Using the REPL

The Keel REPL (Read-Eval-Print Loop) provides an interactive environment for experimenting with Keel code.

Starting the REPL

Start the REPL using the unified CLI:

keel repl

Or using the standalone binary:

keel-repl

You'll see a welcome message and prompt:

keel>

Basic Usage

Enter expressions at the prompt to evaluate them:

> 1 + 2
3

> "Hello, " ++ "World!"
"Hello, World!"

> let x = 42
> x * 2
84

Auto-Imported Standard Library

All standard library modules are available immediately in the REPL without explicit imports:

List — Functional list operations (map, filter, foldl, foldr, length, etc.)
String — String manipulation (length, concat, toUpper, toLower, split, etc.)
Math — Mathematical functions and constants (abs, sqrt, pow, sin, cos, pi, e, etc.)
DateTime — Date and time operations (now, parse, format, addDays, diffHours, etc.)
IO — Console and file I/O (println, readFile, writeFile, etc.)
Http — HTTP client (get, post, send, etc.)
Json — JSON parsing and serialization (parse, stringify, etc.)
DataFrame — Data manipulation with Polars backend
Result — Functional error handling (map, andThen, withDefault, etc.)
Maybe — Optional value handling (map, andThen, withDefault, etc.)

IO and HTTP work by default in the REPL — no environment variables needed.

REPL Commands

The REPL supports special commands, all prefixed with ::

Command	Aliases	Description
`:help [topic]`	`:h`, `:?`	Show help information
`:quit`	`:q`, `:exit`	Exit the REPL
`:clear`	`:cls`	Clear the screen
`:reset`		Reset all REPL state
`:type <expr>`	`:t`	Show the type of an expression
`:info <name>`	`:i`	Show information about a name
`:env [filter]`	`:e`	Show environment (vars/fns/types)
`:modules`	`:mods`	List available and imported modules
`:exports <module>`	`:exp`	Show what a module exports
`:delete <name>`	`:del`, `:undef`	Delete a name from scope
`:load <file>`	`:l`	Load and evaluate a file
`:reload`	`:r`	Reload the last loaded file
`:history [pattern]`	`:hist`	Show or search command history
`:timing [on\|off]`		Toggle execution timing display
`:types [on\|off]`		Toggle type display with results
`:ast <expr>`		Show the AST for an expression
`:bytecode <expr>`	`:bc`	Show bytecode for an expression
`:tutorial`	`:tut`	Start the interactive tutorial

Checking Types

Use :type (or :t for short) to inspect expression types:

> :type 42
Int

> :type "hello"
String

> :type |x: Int| x + 1
Int -> Int

Multi-line Input

The REPL automatically continues to the next line when:

Brackets are unclosed: (, [, {
Line ends with an operator: +, |>, >>, etc.

You can also press Alt+Enter or Shift+Enter to insert a newline, or use explicit blocks with :{ and :}:

> :{
| fn fibonacci : Int -> Int
| fn fibonacci n =
|     case n of
|         0 -> 0
|         1 -> 1
|         _ -> fibonacci (n - 1) + fibonacci (n - 2)
| :}

> fibonacci 10
55

Tab Completion

Press Tab to autocomplete:

Variable and function names
REPL commands
Module names
Module members (e.g., List.ma + Tab → List.map)
Keywords

Syntax Highlighting

The REPL provides syntax highlighting for keywords, types, strings, numbers, and operators as you type.

Keyboard Shortcuts

Shortcut	Action
`Tab`	Trigger completion menu
`Up/Down`	Navigate history
`Alt+Enter`	Insert newline (multi-line input)
`Shift+Enter`	Insert newline (multi-line input)
`Ctrl+C`	Cancel current input
`Ctrl+D`	Exit REPL

Loading Files

Load external Keel files with :load:

> :load examples/math.kl
Loaded: examples/math.kl

> square 5
25

History

The REPL maintains a persistent command history. Use the up and down arrow keys to navigate through previous entries, or use :history to search.

Online Playground

If you don't want to install Keel locally, try the online playground for a quick interactive experience.

Next Steps

Now that you're familiar with the REPL, learn about variables in detail.