Simple Ohm Grammar

2025-03-26

Mar 26, 2025

Here is a simple sample of a DSL, inspired by sketch.systems demo.

This simple example doesn’t parse everything in the demo, but, I hope that this is uncomplicated enough to give a flavour of the simplicity of OhmJS.

The test example is

Green*

tick -> Yellow

Yellow

tick -> Red

Red

tick -> Green

And the grammar is:

sketch {

Main = State+

State = name "*"? Transition+

Transition = name "->" name

name = letter+

}

Further

The sketch-systems demo goes on to show the use of indentation and multi-word state names. Those ideas are not included in the above grammar, for clarity. I’ve tried to address them below.

Note that the more elaborate grammar uses OhmJS’ “space-skipping” features

capitalized and uncapitalized grammar rule names
The use of the “#” operator
It turns out that I didn’t need to use OhmJS’ indentation parsing features, but, they are there if you need to parse something more complicated.

The ultimate example in the sketch-systems demo is:

Powered

power failed -> Red On

Green*

tick -> Yellow

Yellow

tick -> Red

Red

tick -> Green

Unpowered

power restored -> Green

Red On

tick -> Red Off

Red off

tick -> Red On

And, a grammar (untested) that appears to work is:

sketch {

Main = State+

State = StateName "*"? #nl Transition+

Transition = StateName "->" StateName #nl

StateName = name (#" " name)*

name = letter+

nl = "\n"

}

Programming Simplicity

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Programming Simplicity

Simple Ohm Grammar

2025-03-26

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