Read-Eval-Print Loop (REPL)

REPL (as its name implies) loops through every line of the input and compiles it. We use rustyline crate to create our REPL. For each line of input, we can optionally choose to

directly interpret the AST
JIT the AST
compile to our bytecode VM and interpret it

fn main() -> Result<()> {
    let mut rl = DefaultEditor::new()?;
    println!("Calculator prompt. Expressions are line evaluated.");
    loop {
        let readline = rl.readline(">> ");
        match readline {
            Ok(line) => {
                cfg_if! {
                    if #[cfg(any(feature = "jit", feature = "interpreter"))] {
                        match Engine::from_source(&line) {
                            Ok(result) => println!("{}", result),
                            Err(e) => eprintln!("{}", e),
                        };
                    }
                    else if #[cfg(feature = "vm")] {
                        let byte_code = Engine::from_source(&line);
                        println!("byte code: {:?}", byte_code);
                        let mut vm = VM::new(byte_code);
                        vm.run();
                        println!("{}", vm.pop_last());
                    }
                }
            }
            Err(ReadlineError::Interrupted) => {
                println!("CTRL-C");
                break;
            }
            Err(ReadlineError::Eof) => {
                println!("CTRL-D");
                break;
            }
            Err(err) => {
                println!("Error: {:?}", err);
                break;
            }
        }
    }
    Ok(())

Filename: calculator/src/bin/repl.rs

Now, we can use run the REPL and choose different compilation path

cargo run --bin repl --features jit
// OR
cargo run --bin repl --features interpreter
// OR
cargo run --bin repl --features vm

In any of them, you should see the prompt like

Calculator prompt. Expressions are line evaluated.
>>>

waiting for your inputs. Here are some sample outputs of different compilation paths in debug mode.

with --features jit

Calculator prompt. Expressions are line evaluated.
>> 1 + 2
Compiling the source: 1 + 2
[BinaryExpr { op: Plus, lhs: Int(1), rhs: Int(2) }]
Generated LLVM IR: define i32 @jit() {
entry:
  ret i32 3
}

3
>> (1 + 2) - (8 - 10)
Compiling the source: (1 + 2) - (8 - 10)
[BinaryExpr { op: Minus, lhs: BinaryExpr { op: Plus, lhs: Int(1), rhs: Int(2) }, rhs: BinaryExpr { op: Minus, lhs: Int(8), rhs: Int(10) } }]
Generated LLVM IR: define i32 @jit() {
entry:
  ret i32 5
}

5
>>
CTRL-C

with --features vm

Calculator prompt. Expressions are line evaluated.
>> 1 + 2
Compiling the source: 1 + 2
[BinaryExpr { op: Plus, lhs: Int(1), rhs: Int(2) }]
compiling node BinaryExpr { op: Plus, lhs: Int(1), rhs: Int(2) }
added instructions [1, 0, 0] from opcode OpConstant(0)
added instructions [1, 0, 0, 1, 0, 1] from opcode OpConstant(1)
added instructions [1, 0, 0, 1, 0, 1, 3] from opcode OpAdd
added instructions [1, 0, 0, 1, 0, 1, 3, 2] from opcode OpPop
byte code: Bytecode { instructions: [1, 0, 0, 1, 0, 1, 3, 2], constants: [Int(1), Int(2)] }
3
>> (1 + 2) - (8 - 10)
Compiling the source: (1 + 2) - (8 - 10)
[BinaryExpr { op: Minus, lhs: BinaryExpr { op: Plus, lhs: Int(1), rhs: Int(2) }, rhs: BinaryExpr { op: Minus, lhs: Int(8), rhs: Int(10) } }]
compiling node BinaryExpr { op: Minus, lhs: BinaryExpr { op: Plus, lhs: Int(1), rhs: Int(2) }, rhs: BinaryExpr { op: Minus, lhs: Int(8), rhs: Int(10) } }
added instructions [1, 0, 0] from opcode OpConstant(0)
added instructions [1, 0, 0, 1, 0, 1] from opcode OpConstant(1)
added instructions [1, 0, 0, 1, 0, 1, 3] from opcode OpAdd
added instructions [1, 0, 0, 1, 0, 1, 3, 1, 0, 2] from opcode OpConstant(2)
added instructions [1, 0, 0, 1, 0, 1, 3, 1, 0, 2, 1, 0, 3] from opcode OpConstant(3)
added instructions [1, 0, 0, 1, 0, 1, 3, 1, 0, 2, 1, 0, 3, 4] from opcode OpSub
added instructions [1, 0, 0, 1, 0, 1, 3, 1, 0, 2, 1, 0, 3, 4, 4] from opcode OpSub
added instructions [1, 0, 0, 1, 0, 1, 3, 1, 0, 2, 1, 0, 3, 4, 4, 2] from opcode OpPop
byte code: Bytecode { instructions: [1, 0, 0, 1, 0, 1, 3, 1, 0, 2, 1, 0, 3, 4, 4, 2], constants: [Int(1), Int(2), Int(8), Int(10)] }
5
>>>
CTRL-C

Conclusion

This concludes our Calculator chapter. We took advantage of the simplicity of our Calc language to touch on a lot of topics.

Thanks for following along and reading up this far!

Stay tuned for the next chapter where we gradually work our way up to create a statically typed language named creatively as Firstlang :D