\input texinfo   @c -*-texinfo-*-
@c %**start of header
@setfilename distel.info
@settitle Distel User Manual
@c %**end of header

@set EDITION Distel Jungerl
@set UPDATED $Date: 2005/02/20 22:10:15 $
@set VERSION Jungerl

@titlepage
@title Distel User Manual
@subtitle @value{EDITION}, updated @value{UPDATED}
@author Luke Gorrie
@end titlepage

@dircategory Emacs
@direntry
* Distel: (distel).             Erlang development environment.
@end direntry

@c @setchapternewpage off

@contents

@node Top, Introduction, (dir), (dir)
@comment  node-name,  next,  previous,  up

@noindent

@ifinfo
This is the user manual for Distel.
@end ifinfo

@menu
* Introduction::                
* Programming Aids::            
* Applications::                

@detailmenu
 --- The Detailed Node Listing ---

Introduction

* Principles::                  The underlying model of operation.
* Conventions::                 Common conventions for commands.

Programming Aids

* Tags::                        Looking up function definitions.
* Completion::                  Completing module and function names.
* Evaluation::                  Evaluating snippets and reloading modules.
* Refactoring::                 
* Documentation::               Looking up online documentation.

Applications

* Process Manager::             See and manipulate Erlang processes.
* Debugger::                    Debug Erlang programs.
* Interactive Sessions::        Hybrid Emacs buffer / Erlang shell.
* Profiler::                    Profile with fprof.

Debugger

* Basic Commands::              Interpreting modules and setting breakpoints.
* Monitor Buffer::              Monitoring interpreted processes.
* Attach Buffer::               Single-stepping a process.
* Synchronising Breakpoints::   Resynchronising breakpoints after edits.
* Saving and Restoring::        Temporarily saving debugging state.

@end detailmenu
@end menu

@node Introduction, Programming Aids, Top, Top
@chapter Introduction

Distel is a set of Emacs-based programs for interacting with running
Erlang nodes. The purpose is to make everyday Erlang programming tasks
easier -- looking up function definitions, debugging and profiling,
experimenting with code snippets, and so on. It builds on the existing
@code{erlang-mode} to provide more of the features common to
Integrated Development Environments.

This manual describes Distel from the user's point of view. For
details on how Distel works and how to write your own Distel-based
programs, see the paper @cite{Distel: Distributed Emacs Lisp (for
Erlang)} from the proceedings of the 2002 Erlang User Conference. 

The paper is available here;
@url{http://cronqvi.st/distel/gorrie02distel.pdf}.

The Distel home page is here; @url{http://code.google.com/p/distel}.

@ifinfo
The overall principles of Distel's operation and use are described in
the following sections.
@end ifinfo

@menu
* Principles::                  The underlying model of operation.
* Conventions::                 Common conventions for commands.
@end menu

@node Principles, Conventions, Introduction, Introduction
@section Principles of Operation

Distel works by talking to Erlang nodes using the Erlang distribution
protocol. It creates an ``Emacs node,'' similar to the ``C nodes'' of
@code{erl_interface}, and then talks directly to Erlang nodes with RPC
and other forms of message-passing.

Of course, this means that to use Distel you need to have an Erlang
node running. The node should be able to load some supporting modules
for Distel to make RPCs to -- setting this up is simple, and described
in the @file{INSTALL} file in the distribution. Other aspects of the
node's setup, such as which other modules it can find, will also
affect Distel's operation. More on this in the relevant sections.

@node Conventions,  , Principles, Introduction
@section Conventions of Use

Most Distel commands need to know which Erlang node to talk to.
(Distel doesn't start an Erlang node, you have to do that yourself.)
The first command you use will prompt in the minibuffer for the name
of the node to use. You can answer with either a @code{name@@host}
node name, or with just the @code{name} part as an abbreviation for a
node on the local machine.

As a convenience, the node name you enter is cached and then reused in
future commands. If you later want to talk to a different node you can
use the command @code{erl-choose-nodename} (@kbd{C-c C-d n}) to select
a new node to talk to. The currently cached node name is always shown
in the modeline.

Some commands accept a prefix argument to alter their behaviour in some
specific way. You can give a prefix with @kbd{C-u} followed by the
command you want to call. For example, @kbd{C-u M-.} tells the @kbd{M-.}
command to prompt for the function to lookup, instead choosing one by
looking at the source text in the buffer. The effect, if any, of a
prefix on a command is included in the command's documentation. In
addition, many commands take a node name as a parameter and, by default,
the node name is defaulted to the last node name that was
selected. However, if the command is called with a prefix, the node name
will be prompted for.

The command @code{erl-ping} (@kbd{C-c C-d g}) can be used to determine
whether distel can communicate with a specific node. As a side effect,
erl-ping will also load the distel modules into the node.

@node Programming Aids, Applications, Introduction, Top
@chapter Programming Aids

Distel includes a few small sets of related commands to automate
common programming chores. These are described in the following
sections.

@menu
* Tags::                        Looking up function definitions.
* Completion::                  Completing module and function names.
* Evaluation::                  Evaluating snippets and reloading modules.
* Refactoring::                 
* Documentation::               Looking up online documentation.
@end menu

@node Tags, Completion, Programming Aids, Programming Aids
@section Cross-Referencing (Tags)

A ``dynamic tags'' facility effectively makes each function call in an
Erlang source file into a hyperlink to the definition of that
function. For example, if you have a line of code like this:

@example
lists:keysort(2, L).
@end example

You can place the point on the function name, press @kbd{M-.}, and up
pops @file{lists.erl} at the definition of @code{keysort/2}. After you
have perused the definition to your satisfaction, you press @kbd{M-,}
to jump back where you came from. You can also jump through several
(sub)function definitions and then use @kbd{M-,} several times to
unwind step-by-step back to where you came from.

This feature is a dynamic version of a traditional Emacs facility
called ``Tags.''  Whereas Tags needs you to maintain a special
@file{TAGS} file to keep track of definitions, Distel simply asks an
Erlang node, ``Where is the source file for module @code{foo}?'' The
Erlang node makes a well-educated guess at which source file we want
(based on the location and attributes of the beam file for the same
module), and sends back the path. Emacs then opens the file and scans
down to the definition of the function with the right arity.

If you have several versions of the same source file (perhaps
belonging to separate branches in revision control), then Distel will
find the one that matches the code in the Erlang node you're talking
to. So, to work on a particular source tree you just connect to a node
that has the matching code in its code path.

@table @kbd

@item M-.
Jump from a function call to the definition of the function
(@code{erl-find-source-under-point}). If the variable
@code{distel-tags-compliant} is non-nil, or a prefix argument is
given, this command prompts for the function name to lookup.

@item M-,
Jump back from a function definition
(@code{erl-find-source-unwind}). This is a multi-level unwind through
a stack of positions from which you have jumped with @kbd{M-.} The
command is also bound to @kbd{M-*} for consistency with ``etags.''
@end table

To actually find the source file for a particular module, the Erlang
node first ensures that it can load the module, and then tries each of
these locations in order:

@enumerate
@item
Same directory as the beam file.
@item
@file{../src/} from the beam file.
@item
@file{../erl/} from the beam file.
@item
The directory from which the beam file was compiled. We can find this
using @code{module_info/1}, because the compiler records it as an
attribute in the beam file.
@end enumerate

@node Completion, Evaluation, Tags, Programming Aids
@section Completion of Modules and Functions

Completion allows you to write some part of a module or remote
function name and then press @kbd{M-TAB} to have it completed
automatically. When multiple completions exist they are displayed in a
popup buffer, much like Emacs's normal filename completion. The
completion buffer can simply be read to see which completions exist,
or either @kbd{RET} or the middle mouse button can be used to select
one.

@table @kbd

@item M-TAB
Complete the module or function at point. (@code{erl-complete})

@item M-?
Alternative binding for @code{erl-complete}, since @kbd{M-TAB} is
often reserved by window managers.
@end table

@node Evaluation, Refactoring, Completion, Programming Aids
@section Evaluting Erlang Code

Distel includes some simple ways to evaluate Erlang code, described
here. More elaborate interactive evaluation is provided by Interactive
Sessions (@pxref{Interactive Sessions}).

@table @kbd

@item C-c C-d :
Read an Erlang expression from the minibuffer, evaluate it on an Erlang
node, and show the result. If there is an active region in the source
buffer, that region will be the default that is displayed in the
minibuffer. (@code{erl-eval-expression})

@item C-c C-d L
Read a module name from the minibuffer and reload that module in an
Erlang node. (@code{erl-reload-module})

@item C-c C-d r
Reload all loaded modules that are out of date (i.e. the loaded beam is 
compiled before the corresponding beam file.) (@code{erl-reload-modules})
@end table

@node Refactoring, Documentation, Evaluation, Programming Aids
@section Refactoring

Expressions within functions can be automatically ``refactored'' into
their own sub-functions by using the @code{erl-refactor-subfunction}
command (@kbd{C-c C-d f}). The command takes the text of the
expression, determines which variables it needs from the original
function, and then generates the new function and puts it on the kill
ring for insertion by hand (with @code{yank}, @kbd{C-y}). The original
function is rewritten with a call to the subfunction where the
refactored expression used to be.

For example, suppose we want to refactor the following function:

@example
eval_expression(S) ->
    case parse_expr(S) of
        @{ok, Parse@} ->
            case catch erl_eval:exprs(Parse, []) of
                @{value, V, _@} ->
                    @{ok, flatten(io_lib:format("~p", [V]))@};
                @{'EXIT', Reason@} ->
                    @{error, Reason@}
            end;
        @{error, @{_, erl_parse, Err@}@} ->
            @{error, Err@}
    end.
@end example

In this example we will take the inner @code{case} expression and move
it into a new function called @code{try_evaluation}. We do this by
setting the Emacs region (using the mouse or @kbd{C-SPC}) from the
word @code{case} until the end of the word @code{end} -- marking
exactly one whole expression. We then enter @kbd{C-c C-d f} to
refactor, and when prompted for the function name we respond with
``@code{try_evaluation}''. The original function is then rewritten to:

@example
eval_expression(S) ->
    case parse_expr(S) of
       @{ok, Parse@} ->
           try_evaluation(Parse);
       @{error, @{_, erl_parse, Err@}@} ->
           @{error, Err@}
    end.
@end example

And at the front of the kill ring we have the new function definition,
which can be pasted into the buffer wherever we want. The actual
definition we get is:

@example
try_evaluation(Parse) ->
    case catch erl_eval:exprs(Parse, []) of
       @{value, V, _@} ->
           @{ok, flatten(io_lib:format("~p", [V]))@};
       @{'EXIT', Reason@} ->
           @{error, Reason@}
    end.
@end example

@strong{Important note:} This command is not a ``pure'' refactoring,
because although it will import variables from the parent function
into the subfunction, it will not export new bindings created in the
subfunction back to the parent. However, if you follow good
programming practice and never ``export'' variables from inner
expressions, this is not a problem. An example of @emph{bad} code that
will not refactor correctly is this @code{if} expression:

@example
if A < B -> X = true;
   B > A -> X = false
end,
foo(X)
@end example

This is in poor style -- a variable created inside the @code{if} is
used by code at an outer level of nesting. To work with refactoring,
and to be in better style, it should be rewritten like this:

@example
X = if A < B -> true;
       B > A -> false
    end,
foo(X)
@end example

@node Documentation,  , Refactoring, Programming Aids
@section Documentation

Browsing of the OTP documentation is provided by an Erlang program 
called @code{otp_doc}. The OTP html docs are scanned on-the-fly, and
for each M:F/A two pieces of data are extracted (and cached): the 
@strong{signature} and the @strong{link}.

The signature is a string describing the functions name, arguments 
and return value;
@code{lists:usort(List1) -> List2}
Signature(s) are printed in the mini-buffer by the 
@code{erl-find-sig} functions. Note that there is implicit 
wildcarding; running @code{erl-find-sig} on @code{"li:fl"}
will print this in the mini-buffer;
@example
lib:flush_receive() -> void()
lists:flatlength(DeepList) -> int()
lists:flatmap(Fun, List1) -> List2
lists:flatten(DeepList, Tail) -> List
lists:flatten(DeepList) -> List
@end example

The link is used by the @code{erl-find-doc} functions to display the 
full documentation in Emacs (requires the @code{w3m} package.) If 
there is more than one match (see above), a list of candidates will
be written in the mini-buffer;
@example
candidates: lib:flush_receive/0, lists:flatlength/1, lists:flatmap/2, 
lists:flatten/2, lists:flatten/1
@end example

Simple online Erlang documentation is provided via an Erlang program
called @code{fdoc}. The documentation is automatically scanned out of
source files by building a searchable database of the comments
appearing before each function. Naturally, the quality of
documentation provided by this scheme will depend on the style in
which the source files are commented.

@table @kbd
@item C-c C-d d
Describe an Erlang module or function by
name. (@code{erl-fdoc-describe})
@item C-c C-d a
Show apropos information about Erlang functions, by regular
expression. All functions whose names or comments match the regexp are
displayed. (@code{erl-fdoc-apropos})
@end table

With a prefix argument, these commands rebuild the @code{fdoc}
database before searching. This is useful after (re)loading a lot of
modules, since @code{fdoc} only scans the currently loaded modules for
documentation when it builds the database.

@node Applications,  , Programming Aids, Top
@chapter Applications

This chapter describes the larger applications included with Distel.

@menu
* Process Manager::             See and manipulate Erlang processes.
* Debugger::                    Debug Erlang programs.
* Interactive Sessions::        Hybrid Emacs buffer / Erlang shell.
* Profiler::                    Profile with fprof.
@end menu

@node Process Manager, Debugger, Applications, Applications
@section Process Manager

The process manager displays a list of all the processes running on an
Erlang node, and offers commands to manipulate them.

@table @kbd
@item C-c C-d l
Popup a process manager buffer. (@code{erl-process-list})
@end table

Within the process manager's buffer, the following commands are
available:

@table @kbd
@item q
Quit the process manager, and restore the Emacs windows as they were
before it popped up.
@item u
Update the process list.
@item k
Kill a process.
@item RET
Pop up a buffer showing all the information about a process. The
buffer also continuously traces the process by appending events to the
buffer, until the buffer is killed with @kbd{q}.
@item i
Show a piece of information about the process, specified by name. The
name can be any key accepted by the @code{process_info/2} BIF.
@item b
Show a backtrace for a process. The backtrace is a fairly low-level
snapshot of the stack of a process, obtained from
@code{process_info(P, backtrace)}. It may take a little pratice to
learn how to read them.
@item m
Show the contents of a process's mailbox.
@end table

@node Debugger, Interactive Sessions, Process Manager, Applications
@section Debugger
Distel includes a front-end to the Erlang debugger, using the same
backend as the standard Tk-based OTP debugger. The Distel debugger has
three parts: commands in Erlang source buffers for interpreting
modules and configuring breakpoints, a ``Monitor'' buffer listing
processes running interpreted code, and one ``Attach'' buffer for each
process that is being single-stepped.
Note that none of this will work unless the modules are compiled with the 
``+debug_info'' switch to erlc.

@menu
* Basic Commands::              Interpreting modules and setting breakpoints.
* Monitor Buffer::              Monitoring interpreted processes.
* Attach Buffer::               Single-stepping a process.
* Synchronising Breakpoints::   Resynchronising breakpoints after edits.
* Saving and Restoring::        Temporarily saving debugging state.
@end menu

@node Basic Commands, Monitor Buffer, Debugger, Debugger
@subsection Basic Commands

@table @kbd
@item C-c C-d i
Toggle interpretedness of the current buffer's module.
(@code{edb-toggle-interpret})
@item C-x SPC
Toggle a breakpoint on the current line.
(@code{edb-toggle-breakpoint})
@item C-c C-d m
Popup the Monitor buffer. (@code{edb-monitor})
@end table

@node Monitor Buffer, Attach Buffer, Basic Commands, Debugger
@subsection Monitor Buffer

The monitor buffer displays all processes that the debugger knows
about, line-by-line. This includes all processes that have run
interpreted code, and all that are stopped in breakpoints. The current
status of each process is shown -- running, at breakpoint, or
exited. You can attach to a debugged process by pressing @kbd{RET} on
its summary line.

@table @kbd
@item RET
Popup an attach buffer for a process.
@item q
Hide the Monitor buffer and restore Emacs' window configuration to the
way it was before.
@item k
Kill the monitor. This disconnects Emacs from the Erlang node's
debugging state and deletes all the local debugging state
(e.g. breakpoints in buffers.) The next debugger command will
automatically re-attach the monitor.
@end table

@node Attach Buffer, Synchronising Breakpoints, Monitor Buffer, Debugger
@subsection Attach Buffer

An attach buffer corresponds to a particular Erlang process that is
being debugged. It displays the source to the module currently being
executed and, when the process is stopped at a breakpoint, an arrow
showing the next line of execution. The attach buffer is accompanied
by a buffer showing the variable bindings in the current stack frame.

@table @kbd
@item SPC
Step into the next expression. If the expression is a function call,
the debugger will enter that function. (@code{edb-attach-step})
@item n
Step over the next expression, without going down into a subfunction.
(@code{edb-attach-next})
@item c
Continue execution until the next breakpoint.
(@code{edb-attach-continue})
@item u
Show the previous stack frame. (@code{edb-attach-up})
@item d
Show the next stack frame. (@code{edb-attach-down})
@item b
Toggle a breakpoint on the current line.
(@code{edb-toggle-breakpoint})
@item q
Kill the attach buffer. This does not affect the actual Erlang
process.
@item h
Display online help, showing essentially this information.
(@code{edb-attach-help})
@end table

@node Synchronising Breakpoints, Saving and Restoring, Attach Buffer, Debugger
@subsection Synchronising Breakpoints

At any point in time the breakpoints in a particular buffer will be
either ``fresh'' or ``stale,'' depending on whether the buffer has
been modified. Breakpoints are fresh when, as far as Emacs knows, the
buffer's source text (line numbers) correspond with the code in the
Erlang node. After the buffer is modified, the breakpoints become
stale, because edits may change line numbers so that the breakpoints
in Emacs no longer correspond with the actual program. Stale
breakpoints are made fresh by using the @code{edb-synch-breakpoints}
(@kbd{C-c C-d s}) command to reassert that they are correct. This
command is typically used after recompiling and reloading the module.

Fresh breakpoints are marked in red, stale breakpoints are marked in
purple.

@table @kbd
@item C-c C-d s
Synchronise breakpoints by discarding the ones in the Erlang node and
then re-setting them from those in the Emacs buffer.
@end table

@iftex
The overall debugger state machine for Erlang-mode buffers is shown in
this figure:

@image{dbg, 6in}

In the ``Normal'' state, no breakpoints exist. In the ``Interpreted''
state, all breakpoints are fresh. In the ``Out of sync'' state, all
breakpoints are stale. The transitions illustrate how you can navigate
between the states.
@end iftex

Care must be taken to only synchronise breakpoints when the Erlang
node is actually running the same code that is in the Emacs
buffer. Otherwise, the Erlang processes may break in unexpected
places.

When reloading modules during debugging, it is preferable to use the
@code{erl-reload-module} command (@kbd{C-c C-d L}, @pxref{Evaluation})
than to call @code{l(mymodule)} directly in the Erlang shell. This is
because the Distel command is specially coded to make sure reloading
interpreted modules keeps them interpreted, but this doesn't appear to
work correctly in the Erlang shell.

@node Saving and Restoring,  , Synchronising Breakpoints, Debugger
@subsection Saving and Restoring Debugger State

The overall debugger state (set of breakpoints and interpreted
modules) can be temporarily saved inside Emacs and then restored to
the Erlang node. This is particularly useful when you want to restart
the Erlang node and then continue debugging where you left off: you
just save the debug state, restart the node, and then restore.

@table @kbd
@item C-c C-d S
Save the set of breakpoints and interpreted modules inside
Emacs. (@code{edb-save-dbg-state})
@item C-c C-d R
Restore Emacs's saved debugger state to the Erlang
node. (@code{edb-restore-dbg-state})
@end table

@node Interactive Sessions, Profiler, Debugger, Applications
@section Interactive Sessions

Interactive sessions are an Erlang version of the Emacs Lisp
@file{*scratch*} buffer. You can enter arbitrary Erlang expressions
and function definitions in an interactive session buffer and evaluate
them immediately, without creating any files.

@table @kbd
@item C-c C-d e
Display the Interactive Session buffer for an Erlang node, creating it
if necessary. (@code{erl-ie-show-session})
@end table

Within the session buffer, these commands are available:

@table @kbd
@item C-j
Evaluate the Erlang expression on the current line, and insert the
result in-line. (@code{erl-ie-eval-expression})
@item C-M-x
Evaluate the function definition before the point. Once defined, the
function can then be called from expressions in the session buffer,
and can be redefined later.
@end table

@node Profiler,  , Interactive Sessions, Applications
@section Profiler

Distel supports profiling function calls via the OTP @code{fprof}
application. This is a very convenient profiler, in that it doesn't
require any special compiler options or initialisation -- you can use
it whenever you want.

@table @kbd
@item C-c C-d p
Prompt for an Erlang expression, evaluate it with profiling, and then
summarise the results. (@code{fprof})
@item C-c C-d P
Load and display prerecorded profiler data, from a file created by
@code{fprof:analyse/1}. (@code{fprof-analyse})
@end table

After an expression is profiled, the results are popped up in
``profiler results'' buffer. The buffer contains one line to describe
each function that was called, with the following columns:

@table @code
@item Calls
The total number of times the function was called.
@item ACC
The total time (ms) spent in the function, including its callees.
@item Own
The total time (ms) spent by the function itself, excluding time spent
in its callees.
@end table

Furthermore, pressing @kbd{RET} on a summary line in the results
buffer will pop up another buffer showing more information about the
function: how much time it spent on behalf of each of its callers, and
how much time it spent in each of its subfunctions.

@bye