The REPL

If you’ve followed the instructions in Installation, your Emacs is now ready to start playing. Otherwise, i’ll wait for you: when you’re ready, just come back here and proceed to the following sections.


Starting the REPL

To start a Scheme REPL (meaning, a Scheme process offering you a Read-Eval-Print Loop), Geiser provides the generic interactive command run-geiser. If you invoke it (via, as is customary in Emacs, M-x run-geiser), you’ll be saluted by a prompt asking which one of the supported implementations you want to launch—yes, you can stop the asking, see below. Tabbing for completion will offer you, as of this writing, guile and racket. Just choose your poison, and a new REPL buffer will pop up (by default, the REPL will appear in a new window: if that annoys you, just set geiser-repl-use-other-window to nil and the current window will be used).

If all went according to plan, you’ll be facing an implementation-dependent banner, followed by an interactive prompt. Going according to plan includes having the executable of the Scheme you chose in your path. If that’s not the case, you can tell Emacs where it is, as described in a moment. Returning to our REPL, the first thing to notice is that the funny prompt is telling you your current module: its name is the part just after the @ sign (in Guile, that means guile-user, while Racket’s top namespace doesn’t have a name; cf. discussion in Switching context). Other than that, this is pretty much equivalent to having a command-line interpreter in a terminal, with a bunch of add-ons that we’ll be reviewing below. You can start typing sexps right there: Geiser will only dispatch them for evaluation when they’re complete, and will indent new lines properly until then. It will also keep track of your input, maintaining a history file that will be reloaded whenever you restart the REPL.

If you’re not happy with the faces Geiser is using for the REPL’s prompt and evaluated input, you can customise geiser-font-lock-repl-prompt and geiser-font-lock-repl-input to better looking faces.

Connecting to an external Scheme

There’s an alternative way of starting a Geiser REPL: you can connect to an external Scheme process, provided it’s running a REPL server at some known port. How to make that happen depends on the Scheme implementation.

If you use Guile, you just need to start your Guile process (possibly outside Emacs) passing to it the flag --listen. This flag accepts an optional port as argument (as in --listen=1969), if you don’t want to use the default.

In Racket, you have to use the REPL server that comes with Geiser. To that end, put Geiser’s Racket ‘scheme’ directory in Racket’s collection search path and invoke start-geiser (a procedure in the module geiser/server) somewhere in your program, passing it the desired port and, if desired, network interface name. This procedure will start the REPL server in a separate thread. For an example of how to do that, see the script ‘bin/geiser-racket.sh’ in the source distribution, or, if you’ve compiled Geiser, ‘bin/geiser-racket-noinst’ in the build directory, or, if you’ve installed Geiser, ‘geiser-racket’ in ‘<installation-prefix>/bin’. These scripts start a new interactive Racket that is also running a REPL server (they also load the errortrace library to provide better diagnostics, but that’s not strictly needed).

With your external Scheme process running and serving, come back to Emacs and execute M-x geiser-connect, M-x connect-to-guile or M-x connect-to-racket. You’ll be asked for a host and a port, and, voila, you’ll have a Geiser REPL that is served by the remote Scheme process in a dedicated thread, meaning that your external program can go on doing whatever it was doing while you tinker with it from Emacs. Note, however, that all Scheme threads share the heap, so that you’ll be able to interact with those other threads in the running Scheme from Emacs in a variety of ways. For starters, all your (re)definitions will be visible everywhere. That’s dangerous, but will come in handy when you need to debug your running web server.

The connection between Emacs and the Scheme process goes over TCP, so it can be as remote as you need, perhaps with the intervention of an SSH tunnel.


First aids

A quick way of seeing what else Geiser’s REPL can do for you, is to display the corresponding entry up there in your menu bar. No, i don’t normally use menus either; but they can come in handy until you’ve memorized Geiser’s commands, as a learning device. And yes, i usually run Emacs inside a terminal, but one can always use La Carte to access the menus in a convenient enough fashion.

Or just press C-h m and be done with that.

Among the commands at your disposal, we find the familiar input navigation keys, with a couple twists. By default, M-p and M-n are bound to matching items in your input history. That is, they’ll find the previous or next sexp that starts with the current input prefix (defined as the text between the end of the prompt and your current position, a.k.a. point, in the buffer). For going up and down the list unconditionally, just use C-c M-p and C-c M-n. In addition, navigation is sexp-based rather than line-based.

There are also a few commands to twiddle with the Scheme process. C-c C-q will gently ask it to quit, while C-u C-c C-q will mercilessly kill the process (but not before stowing your history in the file system). Unless you’re using a remote REPL, that is, in which case both commands will just sever the connection and leave the remote process alone. If worse comes to worst and the process is dead, C-c C-z will restart it. However, the same shortcut, issued when the REPL is alive, will bring you back to the buffer you came from, as explained in this section.

The remaining commands are meatier, and deserve sections of their own.


Switching context

In tune with Geiser’s modus operandi, evaluations in the REPL take place in the namespace of the current module. As noted above, the REPL’s prompt tells you the name of the current module. To switch to a different one, you can use the command switch-to-geiser-module, bound to C-c C-m. You’ll notice that Geiser simply uses a couple of meta-commands provided by the Scheme REPL (the stock ,m in Guile and the (geiser-defined) ,enter in Racket), and that it doesn’t even try to hide that fact. That means that you can freely use said native ways directly at the REPL, and Geiser will be happy to oblige. In Racket, ,enter works like Racket’s standard enter! form, but you can also provide a path string as its argument (e.g., ,enter "/tmp/foo.rkt" is equivalent to ,enter (file "/tmp/foo.rkt")). Like enter!, ,enter accepts also module names (as in, say, ,enter geiser/main). As mentioned, Guile’s ,m is used as is.

Once you enter a new module, only those bindings visible in its namespace will be available to your evaluations. All Schemes supported by Geiser provide a way to import new modules in the current namespace. Again, there’s a Geiser command, geiser-repl-import-module, to invoke such functionality, bound this time to C-c C-i. And, again, you’ll see Geiser just introducing the native incantation for you, and you’re free to use such incantations by hand whenever you want.

One convenience provided by these two Geiser commands is that completion is available when introducing the new module name, using the <TAB> key. Pressing it at the command’s prompt will offer you a prefix-aware list of available module names.

Which brings me to the next group of REPL commands.


Completion and error handling

We’ve already seen Geiser completion of module names in action at the minibuffer. You won’t be surprised to know that it’s also available at the REPL buffer itself. There, you can use either C-. or M-` to complete module names, and <TAB> or M-<TAB> to complete identifiers. Geiser will know what identifiers are bound in the current module and show you a list of those starting with the prefix at point. Needless to say, this is not a static list, and it will grow as you define or import new bindings in the namespace at hand. If no completion is found, <TAB> will try to complete the prefix after point as a module name.

REPL buffers use Emacs’ compilation mode to highlight errors reported by the Scheme interpreter, and you can use the next-error command (M-g n) to jump to their location. By default, every time you enter a new expression for evaluation old error messages are forgotten, so that M-g n will always jump to errors related to the last evaluation request, if any. If you prefer a not-so-forgetful REPL, set the customization variable geiser-repl-forget-old-errors-p to nil. Note, however, that even when that variable is left as t, you can always jump to an old error by moving to its line at the REPL and pressing <RET>. When your cursor is away from the last prompt, <TAB> will move to the next error in the buffer, and you can use <BACKTAB> everywhere to go to the previous one.


Autodoc and friends

Oftentimes, there’s more you’ll want to know about an identifier besides its name: What module does it belong to? Is it a procedure and, if so, what arguments does it take? Geiser tries to help you answering those questions too.

Actually, if you’ve been playing with the REPL as you read, you might have notice some frantic activity taking place in the echo area every now and then. That was Geiser trying to be helpful (while, hopefully, not being clippy), or, more concretely, what i call, for want of a better name, its autodoc mode. Whenever it’s active (did you notice that A in the mode-line?), Geiser’s gerbils will be scanning what you type and showing (unless you silence them with C-c C-d C-a) information about the identifier nearest to point.

If that identifier corresponds to a variable visible in the current namespace, you’ll see the module it belongs to and its value. For procedures and macros, autodoc will display, instead of their value, the argument names (or an underscore if Geiser cannot determine the name used in the definition). Optional arguments are surrounded by parentheses. When the optional argument has a default value, it’s represented by a list made up of its name and that value. When the argument is a keyword argument, its name has “#:” as a prefix.

If that’s not enough documentation for you, C-c C-d d will open a separate documentation buffer with help on the symbol at point. This buffer will contain implementation-specific information about the identifier (e.g., its docstring for Guile, or its contract, if any, for Racket), and a handy button to open the corresponding manual entry for the symbol, which will open an HTML page (for Racket) or the texinfo manual (for Guile). If you’d rather go directly to the manual, try C-c C-d i, which invokes geiser-doc-look-up-manual as the handy button does.

Geiser can also produce for you a list, classified by kind, of the identifiers exported by a given module: all you need to do is press C-c C-d m, and type or complete the desired module’s name.

The list of exported bindings is shown, again, in a buffer belonging to Geiser’s documentation browser, where you have at your disposal a bunch of navigation commands listed in our cheat-sheet.

We’ll have a bit more to say about the documentation browser in a later section.

If that’s still not enough, Geiser can jump, via M-., to the symbol’s definition. A buffer with the corresponding file will pop up, with its point resting upon the identifier’s defining form. When you’re done inspecting, M-, will bring you back to where you were. As we will see, these commands are also available in Scheme buffers. M-. also works for modules: if your point is on an unambiguous module name, the file where it’s defined will be opened for you.


Seeing is believing

In schemes that support images as values (currently, that means Racket), the REPL will display them inline if you’re using them in a graphics-aware Emacs.

For the terminal, images will appear as buttons: press return on them to invoke an external viewer (configurable via geiser-image-viewer) that will show you the image at hand. You can also ask for the same behaviour on all emacsen by customising geiser-repl-inline-images-p to nil.

Geiser keeps a cache of the last displayed images in the directory geiser-image-cache-dir, which defaults to the system’s temp directory, with up to geiser-image-cache-keep-last files. You can invoke the external image viewer on any of them with M-x geiser-view-last-image, which takes a prefix argument to indicate which image number you want, 0 corresponding to the newest one.


Customization and tips

The looks and ways of the REPL can be fine-tuned via a bunch of customization variables. You can see and modify them all in the corresponding customization group (by using the menu entry or the good old M-x customize-group geiser-repl), or by setting them in your Emacs initialisation files (as a rule, all knobs in Geiser are tunable this way: you don’t need to use customization buffers if you don’t like them).

I’m documenting below a proper subset of those settings, together with some related tips.

Choosing a Scheme implementation

Instead of using the generic run-geiser command, you can start directly your Scheme of choice via run-racket or run-guile. In addition, the variable geiser-active-implementations contains a list of those Schemes Geiser should be aware of. Thus, if you happen to be, say, a racketeer not to be beguiled by other schemes, you can tell Geiser to forget about the richness of the Scheme ecosystem with something like:

 
(setq geiser-active-implementations '(racket))

in your initialisation files.

When starting a new REPL, Geiser assumes, by default, that the corresponding Scheme binary is in your path. If that’s not the case, the variables to tweak are geiser-guile-binary and geiser-racket-binary, which should be set to a string with the full path to the requisite binary.

Before starting the REPL, Geiser will check wether the version of your Scheme interpreter is good enough. This means that it will spend a couple tenths of a second launching and quickly discarding a Scheme process, but also that the error message you’ll get if you’re on the wrong Scheme version will be much more informative. If you one to avoid version checks, just check geiser-repl-skip-version-check-p to t in your configuration.

You can also specify a couple more initialisation parameters. For Guile, geiser-guile-load-path is a list of paths to add to its load path (and its compiled load path) when it’s started, while geiser-guile-init-file is the path to an initialisation file to be loaded on start-up. The equivalent variables for Racket are geiser-racket-collects and geiser-racket-init-file.

Note, however, that specifying geiser-guile-init-file is not equivalent to changing Guile’s initialization file (‘~/.guile’), because the former is loaded using the -l flag, together with -q to disable loading the second. But there are subtle differences in the way Guile loads the initialization file versus how it loads a file specified via the -l flag. If what you want is just loading ‘~/.guile’, leave geiser-guile-init-file alone and set geiser-guile-load-init-file-p to t instead.

Racket startup time

When starting Racket in little computers, Geiser might have to wait a bit more than it expects (which is ten seconds, or ten thousand milliseconds, by default). If you find that Geiser is giving up too quickly and complaining that no prompt was found, try to increase the value of geiser-repl-startup-time to, say, twenty seconds:

 
(setq geiser-repl-startup-time 20000)

If you prefer, you can use the customize interface to, well, customise the above variable’s value.

History

By default, Geiser won’t record duplicates in your input history. If you prefer it did, just set geiser-repl-history-no-dups-p to nil. History entries are persistent across REPL sessions: they’re saved in implementation-specific files whose location is controlled by the variable geiser-repl-history-filename. For example, my Geiser configuration includes the following line:

 
(setq geiser-repl-history-filename "~/.emacs.d/geiser-history")

which makes the files ‘geiser-history.guile’ and ‘geiser-history.racket’ to live inside my home’s ‘.emacs.d’ directory.

Autodoc

If you happen to love peace and quiet and prefer to keep your REPL’s echo area free from autodoc’s noise, geiser-repl-autodoc-p is the customization variable for you: set it to nil and autodoc will be disabled by default in new REPLs. You can always bring the fairies back, on a per-REPL basis, using C-c C-d C-a.

Remote connections

When using connect-to-guile, connect-to-racket or geiser-connect, you’ll be prompted for a host and a port, defaulting to “localhost” and 37146. You can change those defaults customizing geiser-repl-default-host and geiser-repl-default-port, respectively.

Killing REPLs

If you don’t want Emacs to ask for confirmation when you’re about to kill a live REPL buffer (as will happen, for instance, if you’re exiting Emacs before closing all your REPLs), you can set the flag geiser-repl-query-on-kill-p to nil. On a related note, the customizable variable geiser-repl-query-on-exit-p controls whether Geiser should ask for confirmation when you exit the REPL explicitly (via, say, C-c C-q, as opposed to killing the buffer), and is set to nil by default.