Executing get-model or unsat-core depending on solver's decision - z3

Question

I wonder, if there is a possibility in a SMT-LIB 2.0 script to access the last satisfiability decision of a solver (sat, unsat, ...). For example, the following code:
(set-option :produce-unsat-cores true)
(set-option :produce-models true)
(set-logic QF_UF)
(declare-fun p () Bool)
(declare-fun q () Bool)
(declare-fun r () Bool)
(assert (! (=> p q) :named PQ))
(assert (! (=> q r) :named QR))
(assert (! (not (=> p r)) :named nPR))
(check-sat)
(get-model)
(get-unsat-core)
ran in Z3 returns:
unsat
(error "line 15 column 10: model is not available")
(PQ QR nPR)
and ran in MathSAT returns:
unsat
(error "model generation not enabled")
In MathSAT 5 it just breaks on (get-model) and doesn't even reach (get-unsat-core).
Is there any way in SMT-LIB 2.0 language to get-model iff the decision was SAT and unsat-core iff the decision was UNSAT? Solution could for example look like this:
(check-sat)
(ite (= (was-sat) true) (get-model) (get-unsat-core))
I searched SMT-LIB 2.0 language documentation, but I did not found any hint.
EDIT:
I also tried the code below, and unfortunately it did not work.
(ite (= (check-sat) "sat") (get-model) (get-unsat-core))

Answer 1

The SMT language does not let you write commands like this.
The way that tools, such as Boogie, deal with this is to use
a two-way text pipe: It reads back the result from (check-sat).
If the resulting string is "unsat" models are not available, but
cores would be if the check uses asssumptions. If the resulting
string is "sat" the tool can expect that a (get-model) command
succeeds.

Answer 2

As Nikolaj said in his answer, the right way to do this is to parse the solver output and conditionally generate either a (get-model) or a (get-unsat-core) statement.
However, with mathsat you can use the code without the (get-model) statement, and call mathsat with the -model option. For example:
$ cat demo_sat.smt2
(set-option :produce-unsat-cores true)
(set-option :produce-models true)
(set-logic QF_UF)
(declare-fun p () Bool)
(declare-fun q () Bool)
(declare-fun r () Bool)
(assert (! (=> p q) :named PQ))
(assert (! (=> q r) :named QR))
; (assert (! (not (=> p r)) :named nPR))
(check-sat)
(get-unsat-core)
$ mathsat -model demo_sat.smt2
sat
( (p false)
(q false)
(r false) )
(error "no unsatisfiability proof, impossible to compute unsat core")
And in the unsat case:
$ cat demo_unsat.smt2
(set-option :produce-unsat-cores true)
(set-option :produce-models true)
(set-logic QF_UF)
(declare-fun p () Bool)
(declare-fun q () Bool)
(declare-fun r () Bool)
(assert (! (=> p q) :named PQ))
(assert (! (=> q r) :named QR))
(assert (! (not (=> p r)) :named nPR))
(check-sat)
(get-unsat-core)
$ mathsat -model demo_unsat.smt2
unsat
( PQ
QR
nPR )
Unfortunately there does not seem to exist an option like -model for producing unsat cores. So this hack won't work if you want to use it with an incremental problem, unless you are OK with the solver terminating after the first sat result. (Because at the first sat result the solver will exit on the error for (get-unsat-core).)