In this first part,
EasyScript will be extremely simple:
it will only allow addition of integer literals
(so, expressions like 1 + 2 + 3).
We'll need the following Truffle APIs to accomplish this:
The Node class is the abstract superclass of all nodes in your AST interpreter.
It doesn't define any abstract "interpret"
method that you're supposed to override;
instead, you're expected to define it yourself
(the reason Truffle does it that way are specializations,
which are language-specific;
we cover them in later parts of the tutorial).
Because of this,
you almost always define your own abstract superclass of all nodes in your language.
For EasyScript, that's the
EasyScriptNode class.
We also define two concrete subclasses of EasyScriptNode,
IntLiteralNode
and AdditionNode,
which represent integer literals and the plus operator,
respectively.
Each Truffle AST needs to be anchored in a RootNode to be executable.
This is another abstract class that you need to extend,
and, unlike Node,
it does define an abstract execute method that you need to override.
Our implementation is the
EasyScriptRootNode class,
and its execute method simply delegates to the EasyScriptNode's executeInt
that it gets passed through the constructor.
Since the execute method in RootNode takes a Truffle VirtualFrame
as its argument,
we need one more class to execute the AST: a CallTarget.
This is an interface, not an abstract class.
Historically, you created instances of it through a static factory method
createCallTarget() of the TruffleRuntime class,
which is a singleton you acquired using the getRuntime()
method of the Truffle class.
However, since version 22 of GraalVM,
that API has been removed,
and now, you acquire CallTargets directly from RootNodes,
by calling their getCallTarget() method.
In our case, the root node is EasyScriptRootNode.
With a CallTarget reference,
we can finally invoke its calll method with no arguments.
That will create a VirtualFrame instance,
and call the execute method of the RootNode it got passed,
which in turn calls the executeInt method of EasyScriptNode.
All of this is implemented in a simple unit test.