Headline

Generic programming in Haskell with SYB

Motivation

The implementation demonstrates generic programming in Language:Haskell using "Scrap your boilerplate" (SYB) style. Generic programming is useful in so far that several operations of the 101companies features involve traversal over nested data. For instance, totaling all salaries requires the location of salary subterms at all levels in a given company term. Accordingly, operations for totaling and cutting salaries are implemented as generic functions that traverse over the company structure while type-specific cases detect employees and access their salaries. (It happens that the original publication on SYB used these generic programming samples for motivation.) The resulting code is considerably more concise, when compared to a conservative style of functional programming; see Contribution:haskellComposition. Additional operations are implemented to exercise SYB for different idioms of query or transformation and to address additional structural concerns; see the implementations of Feature:Depth, Feature:Mentoring, and Feature:Depth.

Illustration

Several of the operations on companies can be implemented in a very concise manner based on the SYB style of generic programming. For instance, the operation for totaling salaries simply extracts all floats from the given term and reduces them by addition:

-- Total all salaries in a company
total :: Company -> Float
total = everything (+) (extQ (const 0) id)

Architecture

The architecture is exactly the same as the one of Contribution:haskellComposition with one exception. There is an additional module Company.Generics which hosts stand-alone deriving clauses for instances of Data and Typeable classes, as needed for generic programming with Data.Generics.

Usage

The Haskell module Main has to be consulted with GHCi, and the main function has to be applied so that output for the test scenarios is produced. The expected output is available through the file baseline. There is a Makefile with a target test for test automation.

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