NDLI: Open GADTs and declaration-site variance: a problem statement

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Open GADTs and declaration-site variance: a problem statement

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Open GADTs and declaration-site variance: a problem statement

Content Provider	ACM Digital Library
Author	Giarrusso, Paolo G.
Abstract	Generalized algebraic data types (GADTs) allow embedding extensible typed ASTs and transformations on them. Such transformations on typed ASTs are useful for code optimization in deeply embedded DSLs, for instance when using Lightweight Modular Staging (LMS). However, in Scala it is hard to make transformations for typed ASTs type-safe. Therefore, AST transformations in LMS are often not fully typechecked, preventing bugs from being caught early and without extensive testing. We show that writing type-safe transformations in such embeddings is in fact not just hard, but impossible without using unsafe casts or significantly restricting extensibility: Declaration-site variance opens GADTs representing typed ASTs not only to desirable extensions, but also to extensions that introduce exotic terms. We make the problem concrete on an embedding of $λ_{<:}$ through covariant GADTs. We discuss solution approaches, and sketch a Scala extension to address this problem without either introducing unsafe casts or restricting extensibility. We believe a complete solution would significantly ease writing transformations by allowing type-checking to verify them, and thus would ease their development.
Starting Page	1
Ending Page	4
Page Count	4
File Format	PDF
ISBN	9781450320641
DOI	10.1145/2489837.2489842
Language	English
Publisher	Association for Computing Machinery (ACM)
Publisher Date	2013-07-02
Publisher Place	New York
Access Restriction	Subscribed
Subject Keyword	Type-safety Lightweight modular staging Soundness Dsl embedding Scala Lambda-calculus
Content Type	Text
Resource Type	Article

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