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You can build a small programming language in C by implementing a lexer, parser, abstract syntax tree (AST), and tree-walk interpreter in stages. Start with arithmetic and a few statements; do not begin with native code generation. The first useful result is a program that reads source text, reports errors, and evaluates what it understands.
What “from scratch” should mean for a first language
For this project, “from scratch” means writing the language’s lexer, parser, AST, and evaluator yourself in C—not building an industrial compiler, assembler, or optimizing backend at the same time. Keep the language small enough that each part has a clear job and can be tested independently.
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A practical first version can support numeric literals, arithmetic, parentheses, variable declarations, and print or expression statements. Decide the exact syntax before coding. For example, choose whether declarations use a keyword and whether statements end with semicolons; write those rules down as a tiny grammar. The project has no specified syntax or target platform, so these choices belong to you.
How the pieces fit together
A language implementation commonly moves from source text to tokens, from tokens to structured syntax, and from that structure to execution or generated code. LLVM’s staged Kaleidoscope material illustrates this progression, although its implementation is in C++ rather than C.
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- Lexer: reads characters and groups them into tokens such as numbers, identifiers, operators, and punctuation. Record source positions so errors can point to where they occurred.
- Parser: checks whether the token sequence follows your grammar and builds a structured representation.
- AST: represents the meaningful structure of expressions and statements without making later stages repeatedly interpret raw text. LLVM describes an AST as capturing program behavior in a form later stages can interpret.
- Evaluator: visits AST nodes and performs their meaning directly, using an environment or symbol table to store variable values.
The separation matters: changing how a token is recognized should not require rewriting arithmetic evaluation, and adding a new expression form should have a defined place in the parser and evaluator.
Build in stages, and make each stage runnable
1. Write down a small grammar
Start with the syntax you intend to accept, not with a general-purpose language design. A compact grammar might describe numbers, grouped expressions, unary operators, binary arithmetic, variable declarations, and print statements. Decide operator precedence explicitly: multiplication and division should usually bind more tightly than addition and subtraction, while parentheses override the default order.
2. Tokenize source and preserve locations
Implement a lexer that advances through the input and emits tokens with a kind, any associated value, and a source location. Handle whitespace deliberately. When a character cannot begin a valid token, report a lexical error with its position rather than silently skipping it.
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3. Parse expressions and statements
A hand-written parser is a reasonable learning target. One documented approach combines recursive-descent parsing for the grammar with an operator-precedence routine for binary expressions. That lets the parser handle nested constructs while consistently applying precedence. Add parsing for statements only after expressions work.
4. Construct and own AST nodes in C
Define explicit node kinds for the forms your language supports, such as number, unary operation, binary operation, variable reference, declaration, and print statement. Give each node fields appropriate to its kind. In C, decide who owns each allocated node and where it is freed; unclear ownership can turn a small parser into a memory-management problem. Keep the representation simple enough that the evaluator can inspect it without depending on parser internals.
5. Interpret the AST directly
Write an evaluator that recursively visits expression nodes and returns values, then handles statements such as declarations and printing. Keep variable lookup and assignment in a small environment or symbol table. Report runtime problems—such as reading a variable that has not been defined—as language errors, distinct from malformed syntax.
6. Test the behavior and the failures
Use small input programs to test each feature. Include valid expressions, nested grouping, precedence, declarations, and print statements. Also test malformed input and runtime errors. A useful implementation is not only one that produces the expected result for valid programs; it should fail clearly when input violates the grammar or the program requests an unsupported operation.
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A tree-walk interpreter evaluates AST nodes directly, so you can observe the language’s rules without first translating programs to another representation. Code generation is a later layer: it turns the AST into an intermediate representation or another target and brings additional target and toolchain concerns.
Best Value
| Approach | What it does | Why it fits this project |
|---|---|---|
| Tree-walk interpreter | Evaluates AST nodes directly. | Lets you implement and verify language behavior before taking on a backend. |
| Code generation | Translates the AST into an intermediate representation or another target. | Useful as a later learning goal once the front end and language semantics are coherent. |
After the interpreter works, choose a next step based on what you want to learn: bytecode, C output, LLVM IR, or another machine-code backend. LLVM’s Kaleidoscope sequence introduces IR generation and JIT execution after lexer, parser, and AST work; those are extensions, not prerequisites for a first language that runs.
What LLVM’s material can and cannot provide
The official Kaleidoscope tutorial is useful for concepts such as parsing expressions and constructing an AST, but it implements the language in C++ and assumes C++ knowledge. Reading it does not make the project C-only if you copy its implementation; use the concepts to design your own C structures and functions instead.
LLVM also says the tutorial focuses on compiler techniques and LLVM, not software-engineering best practices. Its documentation advises matching tutorial material to the LLVM release being used, because the APIs and examples are version-sensitive. These caveats matter if you later select LLVM as a backend; they do not affect a standalone tree-walk interpreter.
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- Do not add functions, types, modules, optimization, or native compilation until the small language’s existing behavior is clear.
- Keep syntax errors, runtime errors, and implementation failures distinguishable.
- Define the language’s behavior for each supported construct rather than relying on accidental C behavior.
- Do not set a completion-time or code-size target based on someone else’s project; the scope and experience level vary.
For broader compiler-design study, Douglas Thain’s Introduction to Compilers and Language Design is a relevant optional book-length resource. It discusses compiler construction and choices of source and target language or representation.
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