Report statements

Code Example

Runnable Example in Jac and JacLib

Try it!JacPython

# Report Statements

walker Reporter {
    can process with `root entry {
        # Report literal value
        report 42;

        # Report string
        report "hello";

        # Report expression
        report 10 + 20;

        # Report variable
        x = 100;
        report x;

        # Report list
        report [1, 2, 3];

        # Report dictionary
        report {"key": "value", "number": 123};

        # Report complex expression
        report {"result": 5 * 10, "status": "ok"};

        disengage;
    }
}

with entry {
    w = Reporter();
    root spawn w;
}

# Report Statements

walker Reporter {
    can process with `root entry {
        # Report literal value
        report 42;

        # Report string
        report "hello";

        # Report expression
        report 10 + 20;

        # Report variable
        x = 100;
        report x;

        # Report list
        report [1, 2, 3];

        # Report dictionary
        report {"key": "value", "number": 123};

        # Report complex expression
        report {"result": 5 * 10, "status": "ok"};

        disengage;
    }
}

with entry {
    w = Reporter();
    root spawn w;
}

from __future__ import annotations
from jaclang.runtimelib.builtin import *
from jaclang import JacMachineInterface as _jl

class Reporter(_jl.Walker):

    @_jl.entry
    def process(self, here: _jl.Root) -> None:
        _jl.report(42)
        _jl.report('hello')
        _jl.report(10 + 20)
        x = 100
        _jl.report(x)
        _jl.report([1, 2, 3])
        _jl.report({'key': 'value', 'number': 123})
        _jl.report({'result': 5 * 10, 'status': 'ok'})
        _jl.disengage(self)
        return
w = Reporter()
_jl.spawn(_jl.root(), w)

Jac Grammar Snippet

report_stmt: KW_REPORT expression

Description

Report Statements

Report statements are a special feature in Jac's walker-based programming model that allow walkers to send data back to the caller without terminating execution. Unlike return statements which exit a function, report statements enable streaming results during graph traversal.

Basic Report Syntax

The simplest form appears on line 6: report 42;. The report keyword is followed by an expression, and that expression's value is sent back to the caller. The walker continues executing after the report statement.

Reporting Different Value Types

The example demonstrates reporting various types of values:

Line	Expression	Type	Description
6	report 42;	Integer	Reports a literal number
9	report "hello";	String	Reports a text message
12	report 10 + 20;	Expression	Evaluates arithmetic before reporting
16	report x;	Variable	Reports the value stored in variable x (100)
19	report [1, 2, 3];	List	Reports a collection
22	report {"key": "value", "number": 123};	Dictionary	Reports a structured object
25	report {"result": 5 * 10, "status": "ok"};	Complex	Reports dict with computed values

Walker Context

Lines 3-29 define a walker named Reporter with the ability process that executes when entering the root node (line 4). This walker contains multiple report statements that execute sequentially, each sending its value back to the caller.

Execution Flow

graph TD
    A[root spawn Reporter] --> B[Walker enters root node]
    B --> C[report 42]
    C --> D[report "hello"]
    D --> E[report 10 + 20]
    E --> F[report x]
    F --> G[report list]
    G --> H[report dict]
    H --> I[report complex dict]
    I --> J[disengage - walker stops]

When root spawn w; executes on line 33, the walker runs and sends seven values back to the caller before disengaging on line 27.

Report vs Return

Understanding the difference between these two statements is crucial:

Feature	report	return
Terminates execution	No - continues running	Yes - exits immediately
Multiple calls	Yes - can report many times	No - only returns once
Primary usage	Walker abilities	Functions and methods
Value streaming	Yes - sends data during execution	No - single value at end
Graph traversal	Continues to next nodes	Stops traversal

Multiple Reports

Lines 6-25 show a walker ability with seven different report statements. All of these execute in sequence, each sending their respective value back to the caller. This streaming capability is unique to report statements and enables walkers to communicate incremental results during graph traversal.

Use Cases

Report statements excel at: - Collecting data from multiple nodes during a graph walk - Streaming results without waiting for traversal completion - Providing progress updates during long-running traversals - Implementing data aggregation patterns across graph structures - Building APIs that return multiple related values

Practical Example

The walker on lines 3-29 demonstrates a typical pattern: 1. Enter a node (root in this case) 2. Report various computed values (lines 6-25) 3. Disengage to stop the walker (line 27)

The caller can capture all reported values and process them as needed, making report statements ideal for building data collection and query systems.

Integration with Walkers

Report statements work seamlessly with Jac's walker traversal model. As walkers move through a graph using visit statements, they can report findings from each node, building up a collection of results that represents their journey through the data structure.