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Infinite Scroll: Lazy Lists in the Brick TUI Library

YOW! Lambda Jam 2019

The Brick terminal UI library provides a rich library of widgets for building console applications in Haskell. These include a list widget, which uses a packed vector type under the hood: a major problem when working with lists that are very large or expensive to compute.

In this case study I will review, step by step, how I generalised Brick's list widget to admit different underlying container types and achieved lazy loading of list items. The presentation will address several topics including:

  • The advantages of more general (polymorphic) code, including parametricity
  • Ensuring adequate test coverage before refactoring or generalising
  • Maintaining backwards compatibility
  • Assessing and documenting asymptotic performance
  • Using Brick list widget in a real-world application (purebred MUA) for lazy loading where I/O is involved
  • How to evaluate a lazy structure in the background (and why you might want to)
  • Can we really achieve infinite scroll, or is my presentation title just clickbait?

Code examples will abound, and live demonstrations will both justify the work that was done, and show the pleasing results. The presentation uses Haskell exclusively but principles and advice for generalising code apply to many languages.