l3viathan / basil

The basil programming language

Basil

Basil is a message-based programming language. Its name is a reference to both Aceto and OIL.

Concept

Basil has something like a message queue, but it's called the message bag, because the order elements are taken out is "random"; non-guaranteed. There is rudimentary support for expressions and some operators, but for the most part, all a basil program does is wait for messages and react to them by sending other messages.

Syntax

A basil file contains one or more blocks. A block consists of a header, and a set of statements that get executed when a message is received. The header is a space-seperated list of topics this block listens to.

A statement starts with an optional condition, followed by a > character, followed by an action, optionally followed by another > character and another action. A semicolon finalizes a statement:

(condition) ">" (action) ">" (another action) ";"

A condition is an expression. An action is a triplet, seperated by spaces: topic, body, and reply-to. Both topic and reply-to have to be atoms, body can be an arbitrary expression. body and reply-to are optional, but when reply-to is given, body has to be given too.

An expression is either:

• an integer
• a string (enclosed in double-quotes)
• a tuple of one or more values (in Python syntax)
• an atom; a alphanumerical string of characters.
• a complex expression consisting of an operator and one or more operands (prefix notation)

Apart from alphanumerical atoms, there are three special atoms: @, &, and $. They refer to the current body, current reply-to and current topic, respectively.

The usual operators do what one would expect, and there's one more: # is an indexing operator, such that #ab evaluates to the bth element of a.

Semantics

When a message (basically a triplet of topic, body, and reply-to) is being sent, it is being pushed in the bag. When one is being popped, every block that has the message's topic listed in its header gets executed with the message. This may happen in any order.

For every statement, the condition is first evaluated. If it is truthy (not 0 or an empty string or an empty atom) or if it doesn't exist, the first action gets evaluated and sent as a message. Otherwise, this happens with the second action.

At the beginning of the execution of a basil program, a message with the topic "main", an empty body and empty reply-to is pushed in the bag.

Builtins:

Some topics are listened to by built-in blocks (that are not implemented in basil). All built-in topics are lowercase, so it is recommended to choose Titlecase topics in user code to avoid collision (unless collision is intended).

A list of builtins can be seen in the file stdB.py.

Example code

The typical Hello World program just sends a message with the print topic, which the standard library picks up.

main { print "Hello, World!"; }