Аннотация
CONTEXT: Data accessors allow one to read and write components of a data
structure, such as the fields of a record, the variants of a union, or the
elements of a container. These data accessors are collectively known as optics;
they are fundamental to programs that manipulate complex data. INQUIRY:
Individual data accessors for simple data structures are easy to write, for
example as pairs of "getter" and "setter" methods. However, it is not obvious
how to combine data accessors, in such a way that data accessors for a compound
data structure are composed out of smaller data accessors for the parts of that
structure. Generally, one has to write a sequence of statements or declarations
that navigate step by step through the data structure, accessing one level at a
time - which is to say, data accessors are traditionally not first-class
citizens, combinable in their own right. APPROACH: We present a framework for
modular data access, in which individual data accessors for simple data
structures may be freely combined to obtain more complex data accessors for
compound data structures. Data accessors become first-class citizens. The
framework is based around the notion of profunctors, a flexible generalization
of functions. KNOWLEDGE: The language features required are higher-order
functions ("lambdas" or "closures"), parametrized types ("generics" or
äbstract types"), and some mechanism for separating interfaces from
implementations (äbstract classes" or "modules"). We use Haskell as a vehicle
in which to present our constructions, but languages such as Java, C\#, or Scala
that provide the necessary features should work just as well. GROUNDING: We
provide implementations of all our constructions, in the form of a literate
program: the manuscript file for the paper is also the source code for the
program, and the extracted code is available separately for evaluation. We also
prove the essential properties demonstrating that our profunctor-based
representations are precisely equivalent to the more familiar concrete
representations. IMPORTANCE: Our results should pave the way to simpler ways of
writing programs that access the components of compound data structures.
Линки и ресурсы
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