[DOC] Doc for exceptions (#11008)

doc/exceptions.md

@@ -84,15 +84,15 @@ A rescue clause:
 - Ends with the first following `rescue`,
   `else`, `ensure`, or `end` statement.
 
+##### Rescued Exceptions
+
 A `rescue` statement may include one or more classes
 that are to be rescued;
 if none is given, StandardError is assumed.
 
 The rescue clause rescues both the specified class
 (or StandardError if none given) or any of its subclasses;
-(see [Built-In Exception Classes](rdoc-ref:Exception@Built-In+Exception+Classes)
-for the hierarchy of Ruby built-in exception classes):
-
+see [Built-In Exception Class Hierarchy](rdoc-ref:Exception@Built-In+Exception+Class+Hierarchy).
 
 ```
 begin
@@ -133,6 +133,8 @@ rescue FloatDomainError, ZeroDivisionError
 end
 ```
 
+##### Multiple Rescue Clauses
+
 An exception handler may contain multiple rescue clauses;
 in that case, the first clause that rescues the exception does so,
 and those before and after are ignored:
@@ -153,6 +155,8 @@ Output:
 Rescued Errno::ENOENT
 ```
 
+##### Capturing the Rescued \Exception
+
 A `rescue` statement may specify a variable
 whose value becomes the rescued exception
 (an instance of Exception or one of its subclasses:
@@ -173,10 +177,66 @@ ZeroDivisionError
 divided by 0
 ```
 
-In the rescue clause, these global variables are defined:
+##### Global Variables
 
-- `$!`": the current exception instance.
-- `$@`: its backtrace.
+Two read-only global variables always have `nil` value
+except in a rescue clause;
+there:
+
+- `$!`: contains the rescued exception.
+- `$@`: contains its backtrace.
+
+Example:
+
+```
+begin
+  1 / 0
+rescue
+  p $!
+  p $@
+end
+```
+
+Output:
+
+```
+#<ZeroDivisionError: divided by 0>
+["t.rb:2:in `/'", "t.rb:2:in `<main>'"]
+```
+
+##### Cause
+
+In a rescue clause, the method Exception#cause returns the previous value of `$!`,
+which may be `nil`;
+elsewhere, the method returns `nil`.
+
+Example:
+
+```
+begin
+  raise('Boom 0')
+rescue => x0
+  puts "Exception: #{x0.inspect};  $!: #{$!.inspect};  cause: #{x0.cause.inspect}."
+  begin
+    raise('Boom 1')
+  rescue => x1
+    puts "Exception: #{x1.inspect};  $!: #{$!.inspect};  cause: #{x1.cause.inspect}."
+    begin
+      raise('Boom 2')
+    rescue => x2
+      puts "Exception: #{x2.inspect};  $!: #{$!.inspect};  cause: #{x2.cause.inspect}."
+    end
+  end
+end
+```
+
+Output:
+
+```
+Exception: #<RuntimeError: Boom 0>;  $!: #<RuntimeError: Boom 0>;  cause: nil.
+Exception: #<RuntimeError: Boom 1>;  $!: #<RuntimeError: Boom 1>;  cause: #<RuntimeError: Boom 0>.
+Exception: #<RuntimeError: Boom 2>;  $!: #<RuntimeError: Boom 2>;  cause: #<RuntimeError: Boom 1>.
+```
 
 #### Else Clause
 
@@ -349,14 +409,120 @@ not just the part after the point of failure.
 
 ## Raising an \Exception
 
-Raise an exception with method Kernel#raise.
+\Method Kernel#raise raises an exception.
 
 ## Custom Exceptions
 
 To provide additional or alternate information,
-you may create custom exception classes;
-each should be a subclass of one of the built-in exception classes:
+you may create custom exception classes.
+Each should be a subclass of one of the built-in exception classes
+(commonly StandardError or RuntimeError);
+see [Built-In Exception Class Hierarchy](rdoc-ref:Exception@Built-In+Exception+Class+Hierarchy).
 
 ```
 class MyException < StandardError; end
 ```
+
+## Messages
+
+Every `Exception` object has a message,
+which is a string that is set at the time the object is created;
+see Exception.new.
+
+The message cannot be changed, but you can create a similar object with a different message;
+see Exception#exception.
+
+This method returns the message as defined:
+
+- Exception#message.
+
+Two other methods return enhanced versions of the message:
+
+- Exception#detailed_message: adds exception class name, with optional highlighting.
+- Exception#full_message: adds exception class name and backtrace, with optional highlighting.
+
+Each of the two methods above accepts keyword argument `highlight`;
+if the value of keyword `highlight` is `true`,
+the returned string includes bolding and underlining ANSI codes (see below)
+to enhance the appearance of the message.
+
+Any exception class (Ruby or custom) may choose to override either of these methods,
+and may choose to interpret keyword argument <tt>highlight: true</tt>
+to mean that the returned message should contain
+[ANSI codes](https://en.wikipedia.org/wiki/ANSI_escape_code)
+that specify color, bolding, and underlining.
+
+Because the enhanced message may be written to a non-terminal device
+(e.g., into an HTML page),
+it is best to limit the ANSI codes to these widely-supported codes:
+
+- Begin font color:
+
+    | Color   | ANSI Code        |
+    |---------|------------------|
+    | Red     | <tt>\\e[31m</tt> |
+    | Green   | <tt>\\e[32m</tt> |
+    | Yellow  | <tt>\\e[33m</tt> |
+    | Blue    | <tt>\\e[34m</tt> |
+    | Magenta | <tt>\\e[35m</tt> |
+    | Cyan    | <tt>\\e[36m</tt> |
+
+<br>
+
+- Begin font attribute:
+
+    | Attribute | ANSI Code       |
+    |-----------|-----------------|
+    | Bold      | <tt>\\e[1m</tt> |
+    | Underline | <tt>\\e[4m</tt> |
+
+<br>
+
+- End all of the above:
+
+    | Color | ANSI Code       |
+    |-------|-----------------|
+    | Reset | <tt>\\e[0m</tt> |
+
+It's also best to craft a message that is conveniently human-readable,
+even if the ANSI codes are included "as-is"
+(rather than interpreted as font directives).
+
+## Backtraces
+
+A _backtrace_ is a record of the methods currently
+in the [call stack](https://en.wikipedia.org/wiki/Call_stack);
+each such method has been called, but has not yet returned.
+
+These methods return backtrace information:
+
+- Exception#backtrace: returns the backtrace as an array of strings or `nil`.
+- Exception#backtrace_locations: returns the backtrace as an array
+  of Thread::Backtrace::Location objects or `nil`.
+  Each Thread::Backtrace::Location object gives detailed information about a called method.
+
+An `Exception` object stores its backtrace value as one of:
+
+- An array of Thread::Backtrace::Location objects;
+  this is the common case: the exception was raised by the Ruby core or the Ruby standard library.
+  In this case:
+
+    - Exception#backtrace_locations returns the array of Thread::Backtrace::Location objects.
+    - Exception#backtrace returns the array of their string values
+      (`Exception#backtrace_locations.map {|loc| loc.to_s }`).
+
+- An array of strings;
+  this is an uncommon case: the user manually set the backtrace to an array of strings;
+  In this case:
+
+    - Exception#backtrace returns the array of strings.
+    - Exception#backtrace_locations returns `nil`.
+
+- `nil`, in which case both methods return `nil`.
+
+These methods set the backtrace value:
+
+- Exception#set_backtrace: sets the backtrace value to an array of strings, or to `nil`.
+- Kernel#raise: sets the backtrace value to an array of Thread::Backtrace::Location objects,
+  or to an array of strings.
+

error.c

@@ -1469,12 +1469,23 @@ exc_init(VALUE exc, VALUE mesg)
 }
 
 /*
- * call-seq:
- *    Exception.new(msg = nil)        ->  exception
- *    Exception.exception(msg = nil)  ->  exception
+ *  call-seq:
+ *    Exception.new(message = nil) -> exception
+ *
+ *  Returns a new exception object.
+ *
+ *  The given +message+ should be
+ *  a {string-convertible object}[rdoc-ref:implicit_conversion.rdoc@String-Convertible+Objects];
+ *  see method #message;
+ *  if not given, the message is the class name of the new instance
+ *  (which may be the name of a subclass):
+ *
+ *  Examples:
+ *
+ *    Exception.new         # => #<Exception: Exception>
+ *    LoadError.new         # => #<LoadError: LoadError> # Subclass of Exception.
+ *    Exception.new('Boom') # => #<Exception: Boom>
  *
- *  Construct a new Exception object, optionally passing in
- *  a message.
  */
 
 static VALUE
@@ -1490,12 +1501,24 @@ exc_initialize(int argc, VALUE *argv, VALUE exc)
  *  Document-method: exception
  *
  *  call-seq:
- *     exc.exception([string])  ->  an_exception or exc
+ *    exception(message = nil) -> self or new_exception
  *
- *  With no argument, or if the argument is the same as the receiver,
- *  return the receiver. Otherwise, create a new
- *  exception object of the same class as the receiver, but with a
- *  message equal to <code>string.to_str</code>.
+ *  Returns an exception object of the same class as +self+;
+ *  useful for creating a similar exception, but with a different message.
+ *
+ *  With +message+ +nil+, returns +self+:
+ *
+ *    x0 = StandardError.new('Boom') # => #<StandardError: Boom>
+ *    x1 = x0.exception              # => #<StandardError: Boom>
+ *    x0.__id__ == x1.__id__         # => true
+ *
+ *  With {string-convertible object}[rdoc-ref:implicit_conversion.rdoc@String-Convertible+Objects]
+ *  +message+ (even the same as the original message),
+ *  returns a new exception object whose class is the same as +self+,
+ *  and whose message is the given +message+:
+ *
+ *    x1 = x0.exception('Boom') # => #<StandardError: Boom>
+ *    x0..equal?(x1)            # => false
  *
  */
 
@@ -1514,10 +1537,15 @@ exc_exception(int argc, VALUE *argv, VALUE self)
 
 /*
  * call-seq:
- *   exception.to_s   ->  string
+ *   to_s -> string
+ *
+ * Returns a string representation of +self+:
+ *
+ *   x = RuntimeError.new('Boom')
+ *   x.to_s # => "Boom"
+ *   x = RuntimeError.new
+ *   x.to_s # => "RuntimeError"
  *
- * Returns exception's message (or the name of the exception if
- * no message is set).
  */
 
 static VALUE
@@ -1557,10 +1585,10 @@ rb_get_detailed_message(VALUE exc, VALUE opt)
 }
 
 /*
- * call-seq:
- *    Exception.to_tty?   ->  true or false
+ *  call-seq:
+ *    Exception.to_tty? -> true or false
  *
- * Returns +true+ if exception messages will be sent to a tty.
+ *  Returns +true+ if exception messages will be sent to a terminal device.
  */
 static VALUE
 exc_s_to_tty_p(VALUE self)
@@ -1620,20 +1648,51 @@ check_order_keyword(VALUE opt)
 
 /*
  * call-seq:
- *   exception.full_message(highlight: bool, order: [:top or :bottom]) ->  string
+ *   full_message(highlight: true, order: :top) -> string
  *
- * Returns formatted string of _exception_.
- * The returned string is formatted using the same format that Ruby uses
- * when printing an uncaught exceptions to stderr.
+ * Returns an enhanced message string:
  *
- * If _highlight_ is +true+ the default error handler will send the
- * messages to a tty.
+ * - Includes the exception class name.
+ * - If the value of keyword +highlight+ is true (not +nil+ or +false+),
+ *   includes bolding ANSI codes (see below) to enhance the appearance of the message.
+ * - Includes the {backtrace}[rdoc-ref:exceptions.md@Backtraces]:
  *
- * _order_ must be either of +:top+ or +:bottom+, and places the error
- * message and the innermost backtrace come at the top or the bottom.
+ *   - If the value of keyword +order+ is +:top+ (the default),
+ *     lists the error message and the innermost backtrace entry first.
+ *   - If the value of keyword +order+ is +:bottom+,
+ *     lists the error message the the innermost entry last.
  *
- * The default values of these options depend on <code>$stderr</code>
- * and its +tty?+ at the timing of a call.
+ * Example:
+ *
+ *   def baz
+ *     begin
+ *       1 / 0
+ *     rescue => x
+ *       pp x.message
+ *       pp x.full_message(highlight: false).split("\n")
+ *       pp x.full_message.split("\n")
+ *     end
+ *   end
+ *   def bar; baz; end
+ *   def foo; bar; end
+ *   foo
+ *
+ * Output:
+ *
+ *   "divided by 0"
+ *   ["t.rb:3:in `/': divided by 0 (ZeroDivisionError)",
+ *    "\tfrom t.rb:3:in `baz'",
+ *    "\tfrom t.rb:10:in `bar'",
+ *    "\tfrom t.rb:11:in `foo'",
+ *    "\tfrom t.rb:12:in `<main>'"]
+ *   ["t.rb:3:in `/': \e[1mdivided by 0 (\e[1;4mZeroDivisionError\e[m\e[1m)\e[m",
+ *    "\tfrom t.rb:3:in `baz'",
+ *    "\tfrom t.rb:10:in `bar'",
+ *    "\tfrom t.rb:11:in `foo'",
+ *    "\tfrom t.rb:12:in `<main>'"]
+ *
+ * An overrriding method should be careful with ANSI code enhancements;
+ * see {Messages}[rdoc-ref:exceptions.md@Messages].
  */
 
 static VALUE
@@ -1662,10 +1721,11 @@ exc_full_message(int argc, VALUE *argv, VALUE exc)
 
 /*
  * call-seq:
- *   exception.message   ->  string
+ *   message -> string
  *
- * Returns the result of invoking <code>exception.to_s</code>.
- * Normally this returns the exception's message or name.
+ * Returns #to_s.
+ *
+ * See {Messages}[rdoc-ref:exceptions.md@Messages].
  */
 
 static VALUE
@@ -1676,42 +1736,47 @@ exc_message(VALUE exc)
 
 /*
  * call-seq:
- *   exception.detailed_message(highlight: bool, **opt)   ->  string
+ *   detailed_message(highlight: false, **kwargs) -> string
  *
- * Processes a string returned by #message.
+ * Returns the message string with enhancements:
  *
- * It may add the class name of the exception to the end of the first line.
- * Also, when +highlight+ keyword is true, it adds ANSI escape sequences to
- * make the message bold.
+ * - Includes the exception class name in the first line.
+ * - If the value of keyword +highlight+ is +true+,
+ *   includes bolding and underlining ANSI codes (see below)
+ *   to enhance the appearance of the message.
  *
- * If you override this method, it must be tolerant for unknown keyword
- * arguments. All keyword arguments passed to #full_message are delegated
- * to this method.
+ * Examples:
  *
- * This method is overridden by did_you_mean and error_highlight to add
- * their information.
+ *   begin
+ *     1 / 0
+ *   rescue => x
+ *     p x.message
+ *     p x.detailed_message                  # Class name added.
+ *     p x.detailed_message(highlight: true) # Class name, bolding, and underlining added.
+ *   end
  *
- * A user-defined exception class can also define their own
- * +detailed_message+ method to add supplemental information.
- * When +highlight+ is true, it can return a string containing escape
- * sequences, but use widely-supported ones. It is recommended to limit
- * the following codes:
+ * Output:
  *
- * - Reset (+\e[0m+)
- * - Bold (+\e[1m+)
- * - Underline (+\e[4m+)
- * - Foreground color except white and black
- *   - Red (+\e[31m+)
- *   - Green (+\e[32m+)
- *   - Yellow (+\e[33m+)
- *   - Blue (+\e[34m+)
- *   - Magenta (+\e[35m+)
- *   - Cyan (+\e[36m+)
+ *   "divided by 0"
+ *   "divided by 0 (ZeroDivisionError)"
+ *   "\e[1mdivided by 0 (\e[1;4mZeroDivisionError\e[m\e[1m)\e[m"
  *
- * Use escape sequences carefully even if +highlight+ is true.
- * Do not use escape sequences to express essential information;
- * the message should be readable even if all escape sequences are
- * ignored.
+ * This method is overridden by some gems in the Ruby standard library to add information:
+ *
+ * - DidYouMean::Correctable#detailed_message.
+ * - ErrorHighlight::CoreExt#detailed_message.
+ * - SyntaxSuggest#detailed_message.
+ *
+ * An overriding method must be tolerant of passed keyword arguments,
+ * which may include (but may not be limited to):
+ *
+ * - +:highlight+.
+ * - +:did_you_mean+.
+ * - +:error_highlight+.
+ * - +:syntax_suggest+.
+ *
+ * An overrriding method should also be careful with ANSI code enhancements;
+ * see {Messages}[rdoc-ref:exceptions.md@Messages].
  */
 
 static VALUE
@@ -1730,9 +1795,15 @@ exc_detailed_message(int argc, VALUE *argv, VALUE exc)
 
 /*
  * call-seq:
- *   exception.inspect   -> string
+ *   inspect -> string
+ *
+ * Returns a string representation of +self+:
+ *
+ *   x = RuntimeError.new('Boom')
+ *   x.inspect # => "#<RuntimeError: Boom>"
+ *   x = RuntimeError.new
+ *   x.inspect # => "#<RuntimeError: RuntimeError>"
  *
- * Return this exception's class name and message.
  */
 
 static VALUE
@@ -1765,38 +1836,31 @@ exc_inspect(VALUE exc)
 
 /*
  *  call-seq:
- *     exception.backtrace    -> array or nil
+ *    backtrace -> array or nil
  *
- *  Returns any backtrace associated with the exception. The backtrace
- *  is an array of strings, each containing either ``filename:lineNo: in
- *  `method''' or ``filename:lineNo.''
+ *  Returns a backtrace value for +self+;
+ *  the returned value depends on the form of the stored backtrace value:
  *
- *     def a
- *       raise "boom"
- *     end
+ *  - \Array of Thread::Backtrace::Location objects:
+ *    returns the array of strings given by
+ *    <tt>Exception#backtrace_locations.map {|loc| loc.to_s }</tt>.
+ *    This is the normal case, where the backtrace value was stored by Kernel#raise.
+ *  - \Array of strings: returns that array.
+ *    This is the unusual case, where the backtrace value was explicitly
+ *    stored as an array of strings.
+ *  - +nil+: returns +nil+.
  *
- *     def b
- *       a()
- *     end
+ *  Example:
  *
- *     begin
- *       b()
- *     rescue => detail
- *       print detail.backtrace.join("\n")
- *     end
+ *    begin
+ *      1 / 0
+ *    rescue => x
+ *      x.backtrace.take(2)
+ *    end
+ *    # => ["(irb):132:in `/'", "(irb):132:in `<top (required)>'"]
  *
- *  <em>produces:</em>
- *
- *     prog.rb:2:in `a'
- *     prog.rb:6:in `b'
- *     prog.rb:10
- *
- *  In the case no backtrace has been set, +nil+ is returned
- *
- *    ex = StandardError.new
- *    ex.backtrace
- *    #=> nil
-*/
+ *  see {Backtraces}[rdoc-ref:exceptions.md@Backtraces].
+ */
 
 static VALUE
 exc_backtrace(VALUE exc)
@@ -1838,13 +1902,24 @@ rb_get_backtrace(VALUE exc)
 
 /*
  *  call-seq:
- *     exception.backtrace_locations    -> array or nil
+ *    backtrace_locations -> array or nil
  *
- *  Returns any backtrace associated with the exception. This method is
- *  similar to Exception#backtrace, but the backtrace is an array of
- *  Thread::Backtrace::Location.
+ *  Returns a backtrace value for +self+;
+ *  the returned value depends on the form of the stored backtrace value:
  *
- *  This method is not affected by Exception#set_backtrace().
+ *  - \Array of Thread::Backtrace::Location objects: returns that array.
+ *  - \Array of strings or +nil+: returns +nil+.
+ *
+ *  Example:
+ *
+ *    begin
+ *      1 / 0
+ *    rescue => x
+ *      x.backtrace_locations.take(2)
+ *    end
+ *    # => ["(irb):150:in `/'", "(irb):150:in `<top (required)>'"]
+ *
+ *  See {Backtraces}[rdoc-ref:exceptions.md@Backtraces].
  */
 static VALUE
 exc_backtrace_locations(VALUE exc)
@@ -1882,12 +1957,19 @@ rb_check_backtrace(VALUE bt)
 
 /*
  *  call-seq:
- *     exc.set_backtrace(backtrace)   ->  array
+ *    set_backtrace(value) -> value
  *
- *  Sets the backtrace information associated with +exc+. The +backtrace+ must
- *  be an array of Thread::Backtrace::Location objects or an array of String objects
- *  or a single String in the format described in Exception#backtrace.
+ *  Sets the backtrace value for +self+; returns the given +value:
  *
+ *    x = RuntimeError.new('Boom')
+ *    x.set_backtrace(%w[foo bar baz]) # => ["foo", "bar", "baz"]
+ *    x.backtrace                      # => ["foo", "bar", "baz"]
+ *
+ *  The given +value+ must be an array of strings, a single string, or +nil+.
+ *
+ *  Does not affect the value returned by #backtrace_locations.
+ *
+ *  See {Backtraces}[rdoc-ref:exceptions.md@Backtraces].
  */
 
 static VALUE
@@ -1911,12 +1993,35 @@ rb_exc_set_backtrace(VALUE exc, VALUE bt)
 }
 
 /*
- * call-seq:
- *   exception.cause   -> an_exception or nil
+ *  call-seq:
+ *    cause -> exception or nil
+ *
+ *  Returns the previous value of global variable <tt>$!</tt>,
+ *  which may be +nil+
+ *  (see {Global Variables}[rdoc-ref:exceptions.md@Global+Variables]):
+ *
+ *    begin
+ *      raise('Boom 0')
+ *    rescue => x0
+ *      puts "Exception: #{x0};  $!: #{$!};  cause: #{x0.cause.inspect}."
+ *      begin
+ *        raise('Boom 1')
+ *      rescue => x1
+ *        puts "Exception: #{x1};  $!: #{$!};  cause: #{x1.cause}."
+ *        begin
+ *          raise('Boom 2')
+ *        rescue => x2
+ *          puts "Exception: #{x2};  $!: #{$!};  cause: #{x2.cause}."
+ *        end
+ *      end
+ *    end
+ *
+ *  Output:
+ *
+ *    Exception: Boom 0;  $!: Boom 0;  cause: nil.
+ *    Exception: Boom 1;  $!: Boom 1;  cause: Boom 0.
+ *    Exception: Boom 2;  $!: Boom 2;  cause: Boom 1.
  *
- * Returns the previous exception ($!) at the time this exception was raised.
- * This is useful for wrapping exceptions and retaining the original exception
- * information.
  */
 
 static VALUE
@@ -1933,11 +2038,11 @@ try_convert_to_exception(VALUE obj)
 
 /*
  *  call-seq:
- *     exc == obj   -> true or false
+ *    self == object -> true or false
+ *
+ *  Returns whether +object+ is the same class as +self+
+ *  and its #message and #backtrace are equal to those of +self+.
  *
- *  Equality---If <i>obj</i> is not an Exception, returns
- *  <code>false</code>. Otherwise, returns <code>true</code> if <i>exc</i> and
- *  <i>obj</i> share same class, messages, and backtrace.
  */
 
 static VALUE
@@ -3248,66 +3353,30 @@ syserr_eqq(VALUE self, VALUE exc)
 /*
  *  Document-class: Exception
  *
- *  \Class Exception and its subclasses are used to communicate between
- *  Kernel#raise and +rescue+ statements in <code>begin ... end</code> blocks.
+ *  \Class +Exception+ and its subclasses are used to indicate that an error
+ *  or other problem has occurred,
+ *  and may need to be handled.
+ *  See {Exceptions}[rdoc-ref:exceptions.md].
  *
- *  An Exception object carries information about an exception:
- *  - Its type (the exception's class).
- *  - An optional descriptive message.
- *  - Optional backtrace information.
+ *  An +Exception+ object carries certain information:
  *
- *  Some built-in subclasses of Exception have additional methods: e.g., NameError#name.
+ *  - The type (the exception's class),
+ *    commonly StandardError, RuntimeError, or a subclass of one or the other;
+ *    see {Built-In Exception Class Hierarchy}[rdoc-ref:Exception@Built-In+Exception+Class+Hierarchy].
+ *  - An optional descriptive message;
+ *    see methods ::new, #message.
+ *  - Optional backtrace information;
+ *    see methods #backtrace, #backtrace_locations, #set_backtrace.
+ *  - An optional cause;
+ *    see method #cause.
  *
- *  == Defaults
+ *  == Built-In \Exception \Class Hierarchy
  *
- *  Two Ruby statements have default exception classes:
- *  - +raise+: defaults to RuntimeError.
- *  - +rescue+: defaults to StandardError.
- *
- *  == Global Variables
- *
- *  When an exception has been raised but not yet handled (in +rescue+,
- *  +ensure+, +at_exit+ and +END+ blocks), two global variables are set:
- *  - <code>$!</code> contains the current exception.
- *  - <code>$@</code> contains its backtrace.
- *
- *  == Custom Exceptions
- *
- *  To provide additional or alternate information,
- *  a program may create custom exception classes
- *  that derive from the built-in exception classes.
- *
- *  A good practice is for a library to create a single "generic" exception class
- *  (typically a subclass of StandardError or RuntimeError)
- *  and have its other exception classes derive from that class.
- *  This allows the user to rescue the generic exception, thus catching all exceptions
- *  the library may raise even if future versions of the library add new
- *  exception subclasses.
- *
- *  For example:
- *
- *    class MyLibrary
- *      class Error < ::StandardError
- *      end
- *
- *      class WidgetError < Error
- *      end
- *
- *      class FrobError < Error
- *      end
- *
- *    end
- *
- *  To handle both MyLibrary::WidgetError and MyLibrary::FrobError the library
- *  user can rescue MyLibrary::Error.
- *
- *  == Built-In Exception Classes
- *
- *  The built-in subclasses of Exception are:
+ *  The hierarchy of built-in subclasses of class +Exception+:
  *
  *  * NoMemoryError
  *  * ScriptError
- *    * LoadError
+ *    * {LoadError}[https://docs.ruby-lang.org/en/master/LoadError.html]
  *    * NotImplementedError
  *    * SyntaxError
  *  * SecurityError
@@ -3333,13 +3402,14 @@ syserr_eqq(VALUE self, VALUE exc)
  *    * RuntimeError
  *      * FrozenError
  *    * SystemCallError
- *      * Errno::*
+ *      * Errno (and its subclasses, representing system errors)
  *    * ThreadError
  *    * TypeError
  *    * ZeroDivisionError
  *  * SystemExit
  *  * SystemStackError
- *  * fatal
+ *  * {fatal}[https://docs.ruby-lang.org/en/master/fatal.html]
+ *
  */
 
 static VALUE

eval.c

@@ -746,32 +746,91 @@ rb_f_raise(int argc, VALUE *argv)
 
 /*
  *  call-seq:
- *     raise
- *     raise(string, cause: $!)
- *     raise(exception [, string [, array]], cause: $!)
- *     fail
- *     fail(string, cause: $!)
- *     fail(exception [, string [, array]], cause: $!)
+ *    raise(exception, message = exception.to_s, backtrace = nil, cause: $!)
+ *    raise(message = nil, cause: $!)
  *
- *  With no arguments, raises the exception in <code>$!</code> or raises
- *  a RuntimeError if <code>$!</code> is +nil+.  With a single +String+
- *  argument, raises a +RuntimeError+ with the string as a message. Otherwise,
- *  the first parameter should be an +Exception+ class (or another
- *  object that returns an +Exception+ object when sent an +exception+
- *  message).  The optional second parameter sets the message associated with
- *  the exception (accessible via Exception#message), and the third parameter
- *  is an array of callback information (accessible via
- *  Exception#backtrace_locations or Exception#backtrace).
- *  The +cause+ of the generated exception (accessible via Exception#cause)
- *  is automatically set to the "current" exception (<code>$!</code>), if any.
- *  An alternative value, either an +Exception+ object or +nil+, can be
- *  specified via the +:cause+ argument.
+ *  Raises an exception;
+ *  see {Exceptions}[rdoc-ref:exceptions.md].
  *
- *  Exceptions are caught by the +rescue+ clause of
- *  <code>begin...end</code> blocks.
+ *  Argument +exception+ sets the class of the new exception;
+ *  it should be class Exception or one of its subclasses
+ *  (most commonly, RuntimeError or StandardError),
+ *  or an instance of one of those classes:
  *
- *     raise "Failed to create socket"
- *     raise ArgumentError, "No parameters", caller_locations
+ *    begin
+ *      raise(StandardError)
+ *    rescue => x
+ *      p x.class
+ *    end
+ *    # => StandardError
+ *
+ *  Argument +message+ sets the stored message in the new exception,
+ *  which may be retrieved by method Exception#message;
+ *  the message must be
+ *  a {string-convertible object}[rdoc-ref:implicit_conversion.rdoc@String-Convertible+Objects]
+ *  or +nil+:
+ *
+ *    begin
+ *      raise(StandardError, 'Boom')
+ *    rescue => x
+ *      p x.message
+ *    end
+ *    # => "Boom"
+ *
+ *  If argument +message+ is not given,
+ *  the message is the exception class name.
+ *
+ *  See {Messages}[rdoc-ref:exceptions.md@Messages].
+ *
+ *  Argument +backtrace+ sets the stored backtrace in the new exception,
+ *  which may be retrieved by method Exception#backtrace;
+ *  the backtrace must be an array of strings or +nil+:
+ *
+ *    begin
+ *      raise(StandardError, 'Boom', %w[foo bar baz])
+ *    rescue => x
+ *      p x.backtrace
+ *    end
+ *    # => ["foo", "bar", "baz"]
+ *
+ *  If argument +backtrace+ is not given,
+ *  the backtrace is set according to an array of Thread::Backtrace::Location objects,
+ *  as derived from the call stack.
+ *
+ *  See {Backtraces}[rdoc-ref:exceptions.md@Backtraces].
+ *
+ *  Keyword argument +cause+ sets the stored cause in the new exception,
+ *  which may be retrieved by method Exception#cause;
+ *  the cause must be an exception object (Exception or one of its subclasses),
+ *  or +nil+:
+ *
+ *    begin
+ *      raise(StandardError, cause: RuntimeError.new)
+ *    rescue => x
+ *      p x.cause
+ *    end
+ *    # => #<RuntimeError: RuntimeError>
+ *
+ *  If keyword argument +cause+ is not given,
+ *  the cause is the value of <tt>$!</tt>.
+ *
+ *  See {Cause}[rdoc-ref:exceptions.md@Cause].
+ *
+ *  In the alternate calling sequence,
+ *  where argument +exception+ _not_ given,
+ *  raises a new exception of the class given by <tt>$!</tt>,
+ *  or of class RuntimeError if <tt>$!</tt> is +nil+:
+ *
+ *    begin
+ *      raise
+ *    rescue => x
+ *      p x
+ *    end
+ *    # => RuntimeError
+ *
+ *  With argument +exception+ not given,
+ *  argument +message+ and keyword argument +cause+ may be given,
+ *  but argument +backtrace+ may not be given.
  */
 
 static VALUE