March 2, 2011

There are few languages as simple yet as widely used as classical propositional logic—perhaps only elementary arithmetic can claim a similar status. The propositional calculus, as it is also known, is a staple of first-year university logic courses.

As I recall from when I did just such a course, one of the most tedious parts was calculating truth tables for various complex logical expressions. Once one has grasped the basic concept, the rest is purely mechanical, and while it might be useful in the exam, beyond that… well, that’s what computers are for.

Hatt is a command-line program for doing just that: parsing expressions of the propositional calculus and printing their truth tables. Assuming you have the Haskell platform installed, just run the following to install Hatt.

cabal install hatt

Then just run hatt to enter the interactive mode. Here’s an example session.

> (A -> (B | ~C))
A B C | (A -> (B | ~C))
-----------------------
T T T | F
T T F | F
T F T | F
T F F | F
F T T | F
F T F | F
F F T | T
F F F | F
> pretty
Enabling pretty-printing.
> (P <-> (~Q & (R -> S)))
P Q R S | (P ↔ (¬Q ∧ (R → S)))
------------------------------
T T T T | F
T T T F | F
T T F T | F
T T F F | F
T F T T | F
T F T F | F
T F F T | F
T F F F | T
F T T T | T
F T T F | T
F T F T | T
F T F F | T
F F T T | T
F F T F | T
F F F T | T
F F F F | F

The hatt binary isn’t the only thing you get when you install the package. There’s also the Data.Logic.Propositional module, which allows you to plug this functionality—parsing, pretty-printing, truth tables, some simple checks like whether an expression is a tautology or a contradiction—into any Haskell program.

The API isn’t terribly extensive, but it’s enough to play around with, and should be really easy to extend if you wanted, for example, to add the facility to express proofs. I’m always amazed by how much one can accomplish with how little in Haskell: Hatt is under 300 lines of code, and a lot of that is the command-line program which is bloated by help text and the like.

This was, admittedly, a fairly trivial project, but it was a good way to improve my familiarity with Haskell, particularly Parsec. Picking a small, manageable project with precise goals proved to be a fun way to keep my hand in and pick up a few new skills, and I highly recommend it as a technique. If you’re interested in taking a look at the source code, the entire project is available on GitHub under the BSD license.

By Benedict Eastaugh.