Readline: Terminal Interaction

The "readline" collection (not to be confused with MzScheme’s read-line function) provides glue for using GNU’s Readline library with the MzScheme read-eval-print-loop.

1 Normal Use of Readline

(require readline)

(require readline/rep-start)

The readline library installs a Readline-based input port, and hooks the prompt-and-read part of MzScheme’s read-eval-print-loop to interact with it

You can start MzScheme with

  mzscheme -il readline

or evaluate

  (require readline)

in the MzScheme read-eval-print-loop to load Readline manually. You can also put (require readline) in your "~/.mzschemerc", so that MzScheme automatically loads Readline support in interactive mode.

If you want to enable Readline support only sometimes – such as only when you use an xterm, and not when you use an Emacs shell – then you can use dynamic-require, as in the following example:

(when (regexp-match? #rx"xterm"

(getenv "TERM"))

(dynamic-require 'readline #f))

The readline library automatically checks whether the current input port is a terminal, as determined by terminal-port?, and it installs Readline only to replace terminal ports. The readline/rep-start module installs Readline without a terminal check.

By default, Readline’s completion is set to use the visible bindings in the current namespace. This is far from ideal, but it’s better than Readline’s default filename completion which is rarely useful. In addition, the Readline history is stored across invocations in MzScheme’s preferences file, assuming that MzScheme exits normally.

2 Interacting with the Readline-Enabled Input Port

The reading facility that the new input port provides can be customized with the following parameters.

(foo bar) (+ 1

2)

you will see a single prompt in the beginning.

The problem is that the first expression can be (read-line), which normally consumes the rest of the text on the same line. The default value of this parameter is therefore #t, making it mimic plain I/O interactions.

(parameterize ([readline-prompt some-byte-string])

...code-that-reads...)

This expression makes the first call to Readline use the prompt, and subsequent calls will use an all-spaces prompt of the same length (for example, when you’re reading an S-expression). The normal value of readline-prompt is #f for an empty prompt (and spaces after the prompt is used, which is why you should use parameterize to restore it to #f).

A proper solution would be to install a custom output port, too, which keeps track of text that is displayed without a trailing newline. As a cheaper solution, if line-counting is enabled for the terminal’s output-port, then a newline is printed before reading if the column is not 0. (The readline library enables line-counting for the output port.)

Warning: The Readline library uses the output port directly. You should not use it when current-input-port has been modified, or when it was not a terminal port when MzScheme was started (eg, when reading input from a pipe). Expect some problems if you ignore this warning (not too bad, mostly problems with detecting an EOF).

3 Direct Bindings for Readline Hackers

(require readline/readline)

4 License Issues

GNU’s Readline library is covered by the GPL, and that applies to code that links with it. PLT Scheme is LGPL, so this code is not used by default; you should explicitly enable it if you want to. Also, be aware that if you write code that uses this library, it will make your code link to the Readline library when invoked, with the usual GPL implications.