@node The REPL, Between the parens, Installation, Top
@chapter The REPL
@anchor{quick-start}
If you've followed the indications in @ref{Setting it up}, 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.

@menu
* Starting the REPL::
* First aids::
* Switching context::
* Completion and error handling::
* Autodoc and friends::
* Customization and tips::
@end menu

@node Starting the REPL, First aids, The REPL, The REPL
@section Starting the REPL

@cindex REPL
To start a Scheme REPL (meaning, a scheme process offering you a
Read-Eval-Print Loop), Geiser provides the generic interactive command
@command{run-geiser}. If you run it (via, as is customary in Emacs,
@kbd{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 @ref{active-implementations,,below}). Tabbing for completion
will offer you, as of this writing, @code{guile} and @code{racket}. Just
choose your poison, and a new REPL buffer will pop-up.

@imgc{repls}

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 @ref{impl-binary,, below}. 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 @code{guile-user}, while Racket's top namespace doesn't have
a name; cf. @ref{Switching context} below). 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.

@subsubheading Connecting to an external Scheme
@cindex remote REPL
@cindex connect to server
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.

@cindex Guile's REPL server
If you use Guile, you just need to start your Guile process (possibly
outside Emacs) passing to it the flag @code{--listen}. This flag accepts
an optional port as argument (as in @code{--listen=1969}), if you don't
want to use the default.

@cindex Racket's REPL server
In Racket, you have to use the REPL server that comes with Geiser. To
that end, put Geiser's Racket scheme directory in the Racket's
collection search path and invoke @code{start-geiser} (a procedure in
the module @code{geiser/server}) somewhere in your program, passing it
the desired port. This procedure will start the REPL server in a
separate thread. For an example of how to do that, see the script
@file{bin/geiser-racket.sh} in the source distribution, or, if you've
compiled Geiser, @file{bin/geiser-racket-noinst} in the build directory,
or, if you've installed Geiser, @file{geiser-racket} in
@file{<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 @kbd{M-x geiser-connect}, @kbd{M-x connect-to-guile}
or @kbd{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)defintions will be visible everywhere. That's dangerous, but will
come in handy when you need to debug your running webserver.

@cindex remote connections
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.

@node First aids, Switching context, Starting the REPL, The REPL
@section First aids

@img{repl-menu, right}
@cindex REPL commands
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
@uref{http://www.emacswiki.org/emacs/LaCarte, La Carte} to access the
menus in a convenient enough fashion.

Or just press @kbd{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, @kbd{M-p} and
@kbd{M-n} are bound to @i{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. @dfn{point}, in the buffer). For going up and
down the list unconditionally, just use @kbd{C-c M-p} and @kbd{C-c M-n}.
In addition, navigation is sexp- rather than line-based.

There are also a few commands to twiddle with the Scheme process.
@kbd{C-c C-q} will gently ask it to quit, while @kbd{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, @kbd{C-c
C-z} will restart it (but the same shortcut, issued when the REPL is
alive, will bring you back to the buffer you came from, as explained
@ref{switching-repl-buff,,here}).

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

@node Switching context, Completion and error handling, First aids, The REPL
@section Switching context

@cindex current module, in REPL
In tune with Geiser's @ref{current-module,,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
@command{switch-to-geiser-module}, bound to @kbd{C-c C-m}. You'll notice
that Geiser simply uses a couple of meta-commands provided by the Scheme
REPL (the stock @command{,m} in Guile and @command{,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.

@cindex current module, change
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, @command{geiser-repl-import-module}, to
invoke such functionality, bound this time to @kbd{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
@kbd{@key{TAB}} key. Pressing it at the command's prompt will offer you
a prefix-aware list of available module names.

@imgc{mod-completion}

Which brings me to the next group of REPL commands.

@node Completion and error handling, Autodoc and friends, Switching context, The REPL
@section Completion and error handling

@cindex completion, module name
We've already seen Geiser completion of module names in action at the
mini-buffer. You won't be surprised to know that it's also available at
the REPL buffer itself. There, you can use either @kbd{C-.} or
@kbd{M-`} to complete module names, and @kbd{@key{TAB}} or
@kbd{M-@key{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, @kbd{@key{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 @command{next-error} command
(@kbd{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 @kbd{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 @code{geiser-repl-forget-old-errors-p} to
@code{nil}. Note, however, that even when that variable is left as
@kbd{t}, you can always jump to an old error by moving to its line at
the REPL and pressing @kbd{RET}. When your cursor is away from the last
prompt, @kbd{TAB} will move to the next error in the buffer, and you can
use @kbd{BACKTAB} everywhere to go to the previous one.

@node Autodoc and friends, Customization and tips, Completion and error handling, The REPL
@section 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.

@cindex autodoc, in the REPL
Actually, if you've been playing with the REPL as you read, you might
have notice some frantic activity taking place in the minibuffer 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 @dfn{autodoc} mode. Whenever it's active (did you
notice that @i{A} in the mode-line?), Geiser's gerbils will be scanning
what you type and showing (unless you silent them with @kbd{C-c C-a})
information about the identifier nearest to point.

@imgc{repl-autodoc}

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
parenthesis. 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.

@cindex help on identifier
If that's not enough documentation for you, @kbd{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).

@cindex module exports
@anchor{repl-mod} 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 @kbd{C-c C-d m}, and type or complete the desired module's
name.

@imgc{repl-mod}

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 @xref{Documentation browser,,our
cheat-sheet}. We'll have a bit more to say about the documentation
browser in @xref{doc-browser,,a later section}.

@cindex jump, at the REPL
If that's still not enough, Geiser can jump, via @kbd{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, @kbd{M-,} will bring you back to where you were. As we
will see, these commands are also available in scheme buffers. @kbd{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.

@node Customization and tips,  , Autodoc and friends, The REPL
@section Customization and tips

@cindex REPL customization
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 @kbd{M-x customize-group geiser-repl}), or by setting them in your
Emacs initialization files (as a rule, all knobs in Geiser are turnable
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.

@subsubheading Choosing a Scheme implementation
@cindex scheme implementation, choosing
@anchor{choosing-impl}
Instead of using the generic @command{run-geiser} command, you can start
directly your Scheme of choice via @command{run-racket} or
@command{run-guile}. @anchor{active-implementations} In addition, the
variable @code{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
@example
(setq geiser-active-implementations '(racket))
@end example
@noindent in your initialisation files.

@cindex scheme binary
@cindex scheme executable path
@anchor{impl-binary} 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 @code{geiser-guile-binary} and
@code{geiser-racket-binary}, which should be set to a string with the
full path to the requisite binary.

@cindex scheme load path
@cindex scheme init file
@cindex GUILE_LOAD_PATH
@cindex PLTCOLLECTS
You can also specify a couple more initialisation parameters. For Guile,
@code{geiser-guile-load-path} is a list of paths to add to its load path
when it's started, while @code{geiser-guile-init-file} is the path to an
initialisation file to be loaded on startup. The equivalent variables
for Racket are @code{geiser-racket-collects} and
@code{geiser-racket-init-file}.

@subsubheading History

By default, Geiser won't record duplicates in your input history. If you
prefer it did, just set @code{geiser-repl-history-no-dups-p} to
@code{nil}. History entries are persistent across REPL sessions:
they're saved in implementation-specific files whose location is
controlled by the variable @code{geiser-repl-history-filename}. For
example, my Geiser configuration includes the following line:
@example
(setq geiser-repl-history-filename "~/.emacs.d/geiser-history")
@end example
@noindent which makes the files @file{geiser-history.guile} and
@file{geiser-history.racket} to live inside my home's @file{.emacs.d}
directory.

@subsubheading Autodoc

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

@subsubheading Remote connections

@cindex port, default
@cindex host, default
When using @code{connect-to-guile} or @code{geiser-connect}, you'll be
prompted for a host and a port, defaulting to localhost and 37146. You
can change those defaults customizing @code{geiser-repl-default-host}
and @code{geiser-repl-default-port}, respectfully.

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