Your First App

This tutorial builds the smallest useful Repl app — a hello command that runs as both a CLI tool and an interactive REPL.

Create a console project

dotnet new console -n HelloRepl
cd HelloRepl
dotnet add package Repl

Replace Program.cs

using Repl;

var app = ReplApp.Create().UseDefaultInteractive();
app.Map("hello", () => "world");
return app.Run(args);

Run in CLI mode
Terminal window
```
dotnet run -- hello
# world
```
Run in REPL mode
Terminal window
```
dotnet run
# > hello
# world
# > exit
```

Get help for free

dotnet run -- --help
# Commands:
#   hello

What just happened

ReplApp.Create() wires up the DI container, routing, and output pipeline.
UseDefaultInteractive() activates the interactive REPL when no arguments are given.
app.Map("hello", () => "world") registers a route. The return value is rendered automatically.
app.Run(args) dispatches: CLI if args is non-empty, REPL otherwise.

No argument-parsing library. No interactive-loop wiring. No output formatting.

Routes accept typed parameters directly:

app.Map("greet {name}", (string name) => $"Hello, {name}!");
app.Map("add {a:int} {b:int}", (int a, int b) => a + b);

$ myapp greet Alice
Hello, Alice!

$ myapp add 3 4
7

Route constraints like :int, :email, :guid are validated at parse time — invalid input is rejected before your handler runs.

Return any object and Repl renders it:

record Contact(int Id, string Name, string Email);

app.Map("contact {id:int}", (int id) => new Contact(id, "Alice", "alice@example.com"));

$ myapp contact 1
Id  Name   Email
1   Alice  alice@example.com

$ myapp contact 1 --json
{ "id": 1, "name": "Alice", "email": "alice@example.com" }