Foundation instead of in the original English.
\1f
+File: lispref.info, Node: Error Symbols, Prev: Handling Errors, Up: Errors
+
+Error Symbols and Condition Names
+.................................
+
+ When you signal an error, you specify an "error symbol" to specify
+the kind of error you have in mind. Each error has one and only one
+error symbol to categorize it. This is the finest classification of
+errors defined by the XEmacs Lisp language.
+
+ These narrow classifications are grouped into a hierarchy of wider
+classes called "error conditions", identified by "condition names".
+The narrowest such classes belong to the error symbols themselves: each
+error symbol is also a condition name. There are also condition names
+for more extensive classes, up to the condition name `error' which
+takes in all kinds of errors. Thus, each error has one or more
+condition names: `error', the error symbol if that is distinct from
+`error', and perhaps some intermediate classifications.
+
+ In other words, each error condition "inherits" from another error
+condition, with `error' sitting at the top of the inheritance hierarchy.
+
+ - Function: define-error error-symbol error-message &optional
+ inherits-from
+ This function defines a new error, denoted by ERROR-SYMBOL.
+ ERROR-MESSAGE is an informative message explaining the error, and
+ will be printed out when an unhandled error occurs. ERROR-SYMBOL
+ is a sub-error of INHERITS-FROM (which defaults to `error').
+
+ `define-error' internally works by putting on ERROR-SYMBOL an
+ `error-message' property whose value is ERROR-MESSAGE, and an
+ `error-conditions' property that is a list of ERROR-SYMBOL
+ followed by each of its super-errors, up to and including `error'.
+ You will sometimes see code that sets this up directly rather than
+ calling `define-error', but you should _not_ do this yourself,
+ unless you wish to maintain compatibility with FSF Emacs, which
+ does not provide `define-error'.
+
+ Here is how we define a new error symbol, `new-error', that belongs
+to a range of errors called `my-own-errors':
+
+ (define-error 'my-own-errors "A whole range of errors" 'error)
+ (define-error 'new-error "A new error" 'my-own-errors)
+
+`new-error' has three condition names: `new-error', the narrowest
+classification; `my-own-errors', which we imagine is a wider
+classification; and `error', which is the widest of all.
+
+ Note that it is not legal to try to define an error unless its
+super-error is also defined. For instance, attempting to define
+`new-error' before `my-own-errors' are defined will signal an error.
+
+ The error string should start with a capital letter but it should
+not end with a period. This is for consistency with the rest of Emacs.
+
+ Naturally, XEmacs will never signal `new-error' on its own; only an
+explicit call to `signal' (*note Signaling Errors::) in your code can
+do this:
+
+ (signal 'new-error '(x y))
+ error--> A new error: x, y
+
+ This error can be handled through any of the three condition names.
+This example handles `new-error' and any other errors in the class
+`my-own-errors':
+
+ (condition-case foo
+ (bar nil t)
+ (my-own-errors nil))
+
+ The significant way that errors are classified is by their condition
+names--the names used to match errors with handlers. An error symbol
+serves only as a convenient way to specify the intended error message
+and list of condition names. It would be cumbersome to give `signal' a
+list of condition names rather than one error symbol.
+
+ By contrast, using only error symbols without condition names would
+seriously decrease the power of `condition-case'. Condition names make
+it possible to categorize errors at various levels of generality when
+you write an error handler. Using error symbols alone would eliminate
+all but the narrowest level of classification.
+
+ *Note Standard Errors::, for a list of all the standard error symbols
+and their conditions.
+
+\1f
File: lispref.info, Node: Cleanups, Prev: Errors, Up: Nonlocal Exits
Cleaning Up from Nonlocal Exits
appropriate for data pertaining to where the file came from or how to
save it, rather than with how to edit the contents.
-\1f
-File: lispref.info, Node: Default Value, Prev: Creating Buffer-Local, Up: Buffer-Local Variables
-
-The Default Value of a Buffer-Local Variable
---------------------------------------------
-
- The global value of a variable with buffer-local bindings is also
-called the "default" value, because it is the value that is in effect
-except when specifically overridden.
-
- The functions `default-value' and `setq-default' access and change a
-variable's default value regardless of whether the current buffer has a
-buffer-local binding. For example, you could use `setq-default' to
-change the default setting of `paragraph-start' for most buffers; and
-this would work even when you are in a C or Lisp mode buffer that has a
-buffer-local value for this variable.
-
- The special forms `defvar' and `defconst' also set the default value
-(if they set the variable at all), rather than any local value.
-
- - Function: default-value symbol
- This function returns SYMBOL's default value. This is the value
- that is seen in buffers that do not have their own values for this
- variable. If SYMBOL is not buffer-local, this is equivalent to
- `symbol-value' (*note Accessing Variables::).
-
- - Function: default-boundp symbol
- The function `default-boundp' tells you whether SYMBOL's default
- value is nonvoid. If `(default-boundp 'foo)' returns `nil', then
- `(default-value 'foo)' would get an error.
-
- `default-boundp' is to `default-value' as `boundp' is to
- `symbol-value'.
-
- - Special Form: setq-default symbol value
- This sets the default value of SYMBOL to VALUE. It does not
- evaluate SYMBOL, but does evaluate VALUE. The value of the
- `setq-default' form is VALUE.
-
- If a SYMBOL is not buffer-local for the current buffer, and is not
- marked automatically buffer-local, `setq-default' has the same
- effect as `setq'. If SYMBOL is buffer-local for the current
- buffer, then this changes the value that other buffers will see
- (as long as they don't have a buffer-local value), but not the
- value that the current buffer sees.
-
- ;; In buffer `foo':
- (make-local-variable 'local)
- => local
- (setq local 'value-in-foo)
- => value-in-foo
- (setq-default local 'new-default)
- => new-default
- local
- => value-in-foo
- (default-value 'local)
- => new-default
-
- ;; In (the new) buffer `bar':
- local
- => new-default
- (default-value 'local)
- => new-default
- (setq local 'another-default)
- => another-default
- (default-value 'local)
- => another-default
-
- ;; Back in buffer `foo':
- local
- => value-in-foo
- (default-value 'local)
- => another-default
-
- - Function: set-default symbol value
- This function is like `setq-default', except that SYMBOL is
- evaluated.
-
- (set-default (car '(a b c)) 23)
- => 23
- (default-value 'a)
- => 23
-
projects / chise / xemacs-chise.git- / blobdiff