The REPL and script runner (gsi)

gsi is G#'s interactive REPL and single-file script runner. It ships as the Gsharp.Repl .NET global tool and is the fastest way to try the language without creating a project. It is a full-screen, keyboard-first TUI built on Spectre.Console that reuses the same Core compilation engine and LanguageServer analysis as the compiler, so the prompt behaves like a small editor (live syntax colors, completion, hover, and diagnostics) rather than a plain line reader.

Install

gsi requires a .NET 10 runtime.

dotnet tool install --global Gsharp.Repl

Update or remove it with the usual global-tool commands:

dotnet tool update --global Gsharp.Repl
dotnet tool uninstall --global Gsharp.Repl

Command-line usage

Usage: gsi [file.gs] [/r:<assembly> ...] [--engine <name>] [--help] [--version]
  file.gs                Run the given G# script and exit.
  /r:<file>              Reference an assembly (repeatable).
  /reference:<file>      Alias for /r: (also -r: and --reference:).
  --engine <name>        Interactive engine: 'emit' (the default and only
                         engine; also via GSI_ENGINE). The legacy 'evaluator'
                         engine has been removed.
  --help, -h             Show this help and exit.
  --version              Show the gsi version and exit.
  (no args)              Start the interactive REPL.

Interactive REPL

Run gsi with no arguments in an interactive terminal to open the REPL:

gsi

Each submission becomes a transcript "cell" (your input plus its result or diagnostics). Because the session state carries forward, you can build up definitions incrementally:

» let xs = [1, 2, 3]
» import System.Linq
» xs.Select((x int32) -> x * 2).Sum()
= 12

The REPL is organised into keyboard-selectable tabs — REPL, History, Variables, Diagnostics, Help, and Settings — and includes a command palette (open it with :) for session commands:

Command	Effect
reset	Clear all accumulated session state and start fresh.
clear	Clear the current editor input.
theme [name]	Switch the color theme, or cycle themes when no name is given.
load <file.gs>	Evaluate a .gs file into the current session.
exit / quit	Leave the REPL.

gsi needs an interactive terminal; if its input is redirected it exits with a message instead of starting the UI.

Run a script and exit

Pass a .gs file to emit and run it. This is handy for quick experiments and shell one-offs — no .gsproj required:

gsi hello.gs

package Hello

import System

func greet(name string) string {
    return "Hello, $name!"
}

Console.WriteLine(greet("world"))

Diagnostics are written to standard error, and the process exits with a non-zero status if evaluation fails, so gsi file.gs composes cleanly in scripts and CI checks.

Use /r:<path> or /reference:<path> to load an external runtime assembly before evaluation:

gsi /r:/path/to/MyLibrary.dll hello.gs

The installed Gsharp.Repl tool bundles and loads Gsharp.Extensions automatically, so scripts can use its optional and sequence helpers without adding an explicit reference.

How it relates to gsc

Every driver uses the emitter, including gsi file.gs and the interactive REPL. --engine emit explicitly selects the only engine; any other command-line or GSI_ENGINE value is rejected. Both direct gsc modes use the emitter, with /out: only controlling whether the assembly is saved. See the gsc reference and introduction for details.