![](bg6.png)
the previous paragraph.
Another conceivable approach is to implement
composition operators by means of code transfor-
mations. They can be implemented in terms of a
compile-time MOP (Sheard and Jones, 2002), which
means that the transformations can be defined in the
same language as the application code. But still trans-
formations cannot be freely composed because they,
generally, make assumptions on the code structure;
this may have been modified by a previously applied
transformation.
8 CONCLUSIONS
The message of this paper is that, when implementing
an application, dedicated abstractions that fit the prob-
lem domain are needed; and since programs, in gen-
eral, combine multiple problem domains in unfore-
seeable ways, programming languages must not hard-
wire abstractions and composition mechanisms, but
must allow developers to tailor them to their needs.
For this purpose, we propose the Co-op approach
to integrate facilities into programming languages that
allow developers to implement their own composition
mechanisms according to their domain’s abstractions.
They should be implemented as composition opera-
tors which are first class objects in the programming
language such that they can be composed, themselves,
by means of custom composition operators. By dis-
cussing the Co-op language prototypes that allow nor-
mal application objects to act as composition opera-
tors by means of message rewriting, we have shown
that realizing these ideas is feasible.
While the Co-op approach—allowing program-
mers to define their own composition operators as
first-class objects—is quite powerful, it also bears
more complexity. This complexity needs not be ex-
posed to the day-to-day programmer, though, because
the implementation of composition operators can be
provided by a few, well-trained experts through li-
braries. The common programmer simply uses such a
library, instantiating composition operators as needed.
Nevertheless, it is subject to future studies to research
the impact of our proposed programming model in
practice.
ACKNOWLEDGEMENTS
We would like to thank Wilke Havinga and Havva
G
¨
ulay G
¨
urb
¨
uz for their contributions to Co-op. This
research work was partially funded through the
ENOFES project (STW 11850).
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