Vix Reply

Interactive REPL engine for Vix.

Vix Reply powers the vix and vix repl interactive experience. It provides a fast shell for testing expressions, variables, JSON values, runtime helpers, and real C++ snippets powered by the Vix run pipeline.

Start the REPL

Running vix without a command starts the REPL by default:

vix

You can also start it explicitly:

vix repl

To pass arguments into the REPL session, use --:

vix repl -- --port 8080 --mode dev

Inside the REPL, those arguments are available with:

Vix.args()

Example startup

Vix Reply v2.5.6  REPL
gcc 13.3  linux
exit: Ctrl+D | clear: Ctrl+L | help

>>>

What the REPL is for

Use Vix Reply when you want to:

• test small expressions
• evaluate math quickly
• create and inspect variables
• validate JSON values
• inspect environment values
• call simple Vix runtime helpers
• run real C++ snippets from an interactive session
• prototype small ideas before moving them into a C++ file

Vix Reply is inspired by interactive shells from tools like Python, Node.js, and Deno, but adapted to the Vix.cpp workflow.

Basic commands

help
version
pwd
cd <dir>
clear
history
history clear
exit

You can also clear the screen with:

Ctrl+L

To exit, use:

exit

or:

Ctrl+D

Expressions

You can type expressions directly:

>>> 1 + 2
3

>>> 10 * (3 + 4)
70

>>> 100 / 5
20

You can also use calc explicitly:

>>> calc 10 * (2 + 3)
50

Variables

Assign a value:

>>> x = 42
42

Read it back:

>>> x
42

Use it in another expression:

>>> x + 1
43

>>> x * 10
420

String values are supported:

>>> name = "Gaspard"
name = "Gaspard"

>>> name
Gaspard

Other simple values are supported too:

>>> age = 25
age = 25

>>> price = 19.99
price = 19.99

>>> active = true
active = true

JSON values

Vix Reply supports strict JSON values.

Objects

>>> user = {"name":"Gaspard","age":25}
user = {"age":25,"name":"Gaspard"}

>>> user
{"age":25,"name":"Gaspard"}

Arrays

>>> nums = [10,20,30]
nums = [10,20,30]

>>> nums
[10,20,30]

Nested JSON

>>> profile = {"name":"Gaspard","meta":{"country":"UG","verified":true},"tags":["cpp","vix","reply"]}
profile = {"meta":{"country":"UG","verified":true},"name":"Gaspard","tags":["cpp","vix","reply"]}

Access object properties:

>>> user.name
Gaspard

>>> user["name"]
Gaspard

Access arrays:

>>> nums[0]
10

>>> profile.tags[1]
vix

JSON must be valid.

Wrong:

{"name","Gaspard"}

Correct:

{ "name": "Gaspard" }

Printing output

Use print():

>>> print("Hello")
Hello

Use println():

>>> println("Hello world")
Hello world

You can mix strings and values:

>>> x = 3
x = 3

>>> println("x =", x)
x = 3

>>> println("x + 1 =", x + 1)
x + 1 = 4

Supported examples:

print("hello")
println("hello", "world")
println(42)
println(-42)
println(3.14)
println(true)
println(null)

Built-in helpers

Vix Reply exposes simple helpers directly and through the built-in Vix object.

Current working directory

>>> cwd()
/home/user/project

or:

>>> Vix.cwd()
/home/user/project

Change directory

>>> Vix.cd("..")

Process id

>>> pid()
12345

or:

>>> Vix.pid()
12345

Environment variables

>>> Vix.env("HOME")
/home/user

>>> Vix.env("PATH")
/usr/local/bin:/usr/bin:/bin

REPL arguments

>>> Vix.args()
["--port","8080","--mode","dev"]

Create directories

>>> Vix.mkdir("tmp")

Create nested directories:

>>> Vix.mkdir("tmp/logs", true)

Exit with a code

>>> Vix.exit(0)

Value helpers

Length

>>> name = "Gaspard"
name = "Gaspard"

>>> len(name)
7

>>> nums = [10,20,30]
nums = [10,20,30]

>>> len(nums)
3

>>> user = {"name":"Gaspard","age":25}
user = {"age":25,"name":"Gaspard"}

>>> len(user)
2

Convert values

>>> int("42")
42

>>> int(42.0)
42

>>> float("3.14")
3.140000

>>> float(10)
10.000000

>>> str(25)
25

Inspect types

>>> type(user)
object

>>> type(nums)
array

>>> type(user.name)
string

>>> type(nums[0])
int

C++ snippet mode

Vix Reply can run real C++ snippets through the normal Vix run pipeline.

Enter C++ mode:

>>> :cpp
C++ mode. Type :run to execute or :cancel to exit.
cpp>

Example:

>>> :cpp
C++ mode. Type :run to execute or :cancel to exit.
cpp> #include <vector>
...   #include <vix/print.hpp>
...   int main() {
...     vix::print("Hello, world", 2, true, std::vector<int>{1,2,3});
...   }
Hello, world 2 true [1, 2, 3]

Run a Vix HTTP server from the REPL

You can also start a small Vix HTTP server from C++ snippet mode:

>>> :cpp
C++ mode. Type :run to execute or :cancel to exit.
cpp> #include <vix.hpp>
...   using namespace vix;
...
...   int main() {
...     App app;
...
...     app.get("/", [](Request&, Response& res) {
...       res.send("Hello, world");
...     });
...
...     app.run(8080);
...   }

Then test it from another terminal:

curl http://localhost:8080

Expected output:

Hello, world

C++ mode commands

:cpp       Enter C++ snippet mode
:run       Run the current C++ snippet
:cancel    Cancel C++ snippet mode

Important note about C++ mode

Vix Reply is not a full C++ interpreter.

C++ snippet mode writes the snippet to a temporary .cpp file and runs it through vix run. This means the code is validated by the real C++ compiler and uses the normal Vix build, diagnostics, and runtime behavior.

The snippet must be valid C++ code.

For example, callbacks must use valid C++ lambda syntax:

app.get("/", [](Request&, Response& res) {
  res.send("Hello, world");
});

Common mistakes

Invalid JSON syntax

JSON objects require key: value pairs.

Wrong:

{"name","Gaspard"}

Correct:

{ "name": "Gaspard" }

Forgetting quotes around strings

Wrong:

>>> name = Gaspard

Correct:

>>> name = "Gaspard"

Passing REPL arguments without --

Wrong:

vix repl --port 8080

Correct:

vix repl -- --port 8080

Everything after -- is passed to the REPL session and becomes available through Vix.args().

Forgetting to close a C++ function call

Wrong:

app.get("/", [](Request&, Response& res) {
  res.send("Hello, world");
}

Correct:

app.get("/", [](Request&, Response& res) {
  res.send("Hello, world");
});

Common workflow

Start the REPL:

vix

Try simple values:

>>> x = 10
10

>>> x * 2
20

Inspect the environment:

>>> Vix.cwd()
/home/user/project

>>> Vix.env("HOME")
/home/user

Check JSON:

>>> user = {"name":"Ada","role":"developer"}
user = {"name":"Ada","role":"developer"}

>>> user.role
developer

Run C++:

>>> :cpp
cpp> #include <vix/print.hpp>
...   int main() {
...     vix::print("Hello from C++");
...   }
Hello from C++

Exit:

>>> exit

Best practices

• Use the REPL as a scratchpad.
• Use it to validate small expressions.
• Use it to inspect environment values.
• Use it to test JSON values before using them in config files or APIs.
• Use C++ snippet mode to quickly test real C++ code through the Vix pipeline.
• Move larger code into a real .cpp file or project once the idea is validated.

Module layout

include/vix/reply/
  api/
  console/
  core/

src/
  api/
  console/
  core/

Public entry point

The main entry point is:

#include <vix/reply/core/ReplFlow.hpp>

int repl_flow_run(const std::vector<std::string>& replArgs);

The Vix CLI uses this entry point to implement:

vix repl

Next step

Continue with project creation.

Open the vix new guide