How Can I Effectively Prettyprint an Abstract Syntax Tree (AST) Back to Source Code?

Prettyprinting an AST Back to Source Code

When compiling an AST back to source code, the process known as "prettyprinting" can be employed. There are two variations: fidelity printing, which attempts to regenerate the original text as accurately as possible, and nice prettyprinting, which focuses on producing well-formatted code.

To achieve effective prettyprinting, it's crucial to consider various factors, including:

• Literal Value Regeneration: Preserving the exact value of literals, such as floating-point numbers and string escapes, is essential.
• Spacing and Newlines: Maintaining the original spacing and introducing appropriate newlines is crucial for readability.
• Detail Preservation: To ensure fidelity, capturing and regenerating details like literal radix, string quotes, and identifier casing is necessary.
• Comment Handling: Discarding comments during parsing can lead to rejection from users who expect their original comments to be preserved.

Visitor Pattern for Prettyprinting

While the visitor pattern can assist in node manipulation, it's not the most straightforward method for prettyprinting. Instead, a more optimized approach involves iterating the AST from leaves to root, producing text as nodes are visited.

Consider this example for prettyprinting a block of statements:

PrettyPrintBlock:
    Print("{"); PrintNewline();
    PrettyPrint(Node.children[1]); // statements in block
    Print("}"); PrintNewline();

Reengineering Parsers

To effectively capture the necessary information for prettyprinting, it's recommended to employ "reengineering parsers" that collect additional data beyond what traditional parsers gather. This information includes:

• Column-number information for concrete tokens
• Knowledge of quoted string type and escape sequences
• Preserve original casing and formatting

Tools for Prettyprinting

Several tools can aid in the process of prettyprinting:

• DMS Software Reengineering Toolkit: A comprehensive tool featuring a text-box composition approach that allows for arbitrary rearrangement of text blocks.
• PHP Front End: A DMS-based tool specifically designed for PHP prettyprinting.

Conclusion

Prettyprinting an AST back to source code is a nuanced process that requires attention to detail and the consideration of various factors such as literal accuracy, spacing, and comment preservation. By utilizing techniques like visitor pattern and reengineering parsers, it's possible to generate both fidelity and nice prettyprinted code that meets the needs of developers working with the regenerated source.

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