Opinionated Haskell Interoperability

library, system, Propose Tags , Core.Program, Core.Program.Arguments, Core.Program.Execute, Core.Program.Logging, Core.Program.Metadata, Core.Program.Notify, Core.Program.Unlift, Core.System, Core.System.Base, Core.System.External, Core.System.Pretty, command-line, haskell
cabal install core-program


A library helping you write Haskell programs

This library is for writing command-line applications, be they tools or long-running daemons. It provides the following:

  • a Program type encapsulating internals of logging, signal handling, and exception handling;
  • logging handlers for event tracing and application debugging carefully designed so that normal output to console is not corrupted by logging output or error messages;
  • setting up what command-line parameters your program expects in a declarative manner and easily looking up options and arguments at runtime;
  • a mechanism for plumbing top-level application state throughout a program including support for updating that state;
  • support for extracting metadata from the project's .cabal file for use in your program should you need it, for example for use in --version output;
  • simplified UTF-8 text handling via a Rope type backed by a finger-tree and optimized for both conveying large passages of text and appending chunks when building up texts for subsequent output to a file handle, along with code facilitating conversion to other textual types;
  • first class ANSI coloured pretty-printing support;
  • auto-generated --help output that is sensitive to available terminal width;
  • wrappers around key/value maps and sets of elements optimized for common cases and facilitating conversion to other dictionary and collection types;
  • and more!

What's so opinionated about this?

This project started as an effort to record library choices, useful idioms, and recommended practices for teams writing Haskell programs in a large corporate environment. We quickly realized that a cookbook would be far less helpful than a library which simply presented these opinions as a framework application developers could just use out of the box.

Our approach has been to create wrapper layers around existing functionality to bring them together in a useful gestalt. There are some extremely powerful libraries in the Haskell ecosystem, but using them together can sometimes be difficult. Packages date from different eras of Haskell's growth as an industrial language and so they often follow different usage idioms making interoperability a challenge.

The working title for the as-yet unpublished text on this topic is Haskell for Unbelievers and so unbeliever became the name for this package. If you wish to dismiss this library as a giant bikeshedding exercise you would not be wrong. Hopefully you'll like the colour of our bikeshed.

Some goals:

  • Be compatible with the builtin prelude from base. One of the exciting things about Haskell is the degree of experimentation that researchers and engineers are able to do with the language itself, its library ecosystem, and the development tooling used to build and test software. Even the basic functions and syntax of the language are implemented as as a "prelude" library whose code is imported by default into every module. There are some fascinating explorations of different preludes—all with an aim to improve on the well-documented shortcomings of the built-in one. The original Prelude module remains in scope by default, however, and is in use by the majority of other libraries. So we set out to be compatible with (or at least able to ignore) that default and thus unbeliever is not implemented as a custom prelude.

    Nothing prevents you using an alternative prelude in a program built on top of this package, should you wish.

    [We do not argue that the Haskell language should never change. We'd be the first ones to cheer if String was ripped out and tossed on the dungheap of history next to VHS tapes and compters without Esc keys. We can fix the textbooks! But in the present tense, if the first thing a newcomer has to deal with is basic idioms they've been taught not working as they've been shown to expect, it makes for an awkward learning curve.]

  • Facilitate interoperability. One of the biggest annoyances working in Haskell is trying to figure out how to get a value returned from one library converted to the type you need to feed into the next one you are calling. This is common with textual types and when dealing with sets and maps from the various containers libraries. This library chooses one implementation for each of these areas and then supplies a typeclass to permit conversion from our type to the types in common use in other libraries.

  • Make it possible to use package's modules without using qualified imports. Haskell's qualified import mechanism is excellent, but often leads to cumbersome looking code in type signatures and at function call sites. The alternative is picking function names (which do tend to be longer, alas) which do not collide with base and hence do not require being qualified to avoid name collisions.

  • Provide a place to implement common application functionality. Over time, ideas about best practices evolve. Current approaches to structuring programs include an outer layer over IO which carries the application's state and makes it available to inner layers which can be more restricted or better yet pure. This library includes an implementation of that pattern.