Interactive REPL

MLKit includes a read–eval–print loop (REPL) for exploring Standard ML, trying functions, and loading projects. Start it by running mlkit without a source-file argument:

mlkit

Enter declarations and expressions, ending each input with a semicolon:

val greeting = "Hello, MLKit!";
fun square x = x * x;
List.map square [1, 2, 3, 4];

The REPL reports values and types. Definitions remain available to later inputs, so the last expression uses square and produces [1, 4, 9, 16].

Useful commands

Commands also end with a semicolon.

Command	Action
`:help;`	Show the available commands.
`:load project.mlb;`	Load a project described by an ML Basis file.
`:flags;`	Describe compiler flags.
`:help flag;`	Show help for a particular flag.
`:reset;`	Reset the interactive session.
`:quit;`	Exit the REPL.

See ML Basis files for organizing a project and Download for installation instructions. The native arm64 backend also supports the REPL on Apple Silicon.

To try Standard ML directly in a browser, use the SMLtoJs Online SML IDE.