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Build a tiny interpreter first: define a small grammar, turn source text into tokens, parse those tokens into an abstract syntax tree (AST), and evaluate the tree. That gives you a working language without the added complexity of generating machine code. Treat native code generation as a later, optional milestone—not a requirement for a first C project.
What your first language should do
Keep the first version small enough that you can describe its syntax clearly. A useful starting scope is numeric literals, arithmetic, parentheses, variable declarations, and either a print statement or expression statements. You do not need to settle on a platform or native executable format to make this language run: an interpreter can read and evaluate source directly.
Write down the grammar before implementing it. Decide how declarations and expressions look, which operators exist, and how precedence works. A precise, limited grammar makes it possible to build and test each part without accidentally taking on the complexity of a general-purpose language.
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How source code becomes a running program
A language implementation commonly separates reading source from deciding what it means. The front end turns characters into tokens, checks token sequences against a grammar, and builds a structured representation. A later stage either interprets that representation or translates it into another form.
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- Lexing: Read characters and group them into tokens such as numbers, identifiers, operators, and punctuation. Preserve source positions so errors can point to the relevant location.
- Parsing: Check that the tokens follow your grammar. For example, an expression parser should distinguish a valid grouped expression from a missing closing parenthesis.
- AST construction: Build an abstract syntax tree containing the meaningful structure of the program. An AST removes many surface details and gives later stages a more useful representation than raw text. LLVM describes it as a way for later compiler stages to interpret a program’s behavior (LLVM’s parser and AST documentation).
- Evaluation: Walk the AST and compute the result of each expression or execute each statement. Keep variables in a small environment or symbol table.
This order gives you clear boundaries: the lexer reports character-level problems, the parser reports grammar problems, and the evaluator reports runtime problems such as using a variable that has no value.
Build the lexer and parser in manageable steps
Start with tokens and source locations
Give each token a kind and the information it needs, such as a numeric value or identifier text. Store its starting position in the source. Even a simple line-and-column location makes errors substantially easier to diagnose than a generic “invalid input” message.
Parse expressions before statements
Begin with numeric literals and grouping, then add unary and binary operators. A small hand-written parser is a reasonable approach: recursive-descent functions can handle the grammar’s major forms, while a precedence routine handles binary expressions. LLVM’s Kaleidoscope example uses this combination (LLVM’s parser and AST documentation).
Operator precedence is a good early test of whether your parser represents meaning correctly. For example, 2 + 3 * 4 should produce a tree that evaluates multiplication before addition if that is the rule in your grammar. Parentheses should make the intended grouping explicit.
Add statements and an evaluator
Once expressions work, introduce declarations and a print or expression statement. Represent these as distinct AST node kinds, then implement evaluation for each kind. In C, decide how node memory is allocated and released; make ownership rules explicit so that parsing failures and completed programs do not leave unclear responsibility for allocated memory.
Why interpretation is the right first finish line
A tree-walk interpreter evaluates AST nodes directly. It lets you see whether your language’s rules work without first solving how to represent instructions for a target machine or toolchain. This is a practical first milestone, not a requirement imposed by LLVM.
Code generation is a separate next step: it translates the AST into another representation, such as an intermediate representation (IR), or into target-specific code. LLVM’s tutorial sequence places IR generation after lexer, parser, and AST work, and later demonstrates JIT compilation (LLVM’s Kaleidoscope tutorial series; LLVM’s IR generation documentation).
| Approach | What it does | What it makes you handle |
|---|---|---|
| Tree-walk interpreter | Evaluates AST nodes directly. | Language behavior, AST traversal, and runtime state such as variables. |
| Code generation | Translates the AST into IR or another target form. | Target and toolchain concerns in addition to the front end. |
After the interpreter is coherent, choose a next step based on what you want to learn: bytecode, generated C, LLVM IR, or a machine-code backend. None is necessary to call the interpreter a working language.
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Test behavior and errors as you add features
Keep small test programs for each feature, and include invalid inputs alongside valid ones. Tests should establish what the language accepts and how it responds when something goes wrong.
- Valid numeric, grouped, unary, and binary expressions.
- Precedence and associativity cases, including expressions where changing the grouping changes the result.
- Valid declarations and uses of variables.
- Malformed syntax, such as an unfinished expression or missing closing parenthesis.
- Runtime errors, such as referencing an undeclared variable or applying an operation to an unsupported value.
When a test fails, identify which stage owns the failure. A malformed token belongs to lexing; a token sequence that violates the grammar belongs to parsing; a syntactically valid program with an invalid operation belongs to evaluation. That separation keeps fixes focused.
Using LLVM material without turning the project into C++
LLVM’s Kaleidoscope tutorial is useful for understanding the staged design, but its implementation is in C++ and assumes familiarity with C++. It is a conceptual reference, not a C tutorial or a codebase to copy into a C-only project (LLVM’s tutorial introduction; LLVM’s first-language tutorial introduction).
The tutorial says its focus is compiler techniques and LLVM, rather than software-engineering best practices (LLVM’s tutorial introduction). You will need to make your own C design decisions for data structures, memory ownership, error handling, and testing. If you later use LLVM, match the tutorial material to the LLVM release you have installed because its APIs and examples are version-sensitive (LLVM’s tutorial documentation).
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