The zipper is a well known design pattern for providing a cursorlike interface to a data structure. However, the classic treatise by Huet (1) only scratches the surface of some of the potential applications of the zipper. In this work we have taken inspiration from Huet, and built a library suitable as an underpinning for a structured editor for programming languages. We consider a zipper structure that is suitable for traversing heterogeneous data types, encoding routes to other places in the tree (for bookmark or quick-jump functionality), expressing lexically bound information using contexts, and traversals for rendering a program indicating where the cursor is currently focused in the whole.
Strongly-typed functional languages provide a powerful framework for embedding Domain-Specific Languages (DSLs). However, building type-safe functions defined over an embedded DSL can introduce application-specific type constraints that end up being imposed on the DSL data types themselves. At best, these constraints are unwieldy and at worst they can limit the range of DSL expressions that can be built. We present a simple solution to this problem that allows application-specific constraints to be specified at the point of use of a DSL expression rather than when the DSL's embedding types are defined. Our solution applies equally to both tagged and tagless representations and, importantly, also works in the presence of higher-rank types.
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