SRFI 35: Conditions

# Title

Conditions

# Authors

Richard Kelsey and Michael Sperber

# Status

This SRFI is currently in ``final'' status. To see an explanation of each status that a SRFI can hold, see here. It will remain in draft until 2002-10-20, or as amended. to provide input on this SRFI, please mail to `srfi minus 35 at srfi dot schemers dot org`. See instructions here to subscribe to the list. You can access the discussion via the archive of the mailing list. You can access post-finalization messages via the archive of the mailing list.

- Draft: 2002/07/24-2002/10/20
- Revised: 2002/09/20
- Final: 2002/12/1

# Abstract

The SRFI defines constructs for creating and inspecting condition types and values. A condition value encapsulates information about an exceptional situation, or exception. This SRFI also defines a few basic condition types.

# Rationale

Conditions are values that communicate information about exceptional situations between parts of a program. Code that detects an exception may be in a different part of the program than the code that handles it. In fact, the former may have been written independently from the latter. Consequently, to facilitate effective handling of exceptions, conditions must communicate as much information as possible as accurately as possible, and still allow effective handling by code that did not precisely anticipate the nature of the exception that occurred.

This SRFI provides two mechanisms to enable this kind of communication

- subtyping among condition types allows handling code to determine the general nature of an exception even though it does not anticipate its exact nature,
- compound conditions allow an exceptional situation to be described in multiple ways.

# Specification

Conditions are records with named fields. Each condition belongs to one or more condition types. Each condition type specifies a set of field names. A condition belonging to a condition type includes a value for each of the type's field names. These values can be extracted from the condition by using the appropriate field name.

There is a tree of condition types with the distinguished `&condition` as its root. All other condition types have a parent condition type.

A condition belonging to several condition types with a common supertype may have distinct values for the supertype's fields for each type. The type used to access a field determines which of the values is returned. The program can extract each of these field values separately.

## Procedures

`(make-condition-type` id parent field-names`)`

  `Make-condition-type` returns a new condition type. Id must be a symbol that serves as a symbolic name for the condition type. Parent must itself be a condition type. Field-names must be a list of symbols. It identifies the fields of the conditions associated with the condition type.

  Field-names must be disjoint from the field names of parent and its ancestors.

`(condition-type?` thing`)`

  `Condition-type?` is a predicate for condition types: it returns `#t` if thing is a condition type, and `#f` otherwise

`(make-condition` type field-name value ...`)`

  `Make-condition` creates a condition value belonging condition type type. The following arguments must be, in turn, a field name and an arbitrary value. There must be such a pair for each field of type and its direct and indirect supertypes. `Make-condition` returns the condition value, with the argument values associated with their respective fields.

`(condition?` thing`)`

  `Condition?` is a predicate for conditions: it returns `#t` if thing is a condition type, and `#f` otherwise

`(condition-has-type?` condition condition-type`)`

  `Condition-has-type?` tests if condition condition belongs to condition type condition-type. It returns `#t` if any of condition 's types includes condition-type eitherdirectlyorasanancestorand `#f` otherwise.

  It is an error if condition is not a condition, or if condition-type is not a condition type.

`(condition-ref` condition field-name`)`

  Condition must be a condition, and field-name a symbol. Moreover, condition must belong to a condition type which has a field name called field-name, or one of its (direct or indirect) supertypes must have the field. `Condition-ref` returns the value associated with field-name.

  It is an error to refer to a field the condition does not have.

`(make-compound-condition` condition0 condition1 ...`)`

  `Make-compound-condition` returns a compound condition belonging to all condition types that the conditioni belong to.

  `Condition-ref`, when applied to a compound condition will return the value from the first of the conditioni that has such a field.

`(extract-condition` condition condition-type`)`

  Condition must be a condition belonging to condition-type. `Extract-condition` returns a condition of condition type condition-type with the field values specified by condition.

  If condition is a compound condition, `extract-condition` extracts the field values from the subcondition belonging to condition-type that appeared first in the call to `make-compound-condition` that created the condition. The returned condition may be newly created; it is possible for

      (let* ((&c (make-condition-type 'c &condition '()))
             (c0 (make-condition &c))
             (c1 (make-compound-condition c0)))
        (eq? c0 (extract-condition c1 &c)))

  to return false.

## Macros

`(define-condition-type` <condition-type> <supertype> <predicate> <field-spec> ...`)`

  This defines a new condition type. <Condition-type>, <supertypes>, and <predicate> must all be identifiers. `Define-condition-type` defines an identifier <condition-type> to some value describing the condition type. <supertype> must be the name of a previously defined condition type.

  `Define-condition-type` also defines <predicate> to a predicate that identifies conditions associated with that type, or with any of its subtypes.

  Each <field-spec> must be of the form `(` <field> <accessor>`)` where both <field> and <accessor> must be identifiers. `Define-condition-type` defines each <accessor> to a procedure which extracts the value of the named field from a condition associated with this condition type.

`(condition` <type-field-binding> ...`)`

  This creates a condition value. Each <type-field-binding> must be of the form `(` <condition-type> <field-binding> ...`)` Each <field-binding> must be of the form `(` <field> <exp>`)` where <field> is a field identifier from the definition of <condition-type>.

  The <exp> are evaluated in some unspecified order; their values can later be extracted from the condition object via the accessors of the associated condition types or their supertypes.

  The condition returned by `condition` is created by a call of form

      (make-compound-condition
        (make-condition <condition-type> '<field-name> <value>...)
        ...)

  with the condition types retaining their order from the`condition` form. The field names and values are duplicated as necessary as described below.

  Each <type-field-binding> must contain field bindings for all fields of <condition-type> without duplicates. There is an exception to this rule: if a field binding is missing, and the field belongs to a supertype shared with one of the other <type-field-binding> subforms, then the value defaults to that of the first such binding in the `condition` form.

## Standard Conditions

`&condition`

  This is the root of the entire condition type hierarchy. It has a no fields.

`&message`

  This condition type could be defined by

      (define-condition-type &message &condition
        message-condition?
        (message condition-message))

  It carries a message further describing the nature of the condition to humans.

`&serious`

  This condition type could be defined by

      (define-condition-type &serious &condition
        serious-condition?)

  This type describes conditions serious enough that they cannot safely be ignored. This condition type is primarily intended as a supertype of other condition types.

`&error`

  This condition type could be defined by

      (define-condition-type &error &serious
        error?)

  This condition describes errors, typically caused by something that has gone wrong in the interaction of the program with the external world or the user.

# Examples

    (define-condition-type &c &condition
      c?
      (x c-x))

    (define-condition-type &c1 &c
      c1?
      (a c1-a))

    (define-condition-type &c2 &c
      c2?
      (b c2-b))
    (define v1 (make-condition &c1 'x "V1" 'a "a1"))

    (c? v1)        => #t
    (c1? v1)       => #t
    (c2? v1)       => #f
    (c-x v1)       => "V1"
    (c1-a v1)      => "a1"

    (define v2 (condition (&c2
                            (x "V2")
                            (b "b2"))))

    (c? v2)        => #t
    (c1? v2)       => #f
    (c2? v2)       => #t
    (c-x v2)       => "V2"
    (c2-b v2)      => "b2"

    (define v3 (condition (&c1
                           (x "V3/1")
                           (a "a3"))
                          (&c2
                           (b "b3"))))

    (c? v3)        => #t
    (c1? v3)       => #t
    (c2? v3)       => #t
    (c-x v3)       => "V3/1"
    (c1-a v3)      => "a3"
    (c2-b v3)      => "b3"

    (define v4 (make-compound-condition v1 v2))

    (c? v4)        => #t
    (c1? v4)       => #t
    (c2? v4)       => #t
    (c-x v4)       => "V1"
    (c1-a v4)      => "a1"
    (c2-b v4)      => "b2"

    (define v5 (make-compound-condition v2 v3))

    (c? v5)        => #t
    (c1? v5)       => #t
    (c2? v5)       => #t
    (c-x v5)       => "V2"
    (c1-a v5)      => "a3"
    (c2-b v5)      => "b2"

# Reference Implementation

The reference implementation makes use of SRFI 1 ("List Library"), SRFI 9 ("Defining Record Types"), and SRFI 23 ("Error reporting mechanism").

    (define-record-type :condition-type
      (really-make-condition-type name supertype fields all-fields)
      condition-type?
      (name condition-type-name)
      (supertype condition-type-supertype)
      (fields condition-type-fields)
      (all-fields condition-type-all-fields))

    (define (make-condition-type name supertype fields)
      (if (not (symbol? name))
          (error "make-condition-type: name is not a symbol"
                 name))
      (if (not (condition-type? supertype))
          (error "make-condition-type: supertype is not a condition type"
                 supertype))
      (if (not
           (null? (lset-intersection eq?
                                     (condition-type-all-fields supertype)
                                     fields)))
          (error "duplicate field name" ))
      (really-make-condition-type name
                                  supertype
                                  fields
                                  (append (condition-type-all-fields supertype)
                                          fields)))

    (define-syntax define-condition-type
      (syntax-rules ()
        ((define-condition-type ?name ?supertype ?predicate
           (?field1 ?accessor1) ...)
         (begin
           (define ?name
             (make-condition-type '?name
                                  ?supertype
                                  '(?field1 ...)))
           (define (?predicate thing)
             (and (condition? thing)
                  (condition-has-type? thing ?name)))
           (define (?accessor1 condition)
             (condition-ref (extract-condition condition ?name)
                            '?field1))
           ...))))

    (define (condition-subtype? subtype supertype)
      (let recur ((subtype subtype))
        (cond ((not subtype) #f)
              ((eq? subtype supertype) #t)
              (else
               (recur (condition-type-supertype subtype))))))

    (define (condition-type-field-supertype condition-type field)
      (let loop ((condition-type condition-type))
        (cond ((not condition-type) #f)
              ((memq field (condition-type-fields condition-type))
               condition-type)
              (else
               (loop (condition-type-supertype condition-type))))))

    ; The type-field-alist is of the form
    ; ((<type> (<field-name> . <value>) ...) ...)
    (define-record-type :condition
      (really-make-condition type-field-alist)
      condition?
      (type-field-alist condition-type-field-alist))

    (define (make-condition type . field-plist)
      (let ((alist (let label ((plist field-plist))
                     (if (null? plist)
                                '()
                         (cons (cons (car plist)
                                     (cadr plist))
                               (label (cddr plist)))))))
        (if (not (lset= eq?
                        (condition-type-all-fields type)
                        (map car alist)))
            (error "condition fields don't match condition type"))
        (really-make-condition (list (cons type alist)))))

    (define (condition-has-type? condition type)
      (any (lambda (has-type)
             (condition-subtype? has-type type))
           (condition-types condition)))

    (define (condition-ref condition field)
      (type-field-alist-ref (condition-type-field-alist condition)
                            field))

    (define (type-field-alist-ref type-field-alist field)
      (let loop ((type-field-alist type-field-alist))
        (cond ((null? type-field-alist)
               (error "type-field-alist-ref: field not found"
                      type-field-alist field))
              ((assq field (cdr (car type-field-alist)))
               => cdr)
              (else
               (loop (cdr type-field-alist))))))

    (define (make-compound-condition condition-1 . conditions)
      (really-make-condition
       (apply append (map condition-type-field-alist
                          (cons condition-1 conditions)))))

    (define (extract-condition condition type)
      (let ((entry (find (lambda (entry)
                                  (condition-subtype? (car entry) type))
                                (condition-type-field-alist condition))))
        (if (not entry)
            (error "extract-condition: invalid condition type"
                          condition type))
        (really-make-condition
          (list (cons type
                      (map (lambda (field)
                             (assq field (cdr entry)))
                           (condition-type-all-fields type)))))))

    (define-syntax condition
      (syntax-rules ()
        ((condition (?type1 (?field1 ?value1) ...) ...)
         (type-field-alist->condition
          (list
           (cons ?type1
                 (list (cons '?field1 ?value1) ...))
           ...)))))

    (define (type-field-alist->condition type-field-alist)
      (really-make-condition
       (map (lambda (entry)
              (cons (car entry)
                    (map (lambda (field)
                           (or (assq field (cdr entry))
                               (cons field
                                     (type-field-alist-ref type-field-alist field))))
                         (condition-type-all-fields (car entry)))))
            type-field-alist)))

    (define (condition-types condition)
      (map car (condition-type-field-alist condition)))

    (define (check-condition-type-field-alist the-type-field-alist)
      (let loop ((type-field-alist the-type-field-alist))
        (if (not (null? type-field-alist))
            (let* ((entry (car type-field-alist))
                   (type (car entry))
                   (field-alist (cdr entry))
                   (fields (map car field-alist))
                   (all-fields (condition-type-all-fields type)))
              (for-each (lambda (missing-field)
                          (let ((supertype
                                 (condition-type-field-supertype type missing-field)))
                            (if (not
                                 (any (lambda (entry)
                                        (let ((type (car entry)))
                                          (condition-subtype? type supertype)))
                                      the-type-field-alist))
                                (error "missing field in condition construction"
                                       type
                                       missing-field))))
                        (lset-difference eq? all-fields fields))
              (loop (cdr type-field-alist))))))

    (define &condition (really-make-condition-type '&condition
                                                   #f
                                                   '()
                                                   '()))

    (define-condition-type &message &condition
      message-condition?
      (message condition-message))

    (define-condition-type &serious &condition
      serious-condition?)

    (define-condition-type &error &serious
      error?)

# References

- SRFI 12: Exception Handling by William Clinger, R. Kent Dybvig, Matthew Flatt, and Marc Feeley
- Richard Kelsey's 1996 proposal
- Kent Pitman's history paper
- The Conditions chapter from the Common Lisp HyperSpec
- The Conditions chapter by Kent M. Pitman in Common Lisp the Language, 2nd edition by Guy L. Steele
- The Conditions chapter in the Dylan Reference Manual

# Copyright

Copyright (C) Richard Kelsey, Michael Sperber (2002). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.