From d1f74f90cd3bca0fbaa2bedec276ab97c28e81a6 Mon Sep 17 00:00:00 2001
From: Cedric Hutchings <cedhut02@gmail.com>
Date: Sun, 26 Jan 2020 15:29:58 -0500
Subject: [PATCH] Make suggested improvements to mini_repl.rs

---
 examples/mini_repl.rs | 125 +++++++++++++++++++++++++++++++-----------
 1 file changed, 94 insertions(+), 31 deletions(-)

diff --git a/examples/mini_repl.rs b/examples/mini_repl.rs
index 7484a6c75..e199fbd4f 100644
--- a/examples/mini_repl.rs
+++ b/examples/mini_repl.rs
@@ -3,50 +3,113 @@
 ///! This much smaller REPL is still a useful example because it showcases inserting
 ///! values and functions into the Python runtime's scope, and showcases use
 ///! of the compilation mode "Single".
-///! Note that in particular this REPL does a horrible job of showing users their errors
-///! (instead it simply crashes).
-use rustpython_compiler::compile;
-use rustpython_vm::{pyobject::PyResult, PySettings, VirtualMachine};
+use rustpython_compiler as compiler;
+use rustpython_vm as vm;
+// these are needed for special pointer shenanigans to let us share a variable with Python and Rust
+use std::cell::Cell;
+use std::rc::Rc;
 // this needs to be in scope in order to insert things into scope.globals
-use rustpython_vm::pyobject::ItemProtocol;
+use vm::pyobject::ItemProtocol;
 
-fn main() -> PyResult<()> {
-    let mut on = true;
+// This has to be a macro because it uses the py_compile_bytecode macro,
+// which compiles python source to optimized bytecode at compile time, so that
+// the program you're embedding this into doesn't take longer to start up.
+macro_rules! add_python_function {
+    ( $scope:ident, $vm:ident, $src:literal $(,)? ) => {{
+        // this has to be in scope to turn a PyValue into a PyRef
+        // (a PyRef is a special reference that points to something in the VirtualMachine)
+        use vm::pyobject::PyValue;
+
+        // you can safely assume that only one module will be created when passing a source literal
+        // to py_compile_bytecode. However, it is also possible to pass directories, which may
+        // return more modules.
+        let (_, vm::bytecode::FrozenModule { code, .. }): (String, _) =
+            vm::py_compile_bytecode!(source = $src)
+                .into_iter()
+                .collect::<Vec<_>>()
+                .pop()
+                .expect("No modules found in the provided source!");
+
+        // takes the first constant in the file that's a function
+        let def = code
+            .get_constants()
+            .find_map(|c| match c {
+                vm::bytecode::Constant::Code { code } => Some(code),
+                _ => None,
+            })
+            .expect("No functions found in the provided module!");
+
+        // inserts the first function found in the module into the provided scope.
+        $scope.globals.set_item(
+            &def.obj_name,
+            $vm.context().new_pyfunction(
+                vm::obj::objcode::PyCode::new(*def.clone()).into_ref(&$vm),
+                $scope.clone(),
+                None,
+                None,
+            ),
+            &$vm,
+        )
+    }};
+}
+
+fn main() -> vm::pyobject::PyResult<()> {
+    // you can also use a raw pointer instead of Rc<Cell<_>>, but that requires usage of unsafe.
+    // both methods are ways of circumnavigating the fact that Python doesn't respect Rust's borrow
+    // checking rules.
+    let on: Rc<Cell<bool>> = Rc::new(Cell::new(true));
 
     let mut input = String::with_capacity(50);
     let stdin = std::io::stdin();
 
-    let vm = VirtualMachine::new(PySettings::default());
-    let scope = vm.new_scope_with_builtins();
+    let vm = vm::VirtualMachine::new(vm::PySettings::default());
+    let scope: vm::scope::Scope = vm.new_scope_with_builtins();
 
     // typing `quit()` is too long, let's make `on(False)` work instead.
-    scope
-        .globals
-        .set_item(
-            "on",
-            vm.context().new_rustfunc({
-                let on: *mut bool = &mut on;
-                move |b: bool, _: &VirtualMachine| unsafe { *on = b }
-            }),
-            &vm,
-        )
-        .unwrap();
+    scope.globals.set_item(
+        "on",
+        vm.context().new_function({
+            let on = Rc::clone(&on);
+            move |b: bool| on.set(b)
+        }),
+        &vm,
+    )?;
 
-    while on {
+    // let's include a fibonacci function, but let's be lazy and write it in Python
+    add_python_function!(
+        scope,
+        vm,
+        // a fun line to test this with is
+        // ''.join( l * fib(i) for i, l in enumerate('supercalifragilistic') )
+        r#"def fib(n): return n if n <= 1 else fib(n - 1) + fib(n - 2)"#
+    )?;
+
+    while on.get() {
         input.clear();
-        stdin.read_line(&mut input).unwrap();
-
-        let code_obj = vm
-            .compile(&input, compile::Mode::Single, "<embedded>".to_string())
-            .map_err(|err| vm.new_syntax_error(&err))?;
+        stdin
+            .read_line(&mut input)
+            .expect("Failed to read line of input");
 
         // this line also automatically prints the output
         // (note that this is only the case when compile::Mode::Single is passed to vm.compile)
-        let output = vm.run_code_obj(code_obj, scope.clone())?;
-
-        // store the last value in the "last" variable
-        if !vm.is_none(&output) {
-            scope.globals.set_item("last", output, &vm).unwrap();
+        match vm
+            .compile(
+                &input,
+                compiler::compile::Mode::Single,
+                "<embedded>".to_string(),
+            )
+            .map_err(|err| vm.new_syntax_error(&err))
+            .and_then(|code_obj| vm.run_code_obj(code_obj, scope.clone()))
+        {
+            Ok(output) => {
+                // store the last value in the "last" variable
+                if !vm.is_none(&output) {
+                    scope.globals.set_item("last", output, &vm)?;
+                }
+            }
+            Err(e) => {
+                vm::exceptions::print_exception(&vm, &e);
+            }
         }
     }