Interactive REPL

Start an interactive session with expressif (no arguments) in a terminal, or with expressif repl. Double-clicking expressif.exe on Windows also starts an interactive session. A no-argument interactive launch displays a short welcome with help and exit instructions and stays open until you exit.

To run commands from PowerShell, open a terminal in the executable’s folder:

.\expressif.exe
.\expressif.exe --help
.\expressif.exe evaluate '2 | add(1)'

The first command starts the REPL; exit with Ctrl+C or Ctrl+D on an empty line before running the next command. Explicit commands print their output and exit, leaving PowerShell open so you can read the result.

Automatic REPL startup requires both standard input and standard output to be connected to an interactive terminal. With either redirected, or no terminal available, expressif without arguments prints usage and exits without waiting for input. Use an explicit expressif repl invocation when intentionally feeding a session from redirected input.

Evaluate a standalone expression, then start a line with | to apply another pipeline to the current result.

Use expressif repl --output-style pretty --indent 4 to format every result with four-space indentation. --style-output is an alias for --output-style; --pretty and --compact are shortcuts for the corresponding styles. Choose only one style selector. The default is compact output.

Use --output json to serialize evaluation results as JSON throughout the session. --raw explicitly selects the default raw format. --output and --raw are mutually exclusive. Format selection is independent of the style and indentation options.

--indent accepts a space count from 0 to 8, or tab, and requires pretty output. Pretty output defaults to two-space indentation. These settings apply throughout the session, including standalone expressions, subsequent pipelines, and values restored by undo. Configured value types remain inline in pretty output when their complete compact form fits within the preferred line width. The defaults select tuples and 80 characters. The same policy applies to raw and JSON sessions; use the shared or repl-specific inline-types and preferred-line-width configuration settings to change it.

> 2
2
> | add(1) | multiply(4)
12
> :undo
2
> | multiply(5)
10

Use Up and Down to browse expressions and commands submitted in the current interactive session. Recalled input can be edited and runs only when you press Enter. Down past the newest entry restores your unfinished input and cursor. Blank submissions and consecutive identical entries are omitted from history. History is kept only for the current session, including expressions that failed.

:undo restores and displays the result before the last successful submission. One undo removes the entire submitted line, even when it contains several operators. The next pipeline uses the restored value. Saved results are restored directly, without evaluating the previous expressions again. Failed submissions do not add undo steps, and #null is a valid saved result.

Repeated undo walks back through successful submissions, including standalone expressions. Undoing the first submission reports There is no current input.; evaluate a standalone expression before using another pipeline. When history is empty, the command reports Nothing to undo.

In an interactive terminal, Ctrl+Z on an empty input line runs :undo immediately, without Enter. While the line contains text, Ctrl+Z undoes the last text edit instead. Use Left/Right, Home/End, Backspace, and Delete to edit the line. Ctrl+C ends the session; Ctrl+D on an empty line also exits.

With redirected input or output, use the literal :undo command on its own line; keyboard shortcuts are available only in an interactive terminal.

Output preferences can also be saved in configuration, with repl.output-style and repl.indent overriding shared defaults. Explicit CLI options, including --style-output, override configured values.