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cowan

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ConditionsCowan

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== 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` predicates 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 [http://www.r6rs.org/final/html/r6rs-lib/r6rs-lib-Z-H-8.html#node_sec_7.2 the R6RS condition system] and its predecessors [http://srfi.schemers.org/srfi-35/srfi-35.html SRFI 35] and [http://srfi.schemers.org/srfi-36/srfi-36.html SRFI 36], and with [http://srfi.schemers.org/srfi-12/srfi-12.html 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.

== Constructor ==

`(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` is not a string; it is also an error if the value associated with the property name `irritants` is not a list.

== Predicates ==

`(condition? `''obj''`)`

Returns `#t` if ''obj'' is a condition of any type, and `#f` otherwise.

`(condition-of-type? `''obj'' ''sym''`)`

Returns `#t` if ''obj'' is a condition belonging to type ''sym'', 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 there is no such property, returns ''default''.  If ''default'' is not specified, returns `#f`.

`(condition-predicate `''sym''`)`

Returns a predicate which will return `#t` if applied to a condition belonging to type ''sym'', and `#f` otherwise.

`(condition-accessor `''condition'' ''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''`)`

Returns the value of the property named `<property-name>` (from the standard list of property names below) of ''condition'', or `#f` if there is no such property.


== 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 `syntax`, 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.

||`simple`||error created by `error`||
||`file`||file-related error||
||`read`||lexical syntax error||
||`syntax`||Scheme syntax error||
||`domain`||argument has wrong type or value||
||`mismatch`||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`||
||`range`||violation of start-end conditions||
||`input`||input error||
||`output`||output error||
||`invalid-position`||invalid file position||
||`coding`||encoding or decoding error||

== Standard property names ==

||`message`||human-readable description string||
||`irritants`||list of problematic arguments||

time

2012-12-23 03:22:06