Conditions­Cowan

Conditions

This is a WG2 proposal for condition objects. A condition object encapsulates information about an exceptional situation. Typically the rest of the system is notified about a condition using the raise and raise-continuable procedures and their relatives. Conditions are logically independent of the exception system, however: conditions may be used for any purpose, and any object may be passed to the exception system.

The design of this condition system attempts to assume as little as possible about any existing implementation-specific condition system. In particular, there is no specified relationship between conditions and records, as there is in R6RS, nor is there any notion of subtyping required by the system. There are condition types for convenience in dispatching, but they are just symbols and in general entail nothing about what information is encapsulated, as different implementations will provide different kinds of information when creating an implementation-specified condition.

Within the above constraints, I have attempted to make the names as compatible as possible with the R6RS condition system and its predecessors SRFI 35 and SRFI 36, and with SRFI 12.

A condition is said to be "belong to type sym" if (a) it was created by a call to make-condition that was passed the symbol sym as one of its condition types, or (b) it belongs to an implementation-defined set (possibly empty) of conditions of type sym. This allows implementation-dependent condition objects to participate in this condition system.

Constructors

(make-condition symlist ( sym obj ) ...)

Returns a newly allocated condition which belongs to the types whose names are given in symlist. The remainder of the arguments are alternating property names (which are symbols) and values (which can be any object) that specify the information encapsulated by this condition. It is an error if the value associated with the property name message (if it exists) is not a string; it is also an error if the value associated with the property name irritants (if it exists) is not a list.

(alist->condition symlist alist)

Returns a newly allocated condition which belongs to the types whose names are given in symlist. Its properties and their values are specified by alist.

Predicates

(condition? obj)

Returns #t if obj is a condition of any type, and #f otherwise.

(condition-of-type? obj symlist)

Returns #t if obj is a condition belonging to any of the types specified by symlist, and #f otherwise.

(<type>-error? obj)

Returns #t if obj is a condition belonging to type <type> from the list below, and #f otherwise.

Accessors

(condition-types condition)

Returns the list of types to which condition belongs. It is an error to attempt to mutate this list.

(condition-properties condition)

Returns the list of property names associated with this condition. It is an error to attempt to mutate this list.

(condition-ref condition sym [ default ] )

Returns the property value associated with the property named sym of condition. If it has no such property, returns default. If default is not specified, returns #f.

(condition-predicate symlist)

Returns a predicate which will return #t if applied to a condition belonging to any of the types specified in symlist, and #f otherwise.

(condition-accessor sym [ default ] )

Returns an accessor which will return the value of sym if applied to a condition object and default otherwise. If default is not specified, the accessor will return #f.

(condition-<property-name> condition [ default ])

Returns the value of the property named <property-name> (from the standard list of property names below) of condition, or default if it has no such property, or #f if no default is specified.

(condition->alist condition)

Return the properties and values of condition in the form of an alist.

Specific predicates

(error-object? obj)

Returns #t if obj is a condition belonging to type simple, and #f otherwise. Such conditions are normally created only by user code. Part of the small language, but shown here for completeness.

(file-error? obj)

Returns #t if obj is a condition belonging to type file, and #f otherwise. Such conditions may be created by the implementation if there is an error related to file operations; in particular, the inability to open a file for input. Part of the small language, but shown here for completeness.

(read-error? obj)

Returns #t if obj is a condition belonging to type read, and #f otherwise. Such conditions may be created by the implementation if there is an error related to read, such as a lexical syntax error in the input. Part of the small language, but shown here for completeness.

(syntax-error? obj)

Returns #t if obj is a condition belonging to type syntax, and #f otherwise. Such conditions may be created by the implementation if program code is syntactically ill-formed. When such a condition is raised, it may or may not be possible for the exception system to catch it. Part of the small language, but shown here for completeness.

(implementation-restriction? obj)

Returns #t if obj is a condition belonging to type implementation-restriction, and #f otherwise. Such conditions may be created by the implementation if one of its restrictions is exceeded, such as consuming too much memory or trying to compute an exact number too large to represent.

Standard condition types

The following condition types are standardized. Conditions of each type may be created by the implementation in the specified situations as well as any analogous situations. The only constraint is that the implementation must not raise a condition of a specified type unless that type of external situation is in fact present. The list is intended to be comprehensive but not complete: it draws on R6RS, Java, and other sources.

simple	error created by error
assert	error created by assert
file	file-related error
filename	mangled filename
protection	file protection error
input	input error
output	output error
network	socket or network error
closed	I/O operation on closed port
already-exists	file already exists
not-found	file not found
read	textual error during reading
lexical	lexical syntax error
eof	EOF inside a lexical construct
syntax	Scheme syntax error
domain	argument has wrong type or value
type	wrong type
value	wrong value
state	invalid state event
arithmetic	arithmetic error
divide	division by exact zero
fixnum	sufficiently small exact integer expected
conversion	attempted impossible conversion
improper-list	improper lists not supported
circular-list	circular lists not supported
arity	too many or too few arguments
immutability	modifying immutable data
undefined	getting the value of a variable that has not been defined
non-continuable	continuing from an exception raised by raise
match	unsatisfied pattern match
uncaught	uncaught exception
nonexistent	reference to something that does not exist
range	violation of start-end conditions
scheduler	task scheduler error
deadlock	scheduler deadlock
timeout	operation timed out
termination	thread termination
concurrency	invalid concurrent modification
invalid-position	invalid file position
encoding	encoding or decoding error
os	operating system reported error
implementation-restriction	implementation restriction
memory	out of memory
unsupported	unsupported operation
no-infinities	implementation does not support infinities
no-nans	implementation does not support NaNs
continuation	escape procedure invoked when not supported
bignum	exact number too large to represent
version-skew	mismatched versions of code
security	security violation
low-level-error	problem with the implementation
implementation-restriction	the implementation has insufficient resources though the program is correct

Standard property names

message	human-readable description string
irritants	list of problematic arguments
who	an object reporting a problem with another object
what	an object which has a problem
position	the position in what at which the problem occurred
subcondition	a condition embedded in this one which has more details