Create language elements in R at C level

lang.R

## Create language elements in R at C level

## Create class with Rf_lang
inline::cfunction(c(x = "numeric"), '
  SEXP class_call = PROTECT(Rf_lang2(R_ClassSymbol, x));
  UNPROTECT(1);
  return class_call;
') -> class_expr

(e <- class_expr(1L))
eval(e)

## Data frame creation and eval
## Source: https://r.789695.n4.nabble.com/question-regarding-lang2-command-in-C-tp4688070p4688072.html
inline::cfunction(c(age = "numeric", gender = "character"), '
  SEXP call = PROTECT(Rf_lang4(Rf_install("data.frame"), age, gender, Rf_ScalarLogical(FALSE)));
  SET_TAG(CDR(call), Rf_install("age"));
  SET_TAG(CDDR(call), Rf_install("gender"));
  SET_TAG(CDR(CDDR(call)), Rf_install("stringsAsFactors"));
  SEXP evald_df = PROTECT(Rf_eval(call, R_GlobalEnv));
  UNPROTECT(2);
  return evald_df;
') -> make_df

(df <- make_df(c(21, 22), c("male", "female")))
str(df)