A Skeleton Command Interpreter in Ragel for Ruby
03/24/2010 20:35 Filed in: Ragel
Synopsis: Do you need
to write a command interpreter to accept input from
a user or even an automated protocol? Commands must
be parsed and an action taken when the command is
recognized. An error action or message must be
produced when a bad command is given. The following
is a Ragel
definition I developed as a common starting
point for a command interpreter. The target in
this case is generated in Ruby, but could easily
be adapted for C or Java targets. The code
demonstrates how to interpret three fictitious
commands and catch syntactically incorrect
inputs as well. It demonstrates Ragel error
handling actions that apply to command
interpretation. The Ragel code is liberally
commented.
In this example command interpreter grammar there are 3 commands: "Bark", "Shout", "Grunt". Each command takes a numeric parameter up to 5 digits long.
When a specific command is recognized, a "found" action is called based on the type of command recognized. This “found” action is where you would perform some task based on the command that was recognized.
When a command is syntactically incorrect, an error action is called. The command error handler can be filled in to specifically deal with command errors for your application.
Finally, this code also demonstrates the start and end events that can be captured when a command is being parsed - start and end actions are fired at the appropriate times so you can fill in those handlers to do something special when command parsing starts or finishes.
To build the Ragel code and generate the state diagram, you can use this Bash script:
To run the resulting parser (Ragel will generate testcmd.rb):
expected output from running the generated Ruby is
The Ragel code for a simple command interpreter with error handling:
Landon Cox
www.ESawdust.com
Full list of Ragel Recipes
In this example command interpreter grammar there are 3 commands: "Bark", "Shout", "Grunt". Each command takes a numeric parameter up to 5 digits long.
When a specific command is recognized, a "found" action is called based on the type of command recognized. This “found” action is where you would perform some task based on the command that was recognized.
When a command is syntactically incorrect, an error action is called. The command error handler can be filled in to specifically deal with command errors for your application.
Finally, this code also demonstrates the start and end events that can be captured when a command is being parsed - start and end actions are fired at the appropriate times so you can fill in those handlers to do something special when command parsing starts or finishes.
To build the Ragel code and generate the state diagram, you can use this Bash script:
To run the resulting parser (Ragel will generate testcmd.rb):
expected output from running the generated Ruby is
The Ragel code for a simple command interpreter with error handling:
Landon Cox
www.ESawdust.com
Full list of Ragel Recipes
asdfasdf
