Haskell Calculator

A simple four-function visual calculator, implemented in Haskell.

This calculator is capable of evaluating (and visualizing) arithmetic expressions such as "1+1" or "(1+2)*(3+4)".

It supports:

• natural numbers (of arbitrary size);
• addition, subtraction, multiplication, and division;
• sub-expressions nested within parentheses;
• unary negation.

Demonstration

Here's a web-based demonstration built with GHCJS and Reflex.

Implementation

The implementation includes:

• Data structures for encoding arithmetic expressions as trees (see Calculator.Types)
• Functions for parsing arithmetic expressions from textual input (see Calculator.Parsing)
• Functions for pretty-printing arithmetic expressions (see Calculator.Printing)
• Functions for evaluating arithmetic expressions (see Calculator.Evaluation)
• A simple command-line interface (see Calculator.CLI)
• A simple graphical user interface (see Calculator.GUI)

Also included:

• A simple demonstration of using a GADT to restrict the shape of a tree-based data structure (the Exp type).

Building

This project supports both GHC and GHCJS.

The dependencies for GHC and GHCJS are slightly different.

Building with GHC

On Debian-based systems, first install the following dependencies:

• libcairo-dev
• libghc-pango-dev
• libghc-gtk3-dev
• libwebkitgtk-3.0-dev

Then issue the following commands:

stack setup
stack build

Running the command line interface

The command line interface is built with GHC.

To run it, issue the following command:

stack exec calculator-cli

Building the web-based graphical user interface

The web interface is built with GHCJS.

First ensure that you have a recent version of NodeJS installed. (Version 6.6.0 or later should work.)

Then run:

./build-web-gui.sh

If the build is successful, it will notify you of where you can find the build output. For example:

Open the following file with your web browser to view the graphical user interface:
/path/to/haskell/calculator/.stack-work/install/x86_64-linux/lts-7.15/ghcjs-0.2.1.9007015_ghc-8.0.1/bin/calculator-gui.jsexe/index.html

Copy and paste the final line into your web browser to view the user interface.

Running the test suite

The test suite is built with GHC.

To run the suite, issue the following command:

stack test

Experimenting with the calculator library

To load the calculator library into GHCI, issue the following command:

stack ghci