Hatt is a command-line program which prints truth tables
for expressions in classical propositional logic, and a
library allowing its parser, evaluator and truth table
generator to be used in other programs.

# Hatt

Hatt is a command-line program which prints truth tables for expressions in
classical propositional logic, and a library allowing its parser, evaluator and
truth table generator to be used in other programs.

## Installation

Hatt is available from Hackage. To install it with `cabal-install`

, update
your list of known packages and then install Hatt.

```
$ cabal update
$ cabal install hatt
```

To build it from source, `cd`

into the directory containing the Hatt source
files, including `hatt.cabal`

, and run `cabal install`

.

## Valid Hatt expressions

The following are all valid expression forms which can be parsed by Hatt, where
ϕ and ψ are metalinguistic variables standing in for any valid expression. The
parser isn't as smart about parentheses as it could be, so you have to follow
these rules quite literally. This shouldn't be a great hardship, but it does
mean that, for example, while `(A -> B)`

is a valid expression, `A -> B`

isn't.

- Variables:
`P`

, `Q`

, `a`

, `b`

etc.---basically anything in the character
class `[a-zA-Z]`

- Negation:
`~ϕ`

- Conjunction:
`(ϕ & ψ)`

- Disjunction:
`(ϕ | ψ)`

- Conditional:
`(ϕ -> ψ)`

- Biconditional:
`(ϕ <-> ψ)`

## Using the `hatt`

command-line program

The default mode is interactive: you start the program, enter expressions at
the prompt, and their truth tables are printed. Here's an example session.

```
$ hatt
Entering interactive mode. Type `help` if you don't know what to do!
> help
Hatt's interactive mode has several commands.
help
Print this help text.
pretty
Pretty-print expressions using Unicode logic symbols. Only employ this
option if your console is Unicode-aware. If pretty-printing is already
enabled, using this command will disable it.
colour
Colour truth values: green for true, red for false. This feature needs
your console to support ANSI colour codes. If coloured mode is already
enabled, this command will disable it.
exit
Quit the program.
If you don't type in a command, the program will assume you're writing a
logical expression to be evaluated and attempt to parse it.
For example, if you enter "(A -> B)" at the prompt, Hatt will print the
truth table for that expression. Here's an example console session.
> (A | B)
A B | (A | B)
-------------
T T | F
T F | T
F T | T
F F | F
> foobar
Parse error at (line 1, column 2):
unexpected "o"
expecting white space or end of input
> exit
If none of this makes any sense, try reading the README file.
> (A -> B)
A B | (A -> B)
--------------
T T | T
T F | F
F T | T
F F | T
> exit
```

The `--evaluate`

flag lets you pass a single expression to be evaluated
directly.

```
$ hatt --evaluate="(P -> (Q | ~R))"
P Q R | (P -> (Q | ~R))
-----------------------
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
```

By default, `hatt`

will print ASCII representations of expressions. If you have
a Unicode-capable terminal, try passing the `--pretty`

option to pretty-print
expressions using the the more common logical symbols.

```
$ hatt --evaluate="(P -> (Q | ~R))" --pretty
P Q R | (P → (Q ∨ ¬R))
----------------------
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
```

You can enable pretty-printing while in interactive mode by using the `pretty`

command.

If you pass the `--coloured`

flag, `hatt`

will colour the truth values in the
tables which it prints: green for true, red for false. You can enable colouring
during interactive mode by using the `colour`

command.

## Using Hatt in other programs

Hatt exposes the `Data.Logic.Propositional`

module, which provides a simple API
for parsing, evaluating, and printing truth tables.