In CRuby (MRI), Ruby source passes through several representations: the lexer recognizes tokens, a parser organizes them into syntax, the compiler turns that syntax into a RubyVM::InstructionSequence, and the virtual machine executes the instructions. Tools such as Prism, Ripper, and RubyVM::AbstractSyntaxTree let you inspect different parts of that process—but they do not all produce the same kind of output, and an AST is not one universal Ruby format.
What happens between a Ruby file and the VM?
Consider this Ruby code:
x = 1 + 2The line begins as characters in a source file. In CRuby, it is recognized and structured, compiled into VM instructions, then executed. The stages are related but distinct: a syntax tree describes the program’s structure, while an instruction sequence is compiled code for the VM.
- Characters: The source contains the characters
x, spaces,=,1,+, and2. - Tokens: Lexical analysis recognizes meaningful units such as an identifier, assignment operator, integer literals, and plus operator. Whitespace can separate tokens without itself becoming an operation in the expression.
- Syntax: The parser applies Ruby’s grammar to determine that the expression adds two values and assigns the result to
x. It represents the assignment, addition, and operands as a structured syntax tree. - VM instructions: CRuby compiles the parsed program into an instruction sequence. The sequence encodes operations the VM can execute; it is not simply another name for the syntax tree.
- Runtime effect: When the VM executes the sequence, it evaluates
1 + 2and assigns the resulting integer,3, to the local variablex.
That is a conceptual walkthrough, not a promise of a particular token list, tree shape, or disassembly. Those representations and instruction details depend on the API and Ruby version.
What do lexing, parsing, and an AST each do?
Lexing recognizes tokens
A lexer reads the character stream and identifies token boundaries and kinds. Ripper exposes Ruby lexical analysis as well as parser events. For example, Ripper.sexp can return an S-expression for Ruby source:
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require "ripper"
Ripper.sexp('def hello(world)n "Hello, #{world}!"nend')The result is a nested S-expression representing syntax. Ripper can also expose tokens or parser events, so it is useful when a tool needs a stream of lexical or parsing information rather than a Prism-style syntax tree. Its documentation describes it as “a Ruby script parser.”
Parsing applies Ruby grammar
A parser takes the tokens and determines how they fit together under Ruby’s grammar. It distinguishes, for example, an assignment from an argument list and establishes which operations belong inside others. Parsing turns a sequence of tokens into a structured representation that later tools or the compiler can work with.
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An AST represents structure
An abstract syntax tree (AST) captures the meaningful structure of code while leaving out many surface details. The term describes a family of representations, not a single tree format shared by every Ruby tool or implementation. Prism nodes, Ripper’s S-expressions, and MRI’s AST nodes can describe the same source differently.
Which Ruby parser or syntax API should you use?
Choose based on what you need to inspect and where your tool must run. Prism is the official Ruby parser API and is intended for portable parser tooling. Ripper offers token and parser-event access as well as S-expressions. RubyVM::AbstractSyntaxTree exposes MRI’s internal AST and is explicitly experimental and unstable.
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| API | What it exposes | Error handling and portability | Stability and scope |
|---|---|---|---|
Prism |
A syntax tree through Prism.parse. |
Designed to be portable and error-tolerant. Prism is implemented in C99. | Official Ruby parser API. Ruby 3.3 release notes said it was introduced as a default gem, production ready, and usable in place of Ripper for parser tooling. |
Ripper |
Lexical analysis, parser events, and S-expressions such as Ripper.sexp. |
Error tolerance and portability are not stated in the cited Ripper documentation. | Ruby documentation describes it as a Ruby script parser. Its event-oriented and S-expression interfaces differ from a Prism node tree. |
RubyVM::AbstractSyntaxTree |
MRI AST nodes, with an option to retain tokens; tolerant mode can produce error nodes. | It exposes MRI’s implementation representation, rather than a portable tooling interface. | Experimental and unstable. Ruby’s source comments recommend Prism for new code. |
In Ruby 3.3, the Ruby core team’s release announcement described Prism as “a portable, error tolerant, and maintainable recursive descent parser for the Ruby language.” That release established Prism as a production-ready default gem at the time; check the documentation for the Ruby version you target when relying on API availability or compatibility.
Parse with Prism
For a modern syntax-tree parse, use Prism.parse:
require "prism"
result = Prism.parse("x = 1 + 2")
result.valueThe returned parse result contains Prism’s representation of the source. Inspect that tree when your tool needs syntax nodes; do not assume its node classes or shape match Ripper or MRI’s AST.
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Inspect MRI’s AST when you need MRI internals
RubyVM::AbstractSyntaxTree.parse returns MRI AST nodes for the given source:
ast = RubyVM::AbstractSyntaxTree.parse("x = 1 + 2")This can be useful for investigation of CRuby itself, but its experimental, unstable status makes it a poor default for portable tooling. The Ruby source documentation identifies Prism as the official API for parsing Ruby code and recommends it for new code.
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How does CRuby compile and execute a file?
After parsing, CRuby lowers the program into a RubyVM::InstructionSequence, the VM’s representation of compiled instructions. The class documentation describes it as “a compiled sequence of instructions for the Ruby Virtual Machine.” An instruction sequence is executable VM code, unlike an AST, which describes syntax.
To compile a file and inspect its VM instructions:
iseq = RubyVM::InstructionSequence.compile_file("hello.rb")
puts iseq.disasmcompile_file reads, parses, and compiles the source file, and the instruction sequence includes source-location metadata. disasm prints a human-readable disassembly. The API also exposes information through methods such as to_a, child sequences, labels, paths, and source metadata, which can help with debugging or research.
Do not treat a particular disassembly as a stable language-level contract: RubyVM::InstructionSequence is MRI-specific, and instruction details can change across Ruby versions. Use it to understand the CRuby version you are actually running, rather than to define how every Ruby implementation executes code.
Is Prism replacing Ripper?
Prism can be used in place of Ripper for parser tooling, according to the Ruby 3.3 release notes, but that does not mean the APIs are interchangeable or that Ripper’s interfaces have disappeared. Prism provides a syntax tree; Ripper can provide tokens, parser events, and S-expressions. A tool that depends on Ripper’s specific output or event model may need adaptation rather than a simple name change.
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For new parser tooling, Prism is the official Ruby parser API and the RubyVM AST documentation recommends it over the unstable MRI AST interface. If a project already uses Ripper, compare the exact information it consumes with Prism’s nodes before migrating, and check version compatibility for the Ruby releases the project supports.
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