vibed/edu/.nbd/tickets/1eb794.md

+++
title = "§13 Generating C: Atoms and Expressions"
priority = 5
status = "todo"
ticket_type = "task"
dependencies = []
+++

## §13 Generating C: Atoms and Expressions — Stub to fill

File: `edu/src/lisp-compiler.md`, section `### 13. Generating C: Atoms and Expressions`

Replace the stub line with full content. Target 700–900 words. Implement the expression code generator — the recursive function that turns any `Expr` into a C expression string.

## Learning objectives

- Implement `gen_expr` as a recursive function over `Expr`
- Know how each atom type maps to a C literal
- Understand how binary operator calls map to C infix expressions
- Handle `display`, `newline`, `error` as special cases in call generation
- Understand why all output is C *expressions* (not statements) at this level

## Content to write

### Expressions, not statements

In C, everything that produces a value is an expression. At this stage, the code generator works entirely with expressions — `gen_expr` always returns a C expression string that can appear on the right-hand side of an assignment or as a function argument. Statement generation (for sequencing and side effects) comes in §15.

### `gen_expr` — the core function

```rust
/// Generate a C expression from a MiniLisp `Expr`.
///
/// Returns a `String` containing valid C code that evaluates to the
/// same value as the original expression.
pub fn gen_expr(expr: &Expr) -> String {
    match expr {
        Expr::Int(n)    => n.to_string(),
        Expr::Bool(b)   => if *b { "ML_TRUE".to_string() } else { "ML_FALSE".to_string() },
        Expr::Str(s)    => format!("\"{}\"", s.escape_default()),
        Expr::Symbol(name) => mangle(name),
        Expr::If { cond, then, else_ } =>
            format!("({} ? {} : {})", gen_expr(cond), gen_expr(then), gen_expr(else_)),
        Expr::Call { func, args } => gen_call(func, args),
        // These should not appear at expression level — handled as statements in §15
        Expr::Begin(_) | Expr::Define { .. } | Expr::Lambda { .. } | Expr::Let { .. } =>
            panic!("gen_expr called on a statement-level form"),
    }
}
```

Walk through each arm:

**`Int(n)`** → decimal string. `42` → `"42"`, `-7` → `"-7"`.

**`Bool(b)`** → `"ML_TRUE"` or `"ML_FALSE"` (the `#define`s from the preamble).

**`Str(s)`** → a C string literal. Use Rust's `escape_default()` to re-escape the string, then wrap in double quotes. This safely handles embedded newlines and quotes.

**`Symbol(name)`** → `mangle(name)`. A symbol in expression position is a variable reference; mangling produces the correct C identifier.

**`If { cond, then, else_ }`** → C ternary: `(cond ? then : else)`. Parenthesised to avoid operator precedence issues.

**`Call { func, args }`** → delegate to `gen_call`.

### `gen_call` — operator and function calls

```rust
fn gen_call(func: &Expr, args: &[Expr]) -> String {
    // Built-in binary operators
    if let Expr::Symbol(op) = func {
        match op.as_str() {
            "+" | "-" | "*" | "/" => {
                let a = gen_expr(&args[0]);
                let b = gen_expr(&args[1]);
                return format!("({} {} {})", a, op, b);
            }
            "=" => return format!("({} == {})", gen_expr(&args[0]), gen_expr(&args[1])),
            "<" | ">" | "<=" | ">=" => {
                return format!("({} {} {})", gen_expr(&args[0]), op, gen_expr(&args[1]));
            }
            "not" => return format!("(!{})", gen_expr(&args[0])),
            // display / newline / error are statement-level; handled in gen_stmt
            "display" | "newline" | "error" => {
                // When called in expression position (inside an if branch, etc.),
                // emit as a comma expression: (side_effect, 0)
                return format!("({}, 0)", gen_display_stmt(&args[0]));
            }
            _ => {}
        }
    }
    // General function call
    let func_c = gen_expr(func);
    let args_c: Vec<String> = args.iter().map(gen_expr).collect();
    format!("{}({})", func_c, args_c.join(", "))
}
```

Explain the "comma expression" trick for `display` in expression position: `(printf(...), 0)` is valid C — the comma operator evaluates both sides and returns the right-hand value (0 here, which acts as a placeholder integer).

Note that the arity guarantees from §11 mean we can safely index `args[0]` and `args[1]` without bounds checking.

### String escaping

Show the `escape_for_c` helper that the string code path uses:

```rust
fn escape_for_c(s: &str) -> String {
    s.chars().flat_map(|c| match c {
        '"'  => vec!['\\', '"'],
        '\\' => vec!['\\', '\\'],
        '\n' => vec!['\\', 'n'],
        '\t' => vec!['\\', 't'],
        c    => vec![c],
    }).collect()
}
```

Use this instead of `escape_default()` which uses Rust escape syntax (`\u{...}`) that is not valid C.

### Tests

```rust
#[test]
fn test_gen_int() {
    assert_eq!(gen_expr(&Expr::Int(42)), "42");
    assert_eq!(gen_expr(&Expr::Int(-7)), "-7");
}

#[test]
fn test_gen_add() {
    let expr = Expr::Call {
        func: Box::new(Expr::Symbol("+".into())),
        args: vec![Expr::Int(1), Expr::Int(2)],
    };
    assert_eq!(gen_expr(&expr), "(1 + 2)");
}

#[test]
fn test_gen_if() {
    let expr = Expr::If {
        cond:  Box::new(Expr::Bool(true)),
        then:  Box::new(Expr::Int(1)),
        else_: Box::new(Expr::Int(0)),
    };
    assert_eq!(gen_expr(&expr), "(ML_TRUE ? 1 : 0)");
}
```

## Style notes

- Emphasise "C expressions only" at the top — this is the key architectural decision for this section
- Walk through each operator conversion explicitly; readers need to see the `=` → `==` translation noted
- The comma-expression trick for `display` in expression position is an interesting C technique — explain it clearly
- Note that the `panic!` for statement-level forms is a programming error guard, not a user-facing error