mw-parser/README.md

mw-parser

A rule parser for MicroWorld

Usage

Main entry point is (parse-rule string), where string takes a form detailed in grammar, below. If the rule is interpretted correctly the result will be a Clojure anonymous function; if the rule is not interpretted, currently nil is returned and there's no helpful error message.

Generated function and evaluation environment

The generated function is a function of two arguments

_ cell a cell in a world as defined in mw-engine.world, q.v.; _ world the world of which that cell forms part.

It returns a new cell, based on the cell passed.

Actions of the rule will (can only) modify properties of the cell; there are two properties which are special and SHOULD NOT be modified, namely the properties x and y. Currently there is no policing that these properties are not modified.

Grammar

A rule comprises:

• if conditions then actions

Conditions

where conditions is:

• condition
• condition and conditions
• condition or conditions

Note that 'and' takes precedence over or, so

conditionA and conditionB or conditionC and conditionD

is interpreted as

(conditionA and (conditionB or (conditionC and conditionD)))

A condition is one of:

• property is value
• property is not value
• property is in values
• property is not in values
• property is more than numeric-value
• property is less than numeric-value
• number neighbours have property equal to value
• more than number neighbours have property equal to value
• fewer than number neighbours have property equal to value

Actions

Where actions is:

• action
• action and actions

and action is:

• property should be value

Properties

In the above, property is the name of any property of a cell. Any alpha-numeric string of characters can form the name of a property. Actions should NOT refer to the reserved properties x and y.

Values in Conditions

Values in conditions and actions are considered slightly differently. In a condition, a value is one of

• symbolic-value
• numeric-value

The '...more than...' and '...less than...' conditions imply a numeric-value. Thus "if altitude is more than fertility..." is interpreted as meaning "if the value of the property of the current cell called 'altitude' is greater than the value of the property of the current cell called 'fertility'", whereas the apparently similar condition 'if altitude is fertility...' is interpreted as meaning "if the value of the property of the current cell called 'altitude' is the symbol 'fertility'".

Thus symbolic-value is any sequence of alphanumeric characters, whereas numeric-value is one of:

• number
• property

and number is any sequence of the decimal digits 0...9, the minus character '-' and the period character '.', provided that the minus character can only be in the first position, and the period character can only appear once.

Values in Actions

A value in an action is one of

• symbolic-value
• arithmetic-value
• number

where arithmetic-value is:

• property operator numeric-value

and operator is one of the simple arithmetic operators '+', '-', '*' and '/'.

Shorthand

Note that '...members are...' is equivalent to '...members have state equal to...', and 'some neighbours...' is equivalent to 'more than 0 neighbours...'

License

Copyright © 2014 Simon Brooke

Distributed under the GNU General Public License either version 2.0 or (at your option) any later version.