spot/doc/org/tut01.org

Parsing and Printing LTL Formulas

• Shell command
• Python bindings
• C++
  • Simple wrapper for the two parsers
  • Calling the infix parser explicitly
  • Calling the prefix parser explicitly

Our first task is to read formulas and print them in another syntax.

Shell command

Using ltlfilt, you can easily read an LTL formula in one syntax, and output it in another syntax. By default the parser will accept a formula in any infix syntax, but if the input is in the prefix syntax of LBT, you should use --lbt-input. The output syntax is controlled using different options such as (--spin, --lbt, --latex, etc.). Full parentheses can also be requested using -p.

formula='[]<>p0 || <>[]p1'
ltlfilt -f "$formula" --lbt
ltlfilt -f "$formula" --spin -p
ltlfilt --lbt-input -f '& & G p0 p1 p2' --latex

| G F p0 F G p1
([](<>(p0))) || (<>([](p1)))
p_{1} \land p_{2} \land \G p_{0}

The reason the LBT parser has to be explicitly enabled is because of some corner cases that have different meanings in the two syntaxes. (For instance t and f are the true constant in LBT's syntax, but they are considered as atomic propositions in all the other syntaxes.)

Python bindings

Here are the same operation in Python

import spot
f = spot.formula('[]<>p0 || <>[]p1')
print(f.to_str('lbt'))
print(f.to_str('spin', parenth=True))
print(spot.formula('& & G p0 p1 p2').to_str('latex'))

| G F p0 F G p1
([](<>(p0))) || (<>([](p1)))
p_{1} \land p_{2} \land \G p_{0}

The spot.formula function wraps the calls to the two formula parsers of Spot. It first tries to parse the formula using infix syntaxes, and if it fails, it tries to parse it with the prefix parser. (So this this might fail to correctly interpret t or f if you are processing a list of LBT formulas.) Using spot.formula, parse errors are returned as an exception.

C++

Simple wrapper for the two parsers

We first start with the easy parser interface, similar to the one used above in the python bindings. Here parse errors would be returned as exceptions.

  #include <iostream>
  #include "ltlparse/public.hh"
  #include "ltlvisit/print.hh"

  int main()
  {
    const spot::ltl::formula* f = spot::ltl::parse_formula("[]<>p0 || <>[]p1");
    print_lbt_ltl(std::cout, f) << '\n';
    print_spin_ltl(std::cout, f, true) << '\n';
    print_latex_psl(std::cout, spot::ltl::parse_formula("& & G p0 p1 p2"));
    f->destroy();
  }

| G F p0 F G p1
([](<>(p0))) || (<>([](p1)))
p_{1} \land p_{2} \land \G p_{0}

Notice that the different output routines specify in their name the syntax the output, and the type of formula they expect. Here we are only using LTL formulas for demonstration, so those three functions are OK with that (LTL is a subset of PSL as far as Spot is concerned). However if we input PSL formula, it's clear that the above code will be a problem. If you want to add additional checks, you can easily tests whether f->is_ltl_formula() returns true.

Did you notice the calls to f->destroy() at the end? The LTL formula objects are implemented as DAG with sharing of subformulas. Each (sub)formula is therefore reference counted, and currently this is done manually by calling f->clone() and f->destroy() (do not ever delete a formula, always call f->destroy()).

We do not recommend using this parse_formula() interface because of the potential formulas (like f or t) that have different meanings in the two parsers that are tried.

Instead, depending on whether you want to parse formulas with infix syntax, or formulas with prefix syntax, you should call the specific parser. Additionally, this give you control over how to print errors.

Calling the infix parser explicitly

  #include <string>
  #include <iostream>
  #include "ltlparse/public.hh"
  #include "ltlvisit/print.hh"

  int main()
  {
    std::string input = "[]<>p0 || <>[]p1";
    spot::ltl::parse_error_list pel;
    const spot::ltl::formula* f = spot::ltl::parse_infix_psl(input, pel);
    if (spot::ltl::format_parse_errors(std::cerr, input, pel))
      {
        if (f)
          f->destroy();
        return 1;
      }
    print_lbt_ltl(std::cout, f) << '\n';
    print_spin_ltl(std::cout, f, true) << '\n';
    f->destroy();
  }

| G F p0 F G p1
([](<>(p0))) || (<>([](p1)))

So parse_infix_psl() processes input, and stores any diagnostic in pel, which is a list of pairs associating each error to a location. You could iterate over that list to print it by yourself as you wish, or you can call format_parse_errors() to do that for you. Note that as its name implies, this parser can read more than LTL formulas (the fragment of PSL we support is basically LTL extended with regular expressions).

If pel is empty, format_parse_errors() will do nothing and return false.

If pel is non empty, format_parse_errors() will display the errors messages and return true. In the above code, we have decided to aborts the execution in this case.

However the parser usually tries to do some error recovery. For instance if you have input (a U b)) the parser will complain about the extra parenthesis (pel not empty), but it will still return an f that is equivalent to a U b. So you could decide to continue with the "fixed" formula if you wish. Here is an example:

  #include <string>
  #include <iostream>
  #include "ltlparse/public.hh"
  #include "ltlvisit/print.hh"

  int main()
  {
    std::string input = "(a U b))";
    spot::ltl::parse_error_list pel;
    const spot::ltl::formula* f = spot::ltl::parse_infix_psl(input, pel);
    // Use std::cout instead of std::cerr because we can only
    // show the output of std::cout in this documentation.
    (void) spot::ltl::format_parse_errors(std::cout, input, pel);
    if (f == nullptr)
      return 1;
    print_lbt_ltl(std::cout, f) << '\n';
    print_spin_ltl(std::cout, f, true) << '\n';
    f->destroy();
  }

>>> (a U b))
           ^
syntax error, unexpected closing parenthesis

>>> (a U b))
           ^
ignoring trailing garbage

U "a" "b"
(a) U (b)

The formula f is only returned as null when the parser really cannot recover anything.

Calling the prefix parser explicitly

The only difference here is the call to parse_prefix_ltl() instead of parse_infix_psl().

  #include <string>
  #include <iostream>
  #include "ltlparse/public.hh"
  #include "ltlvisit/print.hh"

  int main()
  {
    std::string input = "& & G p0 p1 p2";
    spot::ltl::parse_error_list pel;
    const spot::ltl::formula* f = spot::ltl::parse_prefix_ltl(input, pel);
    if (spot::ltl::format_parse_errors(std::cerr, input, pel))
      {
        if (f)
          f->destroy();
        return 1;
      }
    print_latex_psl(std::cout, f) << '\n';
    f->destroy();
  }

p_{1} \land p_{2} \land \G p_{0}