /2 for separating the head and body rather than :- /2 . Expanding grammar rules is done by expand_term/2, which adds two additional arguments to each term for representing the difference list.
The body of a grammar rule can contain three types of terms. A callable term is interpreted as a reference to a grammar rule. Code between is interpreted as plain Prolog code, and finally, a list is interpreted as a sequence of literals . The Prolog control-constructs ( \+ /1 , -> /2 , ; // 2, , /2 and ! /0 ) can be used in grammar rules.
We illustrate the behaviour by defining a rule set for parsing an integer.
Grammar rule sets are called using the built-in predicates phrase/2 and phrase/3:
phrase ( :DCGBody, ?List ) Equivalent to phrase( DCGBody , InputList , []) . phrase ( :DCGBody, ?List, ?Rest ) True when DCGBody applies to the difference List / Rest . Although DCGBody is typically a callable term that denotes a grammar rule, it can be any term that is valid as the body of a DCG rule.
The example below calls the rule set integer//1 defined in section 4.13 and available from library(library(dcg/basics)) , binding Rest to the remainder of the input after matching the integer.
The next example exploits a complete body. Given the following definition of digit_weight//1 , we can pose the query below.
The SWI-Prolog implementation of phrase/3 verifies that the List and Rest arguments are unbound, bound to the empty list or a list cons cell . Other values raise a type error. 74 The ISO standard allows for both raising a type error and accepting any term as input and output. Note the tail of the list is not checked for performance reasons. The predicate call_dcg/3 is provided to use grammar rules with terms that are not lists.
Note that the syntax for lists of codes changed in SWI-Prolog version 7 (see section 5.2). If a DCG body is translated, both "text" and `text` is a valid code-list literal in version 7. A version 7 string ( "text" ) is not acceptable for the second and third arguments of phrase/3. This is typically not a problem for applications as the input of a DCG rarely appears in the source code. For testing in the toplevel, one must use double quoted text in versions prior to 7 and back quoted text in version 7 or later.
See also portray_text/1, which can be used to print lists of character codes as a string to the top level and debugger to facilitate debugging DCGs that process character codes. The library library(apply_macros) compiles phrase/3 if the argument is sufficiently instantiated, eliminating the runtime overhead of translating DCGBody and meta-calling.
call_dcg ( :DCGBody, ?State0, ?State ) As phrase/3, but without type checking State0 and State . This allows for using DCG rules for threading an arbitrary state variable. This predicate was introduced after type checking was added to phrase/3. 75 After discussion with Samer Abdallah.
A portable solution for threading state through a DCG can be implemented by wrapping the state in a list and use the DCG semicontext facility. Subsequently, the following predicates may be used to access and modify the state: 76 This solution was proposed by Markus Triska.
As stated above, grammar rules are a general interface to difference lists. To illustrate, we show a DCG-based implementation of reverse/2:

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