#+TITLE: Parsing and Printing LTL Formulas
#+SETUPFILE: setup.org
#+HTML_LINK_UP: tut.html

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 [[file:ioltl.org][any infix syntax]], but if the input is in the prefix syntax
of LBT, you should use [[file:ioltl.org][=--lbt-input=]].  The output syntax is controlled
using different options such as (=--spin=, =--lbt=, =--latex=, etc.).
Full parentheses can also be requested using =-p=.

#+BEGIN_SRC sh :results verbatim :exports both
ltlfilt -f '[]<>p0 || <>[]p1' --latex
formula='& & G p0 p1 p2'
ltlfilt --lbt-input -f "$formula" --lbt
ltlfilt --lbt-input -f "$formula" --spin -p
#+END_SRC

#+RESULTS:
: \G \F p_{0} \lor \F \G p_{1}
: & & p1 p2 G p0
: (p1) && (p2) && ([](p0))

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

#+BEGIN_SRC python :results output :exports both
import spot
f = spot.formula('[]<>p0 || <>[]p1')
print(f.to_str('latex'))
f = spot.formula('& & G p0 p1 p2')
print(f.to_str('lbt'))
print(f.to_str('spin', parenth=True))
#+END_SRC

#+RESULTS:
: \G \F p_{0} \lor \F \G p_{1}
: & & p1 p2 G p0
: (p1) && (p2) && ([](p0))

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 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.

#+BEGIN_SRC C++ :results verbatim :exports both
  #include <iostream>
  #include "ltlparse/public.hh"
  #include "ltlvisit/print.hh"

  int main()
  {
    print_latex_psl(std::cout, spot::ltl::parse_formula("[]<>p0 || <>[]p1")) << '\n';
    const spot::ltl::formula* f = spot::ltl::parse_formula("& & G p0 p1 p2");
    print_lbt_ltl(std::cout, f) << '\n';
    print_spin_ltl(std::cout, f, true) << '\n';
    f->destroy();
    return 0;
  }
#+END_SRC

#+RESULTS:
: \G \F p_{0} \lor \F \G p_{1}
: & & p1 p2 G p0
: (p1) && (p2) && ([](p0))

Notice that the different output routines specify in their name the
syntax the output, and the type of formula they can output.  Here we
are only using LTL formulas for demonstration, so those three
functions are OK with that.

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

Here is how to call the infix parser explicitly,:

#+BEGIN_SRC C++ :results verbatim :exports both
  #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_latex_psl(std::cout, f) << '\n';
    f->destroy();
    return 0;
  }
#+END_SRC

#+RESULTS:
: \G \F p_{0} \lor \F \G p_{1}

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:

#+BEGIN_SRC C++ :results verbatim :exports both
  #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_latex_psl(std::cout, f) << '\n';
    f->destroy();
    return 0;
  }
#+END_SRC

#+RESULTS:
: >>> (a U b))
:            ^
: syntax error, unexpected closing parenthesis
:
: >>> (a U b))
:            ^
: ignoring trailing garbage
:
: 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()=.

#+BEGIN_SRC C++ :results verbatim :exports both
  #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_lbt_ltl(std::cout, f) << '\n';
    print_spin_ltl(std::cout, f, true) << '\n';
    f->destroy();
    return 0;
  }
#+END_SRC

#+RESULTS:
: & & p1 p2 G p0
: (p1) && (p2) && ([](p0))


* Additional Comments

** PSL vs LTL

LTL is a subset of PSL as far as Spot is concerned, so you can parse
an LTL formula with =parse_infix_psl()=, and later print it with for
instance =print_spin_ltl()= (which, as its name implies, can only
print LTL formulas).  There is no =parse_infix_ltl()= function because
you can simply use =parse_infix_psl()= to parse LTL formulas.

There is a potential problem if you design a tool that only works with
LTL formulas, but call =parse_infix_psl()= to parse user input.  In
that case, the user might well input a PSL formula and cause problem
down the line.

For instance, let's see what happens if a PSL formulas is passed to
=print_spin_ltl=:

#+BEGIN_SRC C++ :results verbatim :exports both
  #include <string>
  #include <iostream>
  #include "ltlparse/public.hh"
  #include "ltlvisit/print.hh"

  int main()
  {
    std::string input = "{a*;b}<>->(a U (b & GF c))";
    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_spin_ltl(std::cout, f) << '\n';
    f->destroy();
    return 0;
  }
#+END_SRC

#+RESULTS:
: {a[*];b}<>-> (a U (b && []<>c))

The output is a 'best effort' output.  The LTL subformulas have been
rewritten, but the PSL-specific part (the SERE and =<>->= operator)
are output in the only syntax Spot knows, definitively not
Spin-compatible.

If that is unwanted, here are two possible solutions.

The first is to simply diagnose non-LTL formulas.

#+BEGIN_SRC C++ :results verbatim :exports code
  #include <string>
  #include <iostream>
  #include "ltlparse/public.hh"
  #include "ltlvisit/print.hh"

  int main()
  {
    std::string input = "{a*;b}<>->(a U (b & GF c))";
    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;
      }
    if (!f->is_ltl_formula())
      {
        f->destroy();
        std::cerr << "Only LTL formulas are supported.\n";
        return 1;
      }
    print_spin_ltl(std::cout, f) << '\n';
    f->destroy();
    return 0;
  }
#+END_SRC

A second (but slightly weird) idea would be to try to simplify the PSL
formula, and hope that the PSL simplify is able to come up with an
equivalent LTL formula.  This does not always work, so you need to be
prepared to reject the formula any way.  In our example, we are lucky
(maybe because it was carefully chosen...):

#+BEGIN_SRC C++ :results verbatim :exports code
  #include <string>
  #include <iostream>
  #include "ltlparse/public.hh"
  #include "ltlvisit/print.hh"
  #include "ltlvisit/simplify.hh"

  int main()
  {
    std::string input = "{a*;b}<>->(a U (b & GF c))";
    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;
      }
    if (!f->is_ltl_formula())
      {
        spot::ltl::ltl_simplifier simp;
        const formula* g = simp.simplify(f);
        f->destroy();
        f = g;
      }
    if (!f->is_ltl_formula())
      {
        f->destroy();
        std::cerr << "Only LTL formulas are supported.\n";
        return 1;
      }
    print_spin_ltl(std::cout, f) << '\n';
    f->destroy();
    return 0;
  }
#+END_SRC

#+RESULTS:
: a U (b && (a U (b && []<>c)))

** Lenient parsing

In version 6, Spin extended its command-line LTL parser to accept
arbitrary atomic propositions to be specified.  For instance =(a > 4)
U (b < 5)= would be correct input, with =a > 4= and =b < 5= considered
as two atomic propositions.  Of course the atomic proposition could be
arbitrarily complex, and there is no way we can teach Spot about the
syntax for atomic propositions supported by any tool.  The usual
workaround in Spot is to double-quote any arbitrary atomic
proposition:

#+BEGIN_SRC sh :results verbatim :exports both
echo compare
ltlfilt -f '"a > 4" U "b < 5"'
echo and
ltlfilt -f '"a > 4" U "b < 5"' --spin
#+END_SRC

#+RESULTS:
: compare
: "a > 4" U "b < 5"
: and
: (a > 4) U (b < 5)

When the Spin output is requested, these atomic propositions are
atomically output in a way that Spin can parse.

This Spin syntax is not accepted by default by the infix parser, but
it has an option for that.  This is called /lenient parsing/: when the
parser finds a parenthetical block it does not understand, it simply
assume that this block represents an atomic proposition.

#+BEGIN_SRC sh :results verbatim :exports both
ltlfilt --lenient -f '(a > 4) U (b < 5)'
#+END_SRC

#+RESULTS:
: "a > 4" U "b < 5"

Lenient parsing is risky, because any parenthesized sub-formula that
is a syntax-error will be treated as an atomic proposition:

#+BEGIN_SRC sh :results verbatim :exports both
ltlfilt --lenient -f '(a U ) U c'
#+END_SRC

#+RESULTS:
: "a U" U c

In C++ you can enable lenient using one of the Boolean arguments of
=parse_infix_psl()=.
