Staging with Class: A Specification for Typed Template Haskell • Ningning Xie • YOW! 2022

This presentation was recorded at YOW! 2022. #GOTOcon #YOW
https://yowcon.com

Ningning Xie - Research Associate at Department of Computer Science and Technology University of Cambridge ‪@ningningxie3528‬

RESOURCES
  / xnningxie  
https://xnning.github.io

ABSTRACT
Multi-stage programming using typed code quotation is an established technique for writing optimizing code generators with strong type-safety guarantees. Unfortunately, quotation in #Haskell interacts poorly with type classes, making it difficult to write robust multi-stage programs.

In this talk, I will present my recent work which studies this unsound interaction and proposes a resolution, staged type class constraints, which is formalized in a source calculus that elaborates into an explicit core calculus.
I will show type soundness of both calculi, establishing that well-typed, well-staged source programs always elaborate to well-typed, well-staged core programs, and prove beta and eta rules for code quotations. The design allows programmers to incorporate type classes into multi-stage programs with confidence. Although motivated by Haskell, it is also suitable as a foundation for other languages that support both overloading and quotation. [...]

TIMECODES
0:00 Introduction
2:32 Quotations and splices
3:24 Multi-stage programming: example
5:20 Code generation
8:30 Multi-stage programming and type classes
10:53 Multi-stage programming: well-typedness
12:56 Well-stagedness: the level of an expression
14:02 Well-stagedness: the level restriction
15:50 Is the problem with qpower well-stageness?
17:50 Well-staged type classes
18:45 Key idea: staged type class constraints
21:04 Level-indexed constraint resolution
22:32 How to evaluate staged programs?
23:27 Level-indexed Evaluation
24:21 Key idea: splice environments
26:57 Type-directed elaboration
27:53 Integration into #GHC

RECOMMENDED BOOKS
Vitaly Bragilevsky • Haskell in Depth • https://amzn.to/3EXpmbe
Rebecca Skinner • Effective Haskell • https://amzn.to/3VgUKsh
Graham Hutton • Programming in Haskell • https://amzn.to/3Fi8rBC
John Whitington • Haskell from the Very Beginning • https://amzn.to/3VmX68R

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