Functional pearl: Implicit configurations - Or, type classes reflect the values of types

Oleg Kiselyov, Chung Chieh Shan

Research output: Chapter in Book/Report/Conference proceedingConference contribution

14 Citations (Scopus)

Abstract

The configurations problem is to propagate run-time preferences throughout a program, allowing multiple concurrent configuration sets to coexist safely under statically guaranteed separation. This problem is common in all software systems, but particularly acute in Haskell, where currently the most popular solution relies on unsafe operations and compiler pragmas. We solve the configurations problem in Haskell using only stable and widely implemented language features like the type-class system. In our approach, a term expression can refer to run-time configuration parameters as if they were compile-time constants in global scope. Besides supporting such intuitive term notation and statically guaranteeing separation, our solution also helps improve the program's performance by transparently dispatching to specialized code at run-time. We can propagate any type of configuration data-numbers, strings, 10 actions, polymorphic functions, closures, and abstract data types. No previous approach to propagating configurations implicitly in any language provides the same static separation guarantees. The enabling technique behind our solution is to propagate values via types, with the help of polymorphic recursion and higher-rank polymorphism. The technique essentially emulates local type-class instance declarations while preserving coherence. Configuration parameters are propagated throughout the code implicitly as part of type inference rather than explicitly by the programmer. Our technique can be regarded as a portable, coherent, and intuitive alternative to implicit parameters. It motivates adding local instances to Haskell, with a restriction that salvages principal types.

Original languageEnglish
Title of host publicationProceedings of the ACM SIGPLAN 2004 Haskell Workshop, Haskell'04
Pages33-44
Number of pages12
Publication statusPublished - 2004 Dec 1
Externally publishedYes
EventProceedings of the ACM SIGPLAN 2004 Haskell Workshop, Haskell'04 - Snowbird, UT, United States
Duration: 2004 Sep 222004 Sep 22

Publication series

NameProceedings of the ACM SIGPLAN 2004 Haskell Workshop, Haskell'04

Other

OtherProceedings of the ACM SIGPLAN 2004 Haskell Workshop, Haskell'04
CountryUnited States
CitySnowbird, UT
Period04/9/2204/9/22

Keywords

  • Existential types
  • Higher-rank polymorphism
  • Implicit parameters
  • Polymorphic recursion
  • Type classes

ASJC Scopus subject areas

  • Engineering(all)

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