NDLI: Variations on variants

Content Provider	ACM Digital Library
Author	Morris, J. Garrett
Abstract	Extensible variants improve the modularity and expressiveness of programming languages: they allow program functionality to be decomposed into independent blocks, and allow seamless extension of existing code with both new cases of existing data types and new operations over those data types. This paper considers three approaches to providing extensible variants in Haskell. Row typing is a long understood mechanism for typing extensible records and variants, but its adoption would require extension of Haskell's core type system. Alternatively, we might hope to encode extensible variants in terms of existing mechanisms, such as type classes. We describe an encoding of extensible variants using instance chains, a proposed extension of the class system. Unlike many previous encodings of extensible variants, ours does not require the definition of a new type class for each function that consumes variants. Finally, we translate our encoding to use closed type families, an existing feature of GHC. Doing so demonstrates the interpretation of instances chains and functional dependencies in closed type families. One concern with encodings like ours is how completely they match the encoded system. We compare the expressiveness of our encodings with each other and with systems based on row types. We find that, while equivalent terms are typable in each system, both encodings require explicit type annotations to resolve ambiguities in typing not present in row type systems, and the type family implementation retains more constraints in principal types than does the instance chain implementation. We propose a general mechanism to guide the instantiation of ambiguous type variables, show that it eliminates the need for type annotations in our encodings, and discuss conditions under which it preserves coherence.
Starting Page	71
Ending Page	81
Page Count	11
File Format	PDF
ISSN	03621340 15581160
DOI	10.1145/2887747.2804320
Journal	ACM SIGPLAN Notices (SIGP)
Volume Number	50
Issue Number	12
Language	English
Publisher	Association for Computing Machinery (ACM)
Publisher Date	1983-05-01
Publisher Place	New York
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Extensible variants Expression problem Row types
Content Type	Text
Resource Type	Article
Subject	Computer Graphics and Computer-Aided Design Software

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