Fix C codegen for interpolated strings (f-strings)

[dev-dami]

Summary

Fixes string interpolation (f"...") not compiling to valid C code.

Problem

Previously, code like:

let name = "World";
println(f"Hello, {name}!");

Would generate invalid C:

(printf("%lld", ), printf("\n"));  // Empty argument!

Solution

Added proper code generation for InterpolatedString AST nodes:

1. New runtime helpers:

   • carv_int_to_string() - converts int to string
   • carv_float_to_string() - converts float to string
   • carv_bool_to_string() - converts bool to string
   • carv_concat() - concatenates two strings

2. New codegen functions:

   • generateInterpolatedString() - generates C code for f-strings
   • convertToString() - converts any expression to string based on type

3. Type inference: Added InterpolatedString case to inferExprType returning carv_string

Testing

# Compiled mode now works correctly
$ cat test.carv
let name = "World";
let num = 42;
println(f"Hello, {name}! The answer is {num}.");
println(f"2 + 2 = {2 + 2}");

$ ./build/carv build test.carv && ./test
Hello, World! The answer is 42.
2 + 2 = 4

Replaces #18 (which had merge conflicts due to being branched from wrong base).

Summary by CodeRabbit

Release Notes

• New Features
  • String interpolation is now supported, allowing variables and expressions to be embedded within string literals
  • New conversion helpers enable automatic transformation of numeric and boolean types to strings
  • Result constructor API expanded with string-value support and error code handling for enhanced error reporting

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

This pull request adds support for interpolated strings in the C code generator by implementing string interpolation handling, introducing new runtime string conversion helpers (int_to_string, float_to_string, bool_to_string, concat), and exposing additional result constructors (ok_str, err_str, err_code) in the generated C runtime.

Changes

Cohort / File(s)	Summary
String Interpolation Support pkg/codegen/cgen.go	Added generateInterpolatedString() and convertToString() methods to handle AST InterpolatedString nodes; extended generateExpression() dispatch and inferExprType() to recognize InterpolatedString as carv_string type.
Runtime Helper Functions pkg/codegen/cgen.go	Emitted four new C runtime string conversion functions: carv_int_to_string, carv_float_to_string, carv_bool_to_string, and carv_concat for string composition.
Result Constructor Expansion pkg/codegen/cgen.go	Exposed three new result constructors in generated C runtime: carv_ok_str, carv_err_str, and carv_err_code alongside existing carv_ok_int.

Sequence Diagram(s)

sequenceDiagram
    participant Parser as AST Parser
    participant Gen as CGenerator
    participant Conv as convertToString
    participant Runtime as Generated C Runtime
    
    Parser->>Gen: InterpolatedString node
    Gen->>Gen: generateExpression() dispatches
    Gen->>Gen: generateInterpolatedString() called
    Gen->>Conv: convertToString(expr) for each part
    Conv->>Runtime: carv_int_to_string / carv_float_to_string
    Conv->>Runtime: carv_bool_to_string (as needed)
    Conv->>Runtime: carv_concat for composition
    Runtime-->>Gen: Concatenated string result
    Gen-->>Parser: Generated C code

Loading

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• feat: add C code generator #3: Directly related as the base PR that introduced the CGenerator and runtime emission infrastructure that this PR extends with interpolated string support and new runtime functions.

Poem

🐰 A rabbit hops through strings so fine,
With int and float in each design,
We weave them all with carv\_concat's might,
Interpolated whispers, oh what delight!
From AST to C, a merry dance,
The code now strings together in romance! 🌟

✨ Finishing touches

• 📝 Generate docstrings

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