Abstract syntax tree: Difference between revisions

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In [[computer science]], anAn '''abstract syntax tree''' ('''AST'''), is a data structure used in [[computer science]] to represent the structure of a program or justcode '''syntaxsnippet. tree''',It is a [[Tree (data structure)|tree]] representation of the [[abstract syntax|abstract syntactic]] structure of text (often [[source code]]) written in a [[formal language]]. Each node of the tree denotes a construct occurring in the text. It is sometimes called just a '''syntax tree'''.

The syntax is "abstract" in the sense that it does not represent every detail appearing in the real syntax, but rather just the structural or content-related details. For instance, grouping [[Bracket#Parentheses|parentheses]] are implicit in the tree structure, so these do not have to be represented as separate nodes. Likewise, a syntactic construct like an if-condition-then statement may be denoted by means of a single node with three branches.

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* An AST usually contains extra information about the program, due to the consecutive stages of analysis by the compiler. For example, it may store the position of each element in the source code, allowing the compiler to print useful error messages.

ASTs are needed because of the inherent nature of programming languages and their documentation. Languages are often [[syntactic ambiguity|ambiguous]] by nature. In order to avoid this ambiguity, programming languages are often specified as a [[context-free grammar]] (CFG). However, there are often aspects of programming languages that a CFG can't express, but are part of the language and are documented in its specification. These are details that require a context to determine their validity and behaviour. For example, if a language allows new types to be declared, a CFG cannot predict the names of such types nor the way in which they should be used. Even if a language has a predefined set of types, enforcing proper usage usually requires some context. Another example is [[duck typing]], where the type of an element can change depending on context. [[Operator overloading]] is yet another case where correct usage and final function are context-dependent.

=== Design ===

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After verifying correctness, the AST serves as the base for code generation. The AST is often used to generate an intermediate representation (IR), sometimes called an [[intermediate language]], for the code generation.

== Other usages ==

=== AST differencing ===

AST differencing, or for short tree differencing, consists of computing the list of differences between two ASTs.<ref>{{Cite journal |last1=Fluri |first1=Beat |last2=Wursch |first2=Michael |last3=PInzger |first3=Martin |last4=Gall |first4=Harald |date=2007 |title=Change Distilling:Tree Differencing for Fine-Grained Source Code Change Extraction |url=http://dx.doi.org/10.1109/tse.2007.70731 |journal=IEEE Transactions on Software Engineering |volume=33 |issue=11 |pages=725–743 |doi=10.1109/tse.2007.70731 |s2cid=13659557 |issn=0098-5589|url-access=subscription }}</ref> This list of differences is typically called an edit script. The edit script directly refers to the AST of the code. For instance, an edit action may result in the addition of a new AST node representing a function. The problem of computing good AST edit scripts is hard, with two main challenges: handling move actions, and scaling to fine-grained ASTs with thousands of nodes.<ref>{{Cite journal|last1=Falleri|first1=Jean-Rémy|last2=Morandat|first2=Floréal|last3=Blanc|first3=Xavier|last4=Martinez|first4=Matias|last5=Monperrus|first5=Martin|date=2014-09-15|title=Fine-grained and accurate source code differencing|url=https://dl.acm.org/doi/10.1145/2642937.2642982|journal=Proceedings of the 29th ACM/IEEE International Conference on Automated Software Engineering|language=en|___location=Vasteras Sweden|publisher=ACM|pages=313–324|doi=10.1145/2642937.2642982|isbn=978-1-4503-3013-8|s2cid=218737160 }}</ref>

=== Structured merge ===

With proper AST differencing, ASTs can be used for merging versions and branches, this is known as [[Merge (version control)|structured merge]]. The advantage is to avoid spurious conflicts due to lines and formatting.<ref>{{Cite journal|last1=Larsen|first1=Simon|last2=Falleri|first2=Jean-Remy|last3=Baudry|first3=Benoit|last4=Monperrus|first4=Martin|date=2022|title=Spork: Structured Merge for Java with Formatting Preservation|url=https://ieeexplore.ieee.org/document/9684709|journal=IEEE Transactions on Software Engineering|pages=1|doi=10.1109/TSE.2022.3143766|arxiv=2202.05329 |s2cid=246823829 |issn=0098-5589}}</ref>

=== Clone detection ===

An AST is a powerful abstraction to perform code [[clone detection]].<ref>{{Cite journalbook |last1=Koschke |first1=Rainer |last2=Falke |first2=Raimar |last3=Frenzel |first3=Pierre |datetitle=2006 13th Working Conference on Reverse Engineering |titlechapter=Clone Detection Using Abstract Syntax Suffix Trees |date=2006 |chapter-url=http://dx.doi.org/10.1109/wcre.2006.18 |journal=2006 13th Working Conference on Reverse Engineering |pages=253–262 |publisher=IEEE |doi=10.1109/wcre.2006.18|isbn=0-7695-2719-1 |s2cid=6985484 }}</ref>

== See also ==

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* [[Symbol table]]

* [[TreeDL]]

* [[Interpreter (computing)#Abstract syntax tree interpreters|Abstract Syntax Tree Interpreters]]

== References ==

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== Further reading ==

* {{cite web |last=Jones |first=Joel |title=Abstract Syntax Tree Implementation Idioms |url=http://www.hillside.net/plop/plop2003/Papers/Jones-ImplementingASTs.pdf |url-status=dead |archive-url=https://web.archive.org/web/20240721094334/http://hillside.net/plop/plop2003/Papers/Jones-ImplementingASTs.pdf |archive-date=21 July 2024 |access-date=9 November 2011 }} (overview of AST implementation in various language families)

* {{cite conference |last1=Neamtiu |first1=Iulian |last2=Foster |first2=Jeffrey S. |last3=Hicks |first3=Michael |date=May 17, 2005 |title=Understanding Source Code Evolution Using Abstract Syntax Tree Matching |conference=MSR'05 |___location=Saint Louis, Missouri |publisher=ACM |citeseerx=10.1.1.88.5815}}

* {{cite thesis |last=Würsch |first=Michael |degree=Diploma |title=Improving Abstract Syntax Tree based Source Code Change Detection |url=http://www.ifi.uzh.ch/seal/research/tools/archive/changeDetection.html}}

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*{{cite web|url=http://www.eclipse.org/articles/Article-JavaCodeManipulation_AST/index.html|title=Abstract Syntax Tree and Java Code Manipulation in the Eclipse IDE|work=eclipse.org}}

*{{cite web|url=http://www.cs.utah.edu/flux/flick/current/doc/guts/gutsch6.html|title=CAST representation|work=cs.utah.edu}}

*[httphttps://eli-project.sourceforge.net/elionline/idem_3.html eli project]: Abstract Syntax Tree [[Unparsing]]

*{{cite web | url = http://www.omg.org/news/meetings/workshops/ADM_2005_Proceedings_FINAL/T-3_Newcomb.pdf | title = Abstract Syntax Tree Metamodel Standard }}

*{{cite web | url = http://www.omg.org/spec/ASTM/ | title = Architecture‑Driven Modernization — ADM: Abstract Syntax Tree Metamodeling — ASTM }} ([[Object Management Group|OMG]] standard).

*[https://javaparser.org JavaParser]: The JavaParser library provides you with an Abstract Syntax Tree of your Java code. The AST structure then allows you to work with your Java code in an easy programmatic way.

*[https://github.com/INRIA/spoon Spoon]: A library to analyze, transform, rewrite, and transpile Java source code. It parses source files to build a well-designed AST with powerful analysis and transformation API.

*[https://astexplorer.net AST Explorer]: A website to help visualize ASTs in several popular languages such as Go, Python, Java, and JavaScript.

{{Parsers}}