Lowering Visitor: Transforming Expressions into States
To support asynchronous code, we must convert user expression trees into state machine representations that can suspend and resume operations.
This conversion process is known as “lowering”, and it is responsible for transforming flow control constructs (such as if, switch, loops, and awaits) into a state tree that can be used to generate a flattened state machine. This step systematically traverses the expression tree and replaces branching constructs with state nodes that manage control flow using transitions and goto operations. It also identifies variables that will need to be hoisted so that variable scope is correctly maintained.
Introduction
The LoweringVisitor is responsible for transforming an expression tree into discrete states and transitions, that will be used to generate the final state machine.
The purpose of this visitor is to “lower” high-level constructs, such as await, if/else, switch, try\catch, and loops, into individual NodeExpression objects that the state machine can later process.
1. Handling Control Flow Constructs (Branching and Loops)
What is being done?
The LoweringVisitor handles control flow constructs like conditionals (if/else), loops (for, while), and switches (case). Each construct is transformed into a state, and transitions are defined to manage the execution flow between branches and loops.
Problem
Branching and looping introduce multiple execution paths that could require the program to pause and resume execution. These paths must be preserved accurately within a state machine to ensure correct execution flow.
Discussion
The LoweringVisitor uses the VisitBranch method to create distinct states for each branch and loop. For each conditional or loop, a new state is created, and transitions are added to connect these states. The visitor maintains a JoinState that serves as the reconnection point after the conditional or loop is executed. Additionally, the SourceState represents the state before the branch or loop is executed.
Every branching construct must eventually rejoin the main flow of execution. The join state represents the point where diverging branches reunite, ensuring that the state machine continues to execute correctly.
If you think about each unique branch segment (e.g. the ‘if’ or ‘else’ path in a conditional expression) as a single linked list of states, the TailState, is the last node in the conditional or loop path. This tail node must be re-joined to the main execution path; the place where the ‘if’ and ‘else’ branches again begin to execute the same code again. This re-convergance is critical, as branching structures are often nested, and all of the potential paths in a nesting structure must be correctly re-joined.
Each branch (such as if/else or the body of a loop) becomes a distinct state, and the transitions ensure that the state machine can resume from the correct point when execution continues.
Solution
By creating separate states for each branch and loop, and by using JoinState and SourceState, the state machine can accurately manage control flow across complex branching and looping structures. This ensures that execution can pause and resume from the correct points, preserving the integrity of the program’s logic.
2. Handling Try/Catch/Finally Blocks
What is being done?
The LoweringVisitor flattens (try/catch/finally) constructs so they can correctly handle continuations. Each construct is transformed into a mini-state machine within the body of the main state machine, and jump tables are injected to allow continuations to resume from within these nested machines.
Problem
Try/catch/finally blocks introduce complex control flow that must be managed by the state machine. The LoweringVisitor must transform these constructs into smaller state machines that can handle exceptions and ensure that execution continues correctly.
Discussion
The VisitTry handles the try/catch/finally blocks by capturing all the expression within the body of the try in child scope that understands the jump tables and the continuation to the catch block and finally blocks. Additionally because goto statments cannot be used to move into nested scopes it is necessary capture any excpetions and to process the gotos after the scope of the try body.
Solution
By capturing the scope of the try body and processing the gotos after the scope of the try, the state machine can correctly handle returning to the previous state machine after any errors. In the reducing of the Try block, the TryCatchTransition has to create jump table that understand how to handle navigate to the suspension of an await and in case of any nest try/catch/finally blocks the result of any async code. This ensures that the state machine can correctly handle exceptions and continue execution even when there are deeply nested try/catch/finally blocks or when outer block handle errors from nested code.
3. Handling Await
What is being done?
The await expression is used to pause the execution of a method until the awaited task completes. The LoweringVisitor splits the execution into two states: one before the await, and one after the task has completed.
Problem
The await expression introduces natural pauses in execution. Managing this in a state machine requires the program to be split into two distinct states: one before the await, and one after, to resume execution from where it left off.
Discussion
The VisitAwait method handles asynchronous operations by creating two separate states: a state that handles the suspension of execution (before the await), and another state that handles resumption after the awaited task completes. This ensures that the state machine can pause execution, wait for the task to complete, and then resume execution in the appropriate state.
Solution
By breaking the execution into two states, the state machine can handle the suspension and resumption of execution around await expressions. This allows asynchronous operations to be integrated into the state machine seamlessly.
4. Variable Hoisting
What is being done?
In scenarios such as loops or asynchronous operations, variables need to persist across states. To achieve this, the LoweringVisitor hoists variables to a higher scope, ensuring they are accessible after state transitions.
Problem
Variables declared within a state need to persist across multiple states. For example, a variable declared before an await must still be available after the await completes, even though the program has moved to a new state.
Discussion
The LoweringVisitor identifies variables that need to persist across states and hoists them to a higher scope. This ensures that variables are not lost when the state machine transitions from one state to another. By hoisting these variables, the state machine can continue using them after pausing and resuming execution.
Solution
Hoisting variables to a higher scope ensures that they are accessible across multiple states in the state machine. This is critical for maintaining the integrity of the program’s execution, especially in the context of loops and asynchronous operations.
5. Managing Intermediate Expression Values
What is being done?
The LoweringVisitor manages intermediate values produced by expressions and ensures that these values persist across state transitions. This is accomplished by separating the ResultValue (the expression producing the value) from the ResultVariable (where the result is stored).
Problem
Intermediate values need to persist across state transitions to ensure that the program’s logic flows correctly. Without proper management of these values, the state machine could lose track of intermediate results, leading to incorrect behavior.
Discussion
The ResultValue represents the value produced by an expression, while the ResultVariable is the storage for that value. This separation allows the state machine to manage intermediate results effectively. The ResultValue is evaluated before transitioning to a new state, and the result is stored in the ResultVariable for future use.
Solution
By separating the ResultValue and ResultVariable, the state machine can manage intermediate values across state transitions. This ensures that results are not lost when the machine moves from one state to another.
Conclusion
The LoweringVisitor is a critical component in transforming complex control flow expressions into manageable states for a state machine. By handling control flow constructs, asynchronous operations, and intermediate values, the LoweringVisitor ensures that execution can pause and resume as needed, without losing the integrity of the program’s logic.