Pie in Haskell

This is an implementation of Pie, the language from The Little Typer, in Haskell.

How to Use It

Compile the program with cabal v2-build. Run the tests with cabal v2-test. Once it works, use cabal v2-install to install the binary somewhere. Alternatively, you can use the stack build, stack test, and stack install commands.

This implementation of Pie can be run in two modes:

• An interactive REPL
• A batch-mode processor

To start the REPL, invoke pie without a filename as an argument. To process a Pie file in batch mode, use pie FILENAME.

To learn Pie, please consult The Little Typer. If you need to refresh your memory, please consult the language reference.

REPL Commands

An expression

Expressions are treated as examples to be type checked and evaluated.

A declaration

Declarations, such as claims, definitions, and check-same forms, are treated as if they were in a file. Claims and definitions are added to the context, and check-same checks sameness immediately.

:verbose

Switch Pie to verbose mode.

:concise

Switch Pie to concise mode.

:load FILE

Load FILE, throwing away the current context.

Concise and Verbose Modes

In concise mode, Pie emits only the information about the user's program that is explicitly requested. In other words, it will display error messages and the explicit results of examples that the user has requested.

In verbose mode, Pie additionally displays information about the type of each subexpression that it successfully checks. While this information could serve as the basis for a good editor plugin, this has not yet been implemented.

Readline support

The REPL in this implementation of Pie does not support arrow keys or other similar features. You can get rudimentary support for them using rlwrap.

Relationship to the Racket implementation

I wrote this implementation so that people who know Haskell but not Racket would be able to read it and understand how the internals work. This one is probably not as nice to experiment with and/or use as the version written in Racket, as that version has many useful features in DrRacket, including tooltips on every expression showing its type, a pop-up list of TODOs, arrows from variables to their binding sites, auto-indentation support, "go to definition", and automatic renaming of variables. This version is just a batch-mode type checker and REPL.

Design Guidelines

This implementation of Pie is intended to be as clear and simple as possible. Clarity and simplicity are more important than performance, but less important than correctness. To help achieve this, the code adheres to the following dogmas:

1. As few dependencies as possible - currently just base and text. More dependencies are allowed in the test suite.
2. Absolutely no language extensions. Haskell 2010 only.
3. No monad transformers.

Compiler and Library Support

Because it relies on Data.List.NonEmpty, this package requires at least GHC 8.0.1.