let v = e

construct, which evaluates the expression e once, the value can be subsequently referenced by the identifier v. Duplicates in different rules can be factored out by defining auxiliary data features and storing pre-computed expression values in these features. Alternatively, they can be factored (with or without caching) by defining query operations to compute them. For duplicated expression forms with varying arguments, formal parameters of the query operations are defined.

An example of duplicated expressions within a rule occurs in the class diagram rationalisation case study solution using UML-RSDS (Kolahdouz-Rahimi et al., 2014), in the rule:

a : specialization.specific.ownedAttribute and

v = specialization->select(

specific.ownedAttribute->exists( b |
b.name = a.name and
b.type = a.type ) ) and
v.size > 1 implies

Entity->exists( e |
e.name = name + a.name and
a : e.ownedAttribute and
e.specialization = v and

Generalization->exists( g |
g : specialization and
g.specific = e and
v.specific.ownedAttribute->
select( name = a.name )->
isDeleted() ) )

operating on instances of Entity. Here, a let variable v is used to avoid re-computation of the complex expression

specialization->select(

specific.ownedAttribute->exists( b | b.name = a.name and b.type = a.type ) )

which is used in three places in the constraint.

The same concept of factoring can be applied to common update functionality that is repeated in different rules. However, such factoring only improves the modularity and clarity of the specification, and does not improve efficiency. Implementation of update factoring using explicit rule invocation is described in (Kurtev et al, 2006).

(Cuadrado et al, 2008) J. S. Cuadrado, F. Jouault, J. G. Molina, J. Bezivin, Optimization patterns for OCL-based model transformations, MODELS 2008, vol. 5421 LNCS, Springer-Verlag, pp. 273--284, 2008.

(Kolahdouz-Rahimi et al., 2014) S. Kolahdouz-Rahimi, K. Lano, S. Pillay, J. Troya, P. Van Gorp, Evaluation of model transformation approaches for model refactoring, Science of Computer Programming, 2013, http://dx.doi.org/10.1016/j.scico.2013.07.013.

(Kurtev et al, 2006) I. Kurtev, K. Van den Berg, F. Joualt, Rule-based modularisation in model transformation languages illustrated with ATL, Proceedings 2006 ACM Symposium on Applied Computing (SAC 06), ACM Press, pp. 1202--1209, 2006.

(Lano, 2008) K. Lano, A catalogue of UML model transformations, http://www.dcs.kcl.ac.uk/staff/kcl/tcat.pdf, 2006.

(Michie, 1968) D. Michie, Memo Functions and Machine Learning, Nature, vol. 218, pp. 19--22, 1968.