Lambda and StatefulLambda are the primary way to write commands in Mercurial.

These classes are immutable command builders, that allow you to compose a command programmatically, and are easy to read and use.

StatefulLambda is the same as Lambda, but has additional state stored in it, that gets passed to each of its methods to manipulate.

Both Lambda and StatefulLambda start with sensible defaults:

• no requirements
• an empty init method
• an empty execute method
• instantly finishes
• an empty end method
• is interruptible
• allowed to be scheduled in OpModeState.ACTIVE only (start and loop)

Then, you can apply the builder methods to compose a new command, by adding to or replacing components of the current command.

​

Construction

Both Lambda and StatefulLambda are easy to construct:

Our StatefulLambda will have a RefCell with a String in it as its state. Go read the RefCell documentation in Util for how it works. We need a cell or similar in order to obtain interior mutability for a simpler piece of data like a String, but you could use a more complex class you’ve written yourself.

​

Building

There are easy methods to set parts of a Command:

Or add to the pre-existing ones, we’ll demonstrate some of the add methods, but there are equivalents for all the set methods.

Remember, each of these methods are non-mutating. This means than when you call them, you get back a new, different command. In the case of statefulLambda, they still share the same state reference, but other than that they are totally different.

This is a common theme across Dairy, and its not hard to deal with, but you must keep it in mind.

Finally, any command can be converted into a Lambda or StatefulLambda command.

This process is safe for Lambda.from(Command), it does not create a new object if the argument is already a Lambda or a StatefulLambda.

But StatefulLamba.from(Command, STATE) does not have this safety.

Lambda and StatefulLambda are clearly nice and easy ways to compose commands! They are the recommended tool for writing commands in Mercurial.