Setting up specification parameters

Similar to TLC, Apalache requires the specification parameters to be restricted to finite values. In contrast to TLC, there is a way to initialize parameters by writing a symbolic constraint, see Section 5.3.

Using INSTANCE

You can set the specification parameters, using the standard INSTANCE expression of TLA+. For instance, below is the example y2k_instance.tla, which instantiates y2k.tla:

---------------------------- MODULE y2k_instance ----------------------------
(*
 * Another way to instantiate constants for apalache is to
 * use INSTANCE.
 *)
 
VARIABLE
    \* @type: Int;
    year,
    \* @type: Bool;
    hasLicense

The downside of this approach is that you have to declare the variables of the extended specification. This is easy with only two variables, but can quickly become unwieldy.

Convention over configuration

Alternatively, you can extend the base module and use overrides:

---------------------------- MODULE y2k_override ----------------------------
(*
 * One way to instantiate constants for apalache is to use the OVERRIDE prefix. 
 *)
 
EXTENDS y2k

OVERRIDE_BIRTH_YEAR == 80
OVERRIDE_LICENSE_AGE == 18

=============================================================================

ConstInit predicate

This approach is similar to the Init operator, but applied to the constants. We define a special operator, e.g., called ConstInit. For instance, below is the example y2k_cinit.tla:

---------------------------- MODULE y2k_cinit ----------------------------
(*
 * Another way to instantiate constants for apalache is give it constraints
 * on the constants.
 *)
 
EXTENDS y2k

ConstInit ==
    /\ BIRTH_YEAR \in 0..99
    /\ LICENSE_AGE \in 10..99

=============================================================================

To use ConstInit, pass it as the argument to apalache. For instance, for y2k_cinit, we would run the model checker as follows:

$ cd $APALACHE_HOME/test/tla
$ apalache check --inv=Safety \
  --length=20 --cinit=ConstInit y2k_cinit.tla

Parameterized initialization

As a bonus of this approach, Apalache allows one to check a specification over a bounded set of parameters. For example:

CONSTANT N, Values

ConstInit ==
  /\ N \in 3..10
  /\ Values \in SUBSET 0..4
  /\ Values /= {}

The model checker will try the instances for all the combinations of the parameters specified in ConstInit, that is, in our example, it will consider N \in 3..10 and all non-empty value sets that are subsets of 0..4.

Limitation

ConstInit should be a conjunction of assignments and possibly of additional constraints on the constants. For instance, you should not write N = 10 \/ N = 20. However, you can write N \in {10, 20}.

TLC configuration file

We support configuring Apalache via TLC configuration files; these files are produced automatically by TLA Toolbox, for example. TLC configuration files allow one to specify which initialization predicate and transition predicate to employ, which invariants to check, as well as to initialize specification parameters. Some features of the TLC configuration files are not supported yet. Check the manual page on "Syntax of TLC Configuration Files".

If you are checking a file <myspec>.tla, and the file <myspec>.cfg exists in the same directory, it will be picked up by Apalache automatically. You can also explicitly specify which configuration file to use via the --config option.