Expression Evaluation Helpers

Safe wrappers for building and evaluating R function calls from Rust.

🔗Types

🔗Quick Example

use miniextendr_api::expression::{RCall, REnv};
use miniextendr_api::ffi::Rf_mkString;

unsafe {
    // Call paste0("hello", " world") in base
    let result = RCall::new("paste0")
        .arg(Rf_mkString(c"hello".as_ptr()))
        .arg(Rf_mkString(c" world".as_ptr()))
        .eval(REnv::base().as_sexp())?;
}

🔗RSymbol

Wraps R’s Rf_install() for interned symbols. Symbols are never garbage collected, so RSymbol needs no GC protection.

use miniextendr_api::expression::RSymbol;

// From a Rust string (allocates a CString)
let sym = unsafe { RSymbol::new("my_var") };

// From a C string literal (zero allocation)
let sym = unsafe { RSymbol::from_cstr(c"my_var") };

// Use as SEXP
let sexp = sym.as_sexp();

🔗RCall

Builds R function calls with positional and named arguments.

use miniextendr_api::expression::RCall;

unsafe {
    // Positional arguments
    let result = RCall::new("sum")
        .arg(my_vector_sexp)
        .eval(env)?;

    // Named arguments
    let result = RCall::new("paste")
        .arg(x_sexp)
        .arg(y_sexp)
        .named_arg("sep", sep_sexp)
        .eval(env)?;
}

🔗Error Handling

eval() uses R_tryEvalSilent and returns Result<SEXP, String>. On failure, the error message comes from R’s geterrmessage().

match RCall::new("stop").arg(msg_sexp).eval(env) {
    Ok(result) => { /* success */ },
    Err(msg) => { /* msg contains R's error message */ },
}

🔗GC Protection

RCall protects all intermediate SEXPs (the call object and argument list) during construction. The returned SEXP is unprotected – caller must protect it if it will survive across R API calls.

🔗REnv

Provides handles to R’s well-known environments:

use miniextendr_api::expression::REnv;

let global = REnv::global();  // R_GlobalEnv
let base = REnv::base();      // R_BaseEnv
let empty = REnv::empty();    // R_EmptyEnv

// Use as SEXP
let sexp = base.as_sexp();

🔗Safety Requirements

All functions in this module require:

• Being called from the R main thread (they use R API calls)
• unsafe blocks (they call into C)

In #[miniextendr] functions, use these inside unsafe(main_thread) blocks or within ALTREP callbacks (which already run on the main thread).

🔗Use Cases

• S4 slot access: The s4_helpers module uses RCall internally
• Calling R functions from ALTREP callbacks: When elt() needs to call R
• Dynamic dispatch: Building R function calls based on runtime data
• Package interop: Calling functions from other R packages

🔗See Also

• CLASS_SYSTEMS.md – S4 helpers built on RCall
• THREADS.md – Main thread requirements
• GC_PROTECT.md – Protecting returned SEXPs