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Independently extensible solutions to the expression problem
 In Proc. FOOL 12
, 2005
"... The expression problem is fundamental for the development of extensible software. Many (partial) solutions to this problem have been proposed in the past, but the question of how to use different, independent extensions jointly has received less attention so far. This paper proposes solutions to the ..."
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The expression problem is fundamental for the development of extensible software. Many (partial) solutions to this problem have been proposed in the past, but the question of how to use different, independent extensions jointly has received less attention so far. This paper proposes solutions to the expression problem that make it possible to combine independent extensions in a flexible, modular, and typesafe way. The solutions, formulated in the programming language Scala, are affected with only a small implementation overhead and are relatively easy to implement by hand. 1. The Expression Problem Since software evolves over time, it is essential for software systems to be extensible. But the development of extensible software poses many design and implementation problems, especially, if extensions cannot be anticipated. The expression problem is probably the most fundamental one among these problems. It arises when recursively defined datatypes and operations on these types have to be extended simultaneously. The term expression problem was originally coined by Phil Wadler in a post on the JavaGenericity mailing list [34], although it was Cook who first discussed this problem [9]. His work motivated several others to reason about variants of the problem in the following years [18, 27, 17, 12]. In his post to the JavaGenericity mailing list, Wadler also proposed a solution to the problem written in an extended version of GENERIC JAVA [3]. Only later it appeared that this solution could not be typed. For this paper, we paraphrase the problem in the following way: Suppose we have a datatype which is defined by a Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee.