Strategy language for Maude

S. Eker, N. Martí-Oliet, J. Meseguer, I. Pita, R. Rubio, and A. Verdejo

The strategy language

We have developed a basic strategy language for Maude, to be used at the object level, rather than at the metalevel. The strategy language allows the definition of strategy expressions that control the way a term is rewritten. Our design is based on a strict separation between the rewrite rules in system modules and the strategy expressions, that are specified in separate strategy modules. Thus, in our proposal it is not possible to use strategy expressions in the rewrite rules of a system module: they can only be specified in a separate strategy module. In fact, this separation makes possible defining different strategy modules to control in different ways the rewrites of a single system module.

A strategy is described as an operation that, when applied to a given term, produces a set of terms as a result, given that the process is nondeterministic in general. The basic strategies consist of the application of a rule (identified by the corresponding rule label) to a given term, and allowing variables in a rule to be instantiated before its application by means of a substitution. For conditional rules (which may contain rewrite conditions) such rewrite conditions can also be controlled by means of strategies. Basic strategies are combined by means of several combinators, including: regular expression constructions (concatenation, union, and iteration), if-then-else, combinators to control the way subterms of a given term are rewritten, and recursion.

The strategy language is fully available in Maude 3.0, which can be downloaded from the official page. Its manual is the most updated and complete reference of the language. An extended version including a strategy-aware model checker can also be downloaded from here.

We can see how to use the language with an example. The following system module defines a blackboard as a multiset of natural numbers, and a rule selecting two numbers in the blackboard and replacing them by their arithmetic mean.

mod BLACKBOARD is
  pr NAT .
  sort Blackboard .
  subsort Nat < Blackboard .

  vars M N : Nat .

  op __ : Blackboard Blackboard -> Blackboard [assoc comm] .

  rl [play] : M N => (M + N) quo 2 .
endm

With this module loaded we can already use the command for rewriting using a strategy.

Maude> srew 2 4 6 8 using play .
srewrite in BLACKBOARD : 2 4 6 8 using play .

Solution 1
result Blackboard: 3 6 8
Solution 2
result Blackboard: 4 4 8
Solution 3
result Blackboard: 4 5 6
Solution 4
result Blackboard: 2 5 8
Solution 5
result Blackboard: 2 6 6
Solution 6
result Blackboard: 2 4 7

No more solutions.

If we were not interested in all solutions, we can limit the number of answers:

Maude> srew [1] 2 4 6 8 using play .
srewrite [1] in BLACKBOARD : 2 4 6 8 using play .

Solution 1
result Blackboard: 3 6 8
and then we could ask for more solutions
Maude> cont 1 .

Solution 2
result Blackboard: 4 4 8

We can also introduce strategy modules where strategies can be defined. The following module defines a strategy minmin that continuously applies the rewrite rule play to the two minimum values in the blackboard using some auxiliary operations:

smod BLACKBOARD-STRAT is
  protecting BLACKBOARD .

  strat minmin @ Blackboard .

  vars X Y M N : Nat .
  var  B : Blackboard .

  sd minmin := 
     (matchrew B s.t. X := minimum(B) /\ Y := minimum(remove(X,B)) by
              B using play[M <- X , N <- Y] 
     ) ! .

  op minimum : Blackboard -> Nat .
  op remove : Nat Blackboard -> Blackboard .

  eq minimum(N) = N .
  eq minimum(N M) = if (N < M) then N else M fi . 
  eq minimum(N M B) = if (N < M) then minimum(N B) 
                                 else minimum(M B) fi .
  eq remove(X, X B) = B .
  eq remove(X, B) = B [owise] .
endsm
And then the defined strategies can also be used in the strategy commands:
Maude> srew 2000 20 2 200 10 50 using minmin .
srewrite in BLACKBOARD-STRAT : 2000 20 2 200 10 50 using minmin .

Solution 1
result NzNat: 1057

No more solutions.

Rewriting semantics

In order to advance the semantic foundations of the Maude strategy language, we have developed a detailed operational semantics by rewriting to Maude's strategy language. This is most fitting, since rewriting logic is a general semantic framework in which strategies themselves should not be thought of as extra-logical constructs, but should instead be expressed inside the logic. This idea of giving an operational, rewriting semantics to the strategy language is achieved in this paper by means of a theory transformation.

By taking advantage of the reflective properties of rewriting logic, which allow to consider metalevel entities such as theories as usual data, the transformation could be implemented as an operation from rewrite theories to rewrite theories, specified itself in rewriting logic. Instead of doing this, we have written a parameterized module that extends the metalevel with the rewriting semantics of the strategy language in a generic and quite efficient way.

Model checking

The Maude with strategies version, which can be downloaded in this page, includes a model checker capable of verifying systems controlled by the strategy language. To invoke the model checker, the user has to prepare the model and atomic propositions as if it were the standard model checker, but also define the relevant strategies in a strategy module. The STRATEGY-MODEL-CHECKER module in model-checker.maude includes the operator

op modelCheck : State Formula Qid -> ModelCheckerResult [special (...)] .

that additionally receives a third argument for the name of the strategy that will control the model behavior.

Moreover, CTL* and μ-calculus properties can also be checked using a Maude language module for the LTSmin model checker, which is included as libmaudemc.so in the release packages. Since the current version of LTSmin does not support both edge and state labels in μ-calculus formulae, a modified version ready to use can be downloaded from the download section. The recommended way of using the external model checker is through the umaudemc tool included in the release packages, where other external backends are supported like NuSMV, pyModelChecking and our own implementations.

Some examples and documentation are available:

Examples

Here you can find some examples. Those using parameterization or the strategy-aware model checker are gathered together.

First examples

Parameterization

Meta-level and language extensions

Model checking

Moreover, the language has been applied by different authors to various interesting examples. Their source code has been updated to the current language version.

Publications

Downloads

While the strategy language is completely implemented in Maude 3.0, the extension including the model checker for strategy-controlled systems can be downloaded from the following links:

The initial prototype of the strategy language (updated February 27 2011) is also available. It is implemented on top of Full Maude, and lacks strategy module importation and parameterization support. For model checking branching-time properties, some external tools from the projects LTSmin and mCRL2 are required, which can be downloaded directly from here for Linux and macOS (or compiled from source).

Any questions or comments are welcome. Alberto Verdejo