haskell-atob

An interpreter for the esoteric language A=B (pronounced "a to b"), written entirely in Haskell.

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What is A=B?

A=B is a minimalist esoteric programming language that operates solely through text replacement rules. The program consists of a series of transformation instructions, and an input string is passed in at runtime. The instructions modify the string in a loop until no more transformations can be made.

How it works

Each line of an A=B program contains:

string1=string2

• If string1 exists in the current string, it is replaced by string2.
• If not, the program moves to the next line.
• When a rule is applied, the interpreter restarts from the top.
• The program halts only when no rule applies anymore.

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Special Syntax

A=B features several special keywords that add conditional behavior to replacement rules:

Syntax	Meaning
(start)string1=string2	Match string1 only at the start of the input string
(end)string1=string2	Match string1 only at the end of the input string
string1=(start)string2	Insert string2 at start after removing string1
string1=(end)string2	Insert string2 at end after removing string1
(start)string1=(end)string2	Match string1 at start, delete it, append string2 at end
string1=(return)string2	If string1 is found, replace it with string2 and terminate
(once)string1=string2	Instruction can only be executed once during entire execution

Combination Rules

If the string ends in abc, remove it and add xyz to the front — but only once.

• You cannot combine (start), (end), or (return) as this makes no sense.
• You can only use (once) on the left-hand-side of the instruction.
• You can only use (return) on the right-hand-side of the instruction.
• (once) must come before (start)/(end) if used in combination.
  • A=B doesn't traditionally support combining (once) with another keyword on LHS; however, it makes sense to allow this functionality

Combined Example

(once)(end)abc=(start)xyz

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Running the Interpreter

Make sure you have Stack installed.

Using the to Wrapper Script (Recommended)

A simple Zsh wrapper script is provided to simplify running programs.

1. Make it Executable:

   chmod +x to

2. Run a Program:

   ./to programname inputstring

Example Use Case

To check if a string is a palindrome using the provided palindrome.to program:

%./to code/palindrome abcabc
false

Using stack directly:

stack run -- programname inputstring

Note: Omit the .to extension when passing programname.
Don't use quotation marks around the input string.

File Requirements:

• Program files must end with extension .to.
• Input and instruction strings must be:
  • no spaces
  • no reserved characters: ( ) = #
  • No empty input strings, but empty instruction strings are permitted (and useful!)

Comments

Use # at the beginning or middle of a line for a comment. Only one instruction per line.

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Project Structure

haskell-atob/
├── code/            # The .to A=B program files used for tests
├── test/            # Integration tests' input/output cases
├── app/             # Main
├── src/             # Parsing and interpreter logic
├── README.md
└── package.yaml

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Testing

Test cases are located in test/cases/. Each .txt file contains alternating lines:

input1
expected_output1
input2
expected_output2
...

The interpreter is tested using these pairs to ensure correctness.

To run tests:

stack test

You can also see the code solutions to these tests under code/ (all of which primarily assume only "a", "b", and "c" are used to makeup input strings)

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License

See the LICENSE file for details about the license.

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Author

Made by Edward when I definitely should've been studying for exams.