On this page:
10.2.1 Error Message Conventions
10.2.2 Raising Exceptions
raise
error
raise-user-error
raise-argument-error
raise-argument-error*
raise-result-error
raise-result-error*
raise-arguments-error
raise-arguments-error*
raise-range-error
raise-range-error*
raise-type-error
raise-mismatch-error
raise-arity-error
raise-arity-error*
raise-arity-mask-error
raise-arity-mask-error*
raise-result-arity-error
raise-result-arity-error*
raise-syntax-error
unquoted-printing-string?
unquoted-printing-string
unquoted-printing-string-value
10.2.3 Handling Exceptions
call-with-exception-handler
uncaught-exception-handler
with-handlers
with-handlers*
10.2.4 Configuring Default Handling
error-escape-handler
error-display-handler
error-print-width
error-print-context-length
error-print-source-location
error-value->string-handler
error-syntax->string-handler
10.2.5 Built-in Exception Types
exn
exn:  fail
exn:  fail:  contract
exn:  fail:  contract:  arity
exn:  fail:  contract:  divide-by-zero
exn:  fail:  contract:  non-fixnum-result
exn:  fail:  contract:  continuation
exn:  fail:  contract:  variable
exn:  fail:  syntax
exn:  fail:  syntax:  unbound
exn:  fail:  syntax:  missing-module
exn:  fail:  read
exn:  fail:  read:  eof
exn:  fail:  read:  non-char
exn:  fail:  filesystem
exn:  fail:  filesystem:  exists
exn:  fail:  filesystem:  version
exn:  fail:  filesystem:  errno
exn:  fail:  filesystem:  missing-module
exn:  fail:  network
exn:  fail:  network:  errno
exn:  fail:  out-of-memory
exn:  fail:  unsupported
exn:  fail:  user
exn:  break
exn:  break:  hang-up
exn:  break:  terminate
prop:  exn:  srclocs
exn:  srclocs?
exn:  srclocs-accessor
srcloc
srcloc->string
prop:  exn:  missing-module
exn:  missing-module?
exn:  missing-module-accessor
10.2.6 Additional Exception Functions
exn->string
10.2.7 Realms and Error Message Adjusters
error-message->adjusted-string
error-contract->adjusted-string
current-error-message-adjuster
error-message-adjuster-key

10.2 Exceptions🔗

+Exceptions in The Racket Guide introduces exceptions.

See Exceptions for information on the Racket exception model. It is based on a proposal by Friedman, Haynes, and Dybvig [Friedman95].

Whenever a primitive error occurs in Racket, an exception is raised. The value that is passed to the current exception handler for a primitive error is always an instance of the exn structure type. Every exn structure value has a message field that is a string, the primitive error message. The default exception handler recognizes exception values with the exn? predicate and passes the error message to the current error display handler (see error-display-handler).

Primitive procedures that accept a procedure argument with a particular required arity (e.g., call-with-input-file, call/cc) check the argument’s arity immediately, raising exn:fail:contract if the arity is incorrect.

10.2.1 Error Message Conventions🔗

Racket’s error message convention is to produce error messages with the following shape:

srcloc: name: message;
 continued-message ...
  field: detail
  ...

The message starts with an optional source location, srcloc, which is followed by a colon and space when present. The message continues with an optional name that usually identifies the complaining function, syntactic form, or other entity, but may also refer to an entity being complained about; the name is also followed by a colon and space when present.

The message should be relatively short, and it should be largely independent of specific values that triggered the error. More detailed explanation that requires multiple lines should continue with each line indented by a single space, in which case message should end in a semi-colon (but the semi-colon should be omitted if continued-message is not present). Message text should be lowercase—using semi-colons to separate sentences if needed, although long explanations may be better deferred to extra fields.

Specific values that triggered the error or other helpful information should appear in separate field lines, each of which is indented by two spaces. If a detail is especially long or takes multiple lines, it should start on its own line after the field label, and each of its lines should be indented by three spaces. Field names should be all lowercase.

A field name should end with ... if the field provides relatively detailed information that might be distracting in common cases but useful in others. For example, when a contract failure is reported for a particular argument of a function, other arguments to the function might be shown in an “other arguments...” field. The intent is that fields whose names end in ... might be hidden by default in an environment such as DrRacket.

Make field names as short as possible, relying on message or continued message text to clarify the meaning for a field. For example, prefer “given” to “given turtle” as a field name, where message is something like “given turtle is too sleepy” to clarify that “given” refers to a turtle.

10.2.2 Raising Exceptions🔗

procedure

(raise v [barrier?])  any

  v : any/c
  barrier? : any/c = #t
Raises an exception, where v represents the exception being raised. The v argument can be anything; it is passed to the current exception handler.

If barrier? is true, then the call to the exception handler is protected by a continuation barrier, so that multiple returns/escapes are impossible. All exceptions raised by racket functions effectively use raise with a #t value for barrier?.

Breaks are disabled from the time the exception is raised until the exception handler obtains control, and the handler itself is parameterize-breaked to disable breaks initially; see Breaks for more information on breaks.

Examples:
> (with-handlers ([number? (lambda (n)
                             (+ n 5))])
    (raise 18 #t))

23

> (struct my-exception exn:fail:user ())
> (with-handlers ([my-exception? (lambda (e)
                                   #f)])
    (+ 5 (raise (my-exception
                 "failed"
                 (current-continuation-marks)))))

#f

> (raise 'failed #t)

uncaught exception: failed

procedure

(error message-sym)  any

  message-sym : symbol?
(error message-str v ...)  any
  message-str : string?
  v : any/c
(error who-sym format-str v ...)  any
  who-sym : symbol?
  format-str : string?
  v : any/c
Raises the exception exn:fail, which contains an error string. The different forms produce the error string in different ways:

In all cases, the constructed message string is passed to make-exn:fail, and the resulting exception is raised.

Examples:
> (error 'failed)

error: failed

> (error "failed" 23 'pizza (list 1 2 3))

failed 23 'pizza '(1 2 3)

> (error 'method-a "failed because ~a" "no argument supplied")

method-a: failed because no argument supplied

procedure

(raise-user-error message-sym)  any

  message-sym : symbol?
(raise-user-error message-str v ...)  any
  message-str : string?
  v : any/c
(raise-user-error who-sym format-str v ...)  any
  who-sym : symbol?
  format-str : string?
  v : any/c
Like error, but constructs an exception with make-exn:fail:user instead of make-exn:fail. The default error display handler does not show a “stack trace” for exn:fail:user exceptions (see Continuation Marks), so raise-user-error should be used for errors that are intended for end users.

procedure

(raise-argument-error name expected v)  any

  name : symbol?
  expected : string?
  v : any/c
(raise-argument-error name    
  expected    
  bad-pos    
  v ...)  any
  name : symbol?
  expected : string?
  bad-pos : exact-nonnegative-integer?
  v : any/c
Creates an exn:fail:contract value and raises it as an exception. The name argument is used as the source procedure’s name in the error message. The expected argument is used as a description of the expected contract (i.e., as a string, but the string is intended to contain a contract expression).

In the first form, v is the value received by the procedure that does not have the expected type.

In the second form, the bad argument is indicated by an index bad-pos (counting from 0), and all of the original arguments v are provided (in order). The resulting error message names the bad argument and also lists the other arguments. If bad-pos is not less than the number of vs, the exn:fail:contract exception is raised.

The error message generated by raise-argument-error is adjusted via error-contract->adjusted-string and then error-message->adjusted-string using the default 'racket realm.

Examples:
> (define (feed-machine bits)
    (unless (integer? bits)
      (raise-argument-error 'feed-machine "integer?" bits))
    "fed the machine")
> (feed-machine 'turkey)

feed-machine: contract violation

  expected: integer?

  given: 'turkey

> (define (feed-cow animal)
    (unless (eq? animal 'cow)
      (raise-argument-error 'feed-cow "'cow" animal))
    "fed the cow")
> (feed-cow 'turkey)

feed-cow: contract violation

  expected: 'cow

  given: 'turkey

> (define (feed-animals cow sheep goose cat)
    (unless (eq? goose 'goose)
      (raise-argument-error 'feed-animals "'goose" 2 cow sheep goose cat))
    "fed the animals")
> (feed-animals 'cow 'sheep 'dog 'cat)

feed-animals: contract violation

  expected: 'goose

  given: 'dog

  argument position: 3rd

  other arguments...:

   'cow

   'sheep

   'cat

procedure

(raise-argument-error* name realm expected v)  any

  name : symbol?
  realm : symbol?
  expected : string?
  v : any/c
(raise-argument-error* name    
  realm    
  expected    
  bad-pos    
  v ...)  any
  name : symbol?
  realm : symbol?
  expected : string?
  bad-pos : exact-nonnegative-integer?
  v : any/c
Like raise-argument-error, but using the given realm for error-message adjustments.

Added in version 8.4.0.2 of package base.

procedure

(raise-result-error name expected v)  any

  name : symbol?
  expected : string?
  v : any/c
(raise-result-error name    
  expected    
  bad-pos    
  v ...)  any
  name : symbol?
  expected : string?
  bad-pos : exact-nonnegative-integer?
  v : any/c
Like raise-argument-error, but the error message describe v as a “result” instead of an “argument.”

procedure

(raise-result-error* name realm expected v)  any

  name : symbol?
  realm : symbol?
  expected : string?
  v : any/c
(raise-result-error* name    
  realm    
  expected    
  bad-pos    
  v ...)  any
  name : symbol?
  realm : symbol?
  expected : string?
  bad-pos : exact-nonnegative-integer?
  v : any/c
Like raise-result-error, but using the given realm for error-message adjustments.

Added in version 8.4.0.2 of package base.

procedure

(raise-arguments-error name    
  message    
  field    
  v ...    
  ...)  any
  name : symbol?
  message : string?
  field : string?
  v : any/c
Creates an exn:fail:contract value and raises it as an exception. The name is used as the source procedure’s name in the error message. The message is the error message; if message contains newline characters, each extra line should be suitably indented (with one extra space at the start of each line), but it should not end with a newline character. Each field must have a corresponding v, and the two are rendered on their own line in the error message; each v is formatted using the error value conversion handler (see error-value->string-handler), unless v is a unquoted-printing string, in which case the string content is displayed without using the error value conversion handler.

The error message generated by raise-arguments-error is adjusted via error-message->adjusted-string using the default 'racket realm.

Example:
> (raise-arguments-error 'eat
                         "fish is smaller than its given meal"
                         "fish" 12
                         "meal" 13)

eat: fish is smaller than its given meal

  fish: 12

  meal: 13

procedure

(raise-arguments-error* name    
  realm    
  message    
  field    
  v ...    
  ...)  any
  name : symbol?
  realm : symbol?
  message : string?
  field : string?
  v : any/c
Like raise-arguments-error, but using the given realm for error-message adjustments.

Added in version 8.4.0.2 of package base.

procedure

(raise-range-error name    
  type-description    
  index-prefix    
  index    
  in-value    
  lower-bound    
  upper-bound    
  [alt-lower-bound])  any
  name : symbol?
  type-description : string?
  index-prefix : string?
  index : exact-integer?
  in-value : any/c
  lower-bound : exact-integer?
  upper-bound : exact-integer?
  alt-lower-bound : (or/c #f exact-integer?) = #f
Creates an exn:fail:contract value and raises it as an exception to report an out-of-range error. The type-description string describes the value for which the index is meant to select an element, and index-prefix is a prefix for the word “index.” The index argument is the rejected index. The in-value argument is the value for which the index was meant. The lower-bound and upper-bound arguments specify the valid range of indices, inclusive; if upper-bound is below lower-bound, the value is characterized as “empty.” If alt-lower-bound is not #f, and if index is between alt-lower-bound and upper-bound, then the error is report as index being less than the “starting” index lower-bound.

Since upper-bound is inclusive, a typical value is one less than the size of a collection—for example, (sub1 (vector-length vec)), (sub1 (length lst)), and so on.

The error message generated by raise-range-error is adjusted via error-message->adjusted-string using the default 'racket realm.

Examples:
> (raise-range-error 'vector-ref "vector" "starting " 5 #(1 2 3 4) 0 3)

vector-ref: starting index is out of range

  starting index: 5

  valid range: [0, 3]

  vector: '#(1 2 3 4)

> (raise-range-error 'vector-ref "vector" "ending " 5 #(1 2 3 4) 0 3)

vector-ref: ending index is out of range

  ending index: 5

  valid range: [0, 3]

  vector: '#(1 2 3 4)

> (raise-range-error 'vector-ref "vector" "" 3 #() 0 -1)

vector-ref: index is out of range for empty vector

  index: 3

> (raise-range-error 'vector-ref "vector" "ending " 1 #(1 2 3 4) 2 3 0)

vector-ref: ending index is smaller than starting index

  ending index: 1

  starting index: 2

  valid range: [0, 3]

  vector: '#(1 2 3 4)

procedure

(raise-range-error* name    
  realm    
  type-description    
  index-prefix    
  index    
  in-value    
  lower-bound    
  upper-bound    
  [alt-lower-bound])  any
  name : symbol?
  realm : symbol?
  type-description : string?
  index-prefix : string?
  index : exact-integer?
  in-value : any/c
  lower-bound : exact-integer?
  upper-bound : exact-integer?
  alt-lower-bound : (or/c #f exact-integer?) = #f
Like raise-range-error, but using the given realm for error-message adjustments.

Added in version 8.4.0.2 of package base.

procedure

(raise-type-error name expected v)  any

  name : symbol?
  expected : string?
  v : any/c
(raise-type-error name expected bad-pos v ...)  any
  name : symbol?
  expected : string?
  bad-pos : exact-nonnegative-integer?
  v : any/c
Like raise-argument-error, but with Racket’s old formatting conventions, and where expected is used as a “type” description instead of a contract expression. Use raise-argument-error or raise-result-error, instead.

The error message generated by raise-type-error is adjusted via error-message->adjusted-string using the default 'racket realm.

procedure

(raise-mismatch-error name    
  message    
  v ...+    
  ...+)  any
  name : symbol?
  message : string?
  v : any/c
Similar to raise-arguments-error, but using Racket’s old formatting conventions, with a required v immediately after the first message string, and with further message strings that are spliced into the message without line breaks or space. Use raise-arguments-error, instead.

The error message generated by raise-mismatch-error is adjusted via error-message->adjusted-string using the default 'racket realm.

procedure

(raise-arity-error name arity-v arg-v ...)  any

  name : (or/c symbol? procedure?)
  arity-v : 
(or/c exact-nonnegative-integer?
      arity-at-least?
      (listof
       (or/c exact-nonnegative-integer?
             arity-at-least?)))
  arg-v : any/c
Creates an exn:fail:contract:arity value and raises it as an exception. The name is used for the source procedure’s name in the error message.

The arity-v value must be a possible result from procedure-arity, except that it does not have to be normalized (see procedure-arity? for the details of normalized arities); raise-arity-error will normalize the arity and use the normalized form in the error message. If name is a procedure, its actual arity is ignored.

The arg-v arguments are the actual supplied arguments, which are shown in the error message (using the error value conversion handler; see error-value->string-handler); also, the number of supplied arg-vs is explicitly mentioned in the message.

The error message generated by raise-arity-error is adjusted via error-message->adjusted-string using the default 'racket realm.

Example:
> (raise-arity-error 'unite (arity-at-least 13) "Virginia" "Maryland")

unite: arity mismatch;

 the expected number of arguments does not match the given

number

  expected: at least 13

  given: 2

  arguments...:

   "Virginia"

   "Maryland"

procedure

(raise-arity-error* name    
  realm    
  arity-v    
  arg-v ...)  any
  name : (or/c symbol? procedure?)
  realm : symbol?
  arity-v : 
(or/c exact-nonnegative-integer?
      arity-at-least?
      (listof
       (or/c exact-nonnegative-integer?
             arity-at-least?)))
  arg-v : any/c

Like raise-arity-error, but using the given realm for error-message adjustments.

Added in version 8.4.0.2 of package base.

procedure

(raise-arity-mask-error name mask arg-v ...)  any

  name : (or/c symbol? procedure?)
  mask : exact-integer?
  arg-v : any/c
The same as raise-arity-error, but using the arity representation described with procedure-arity-mask.

Added in version 7.0.0.11 of package base.

procedure

(raise-arity-mask-error* name    
  realm    
  mask    
  arg-v ...)  any
  name : (or/c symbol? procedure?)
  realm : symbol?
  mask : exact-integer?
  arg-v : any/c
Like raise-arity-mask-error, but using the given realm for error-message adjustments.

Added in version 8.4.0.2 of package base.

procedure

(raise-result-arity-error name    
  arity-v    
  detail-str    
  result-v ...)  any
  name : (or/c symbol? #f)
  arity-v : exact-nonnegative-integer?
  detail-str : (or/c string? #f)
  result-v : any/c
Like raise-arity-error, but reports a “result” mismatch instead of an “argument” mismatch. The name argument can be #f to omit an initial source for the error. The detail-str argument, if non-#f, should be a string that starts with a newline, since it is added near the end of the generated error message.

The error message generated by raise-result-arity-error is adjusted via error-message->adjusted-string using the default 'racket realm.

Example:
> (raise-result-arity-error 'let-values 2 "\n  in: example" 'a 2.0 "three")

let-values: result arity mismatch;

 expected number of values not received

  expected: 2

  received: 3

  in: example

  arguments...:

   'a

   2.0

   "three"

Added in version 6.90.0.26 of package base.

procedure

(raise-result-arity-error* name    
  realm    
  arity-v    
  detail-str    
  result-v ...)  any
  name : (or/c symbol? #f)
  realm : symbol?
  arity-v : exact-nonnegative-integer?
  detail-str : (or/c string? #f)
  result-v : any/c
Like raise-result-arity-error, but using the given realm for error-message adjustments.

Added in version 8.4.0.2 of package base.

procedure

(raise-syntax-error name    
  message    
  [expr    
  sub-expr    
  extra-sources    
  message-suffix    
  #:exn exn])  any
  name : (or/c symbol? #f)
  message : string?
  expr : any/c = #f
  sub-expr : any/c = #f
  extra-sources : (listof syntax?) = null
  message-suffix : string? = ""
  exn : 
(-> string?
    continuation-mark-set?
    (listof syntax?)
    exn:fail:syntax?)
 = exn:fail:syntax
Creates an exn:fail:syntax? value and raises it as an exception. Macros use this procedure to report syntax errors.

The name argument is usually #f when expr is provided; it is described in more detail below. The message is used as the main body of the error message; if message contains newline characters, each new line should be suitably indented (with one space at the start), and it should not end with a newline character.

The optional expr argument is the erroneous source syntax object or S-expression (but the expression #f cannot be represented by itself; it must be wrapped as a syntax object). The optional sub-expr argument is a syntax object or S-expression (again, #f cannot represent itself) within expr that more precisely locates the error. Both may appear in the generated error-message text if error-print-source-location is #t. Source location information in the error-message text is similarly extracted from sub-expr or expr when at least one is a syntax object and error-print-source-location is #t.

If sub-expr is provided and not #f, it is used (in syntax form) for the exprs field of the generated exception record, else the expr is used if provided and not #f. In either case, the syntax object is consed onto extra-sources to produce the exprs field, or extra-sources is used directly for exprs if neither expr nor sub-expr is provided and not #f. The extra-sources argument is also used directly for exprs in the unusual case that the sub-expr or expr that would be included in exprs cannot be converted to a syntax object (because it contains a cycle).

The form name used in the generated error message is determined through a combination of the name, expr, and sub-expr arguments:

The message-suffix string is appended to the end of the error message. If not "", it should normally start with a newline and two spaces to add extra fields to the message (see Error Message Conventions).

If specified, exn should be a constructor or function that has the same signature as the exn:fail:syntax constructor.

Examples:
> (raise-syntax-error #f "bad syntax" '(bad syntax))

?: bad syntax

  in: (bad syntax)

> (raise-syntax-error #f "unbound identifier" 'unbound-id #:exn exn:fail:syntax:unbound)

?: unbound identifier

  in: unbound-id

Changed in version 6.90.0.18 of package base: Added the message-suffix optional argument.
Changed in version 8.4.0.6: Added the exn optional argument.

An unquoted-printing string wraps a string and prints, writes, and displays the same way that the string displays. An unquoted-printing string is especially useful with raise-arguments-error to serve as a field “value” that causes literal text to be printed as the field content.

The unquoted-printing-string? procedure returns #t if v is a unquoted-printing string, #f otherwise. The unquoted-printing-string creates a unquoted-printing string value that encapsulates the string s, and unquoted-printing-string-value returns the string within a unquoted-printing string.

Added in version 6.10.0.2 of package base.

10.2.3 Handling Exceptions🔗

procedure

(call-with-exception-handler f thunk)  any

  f : (any/c . -> . any)
  thunk : (-> any)
Installs f as the exception handler for the dynamic extent of the call to thunk. If an exception is raised during the evaluation of thunk (in an extension of the current continuation that does not have its own exception handler), then f is applied to the raised value in the continuation of the raise call (but the continuation is normally extended with a continuation barrier; see Prompts, Delimited Continuations, and Barriers and raise).

Any procedure that takes one argument can be an exception handler. Normally, an exception handler escapes from the context of the raise call via abort-current-continuation or some other escape mechanism. To propagate an exception to the “previous” exception handler—that is, the exception handler associated with the rest of the continuation after the point where the called exception handler was associated with the continuation—an exception handler can simply return a result instead of escaping, in which case the raise call propagates the value to the previous exception handler (still in the dynamic extent of the call to raise, and under the same barrier, if any). If an exception handler returns a result and no previous handler is available, the uncaught-exception handler is used.

A call to an exception handler is parameterize-breaked to disable breaks, and it is wrapped with call-with-exception-handler to install an exception handler that reports both the original and newly raised exceptions via the error display handler and then escapes via the error escape handler.

parameter

(uncaught-exception-handler)  (any/c . -> . any)

(uncaught-exception-handler f)  void?
  f : (any/c . -> . any)
A parameter that determines an uncaught-exception handler used by raise when the relevant continuation has no exception handler installed with call-with-exception-handler or with-handlers. Unlike exception handlers installed with call-with-exception-handler, the uncaught-exception handler must not return a value when called by raise; if it returns, an exception is raised (to be handled by an exception handler that reports both the original and newly raised exception).

The default uncaught-exception handler prints an error message using the current error display handler (see error-display-handler), unless the argument to the handler is an instance of exn:break:hang-up. If the argument to the handler is an instance of exn:break:hang-up or exn:break:terminate, the default uncaught-exception handler then calls the exit handler with 1, which normally exits or escapes. For any argument, the default uncaught-exception handler then escapes by calling the current error escape handler (see error-escape-handler). The call to each handler is parameterized to set error-display-handler to the default error display handler, and it is parameterize-breaked to disable breaks. The call to the error escape handler is further parameterized to set error-escape-handler to the default error escape handler; if the error escape handler returns, then the default error escape handler is called.

When the current error display handler is the default handler, then the error-display call is parameterized to install an emergency error display handler that logs an error (see log-error) and never fails.

syntax

(with-handlers ([pred-expr handler-expr] ...)
  body ...+)
Evaluates each pred-expr and handler-expr in the order that they are specified, and then evaluates the bodys with a new exception handler during its dynamic extent.

The new exception handler processes an exception only if one of the pred-expr procedures returns a true value when applied to the exception, otherwise the exception handler is invoked from the continuation of the with-handlers expression (by raising the exception again). If an exception is handled by one of the handler-expr procedures, the result of the entire with-handlers expression is the return value of the handler.

When an exception is raised during the evaluation of bodys, each predicate procedure pred-expr is applied to the exception value; if a predicate returns a true value, the corresponding handler-expr procedure is invoked with the exception as an argument. The predicates are tried in the order that they are specified.

Before any predicate or handler procedure is invoked, the continuation of the entire with-handlers expression is restored, but also parameterize-breaked to disable breaks. Thus, breaks are disabled by default during the predicate and handler procedures (see Breaks), and the exception handler is the one from the continuation of the with-handlers expression.

The exn:fail? procedure is useful as a handler predicate to catch all error exceptions. Avoid using (lambda (x) #t) as a predicate, because the exn:break exception typically should not be caught (unless it will be re-raised to cooperatively break). Beware, also, of catching and discarding exceptions, because discarding an error message can make debugging unnecessarily difficult; instead of discarding an error message, consider logging it via log-error or a logging form created by define-logger.

Examples:
> (with-handlers ([exn:fail:syntax?
                   (λ (e) (displayln "got a syntax error"))])
    (raise-syntax-error #f "a syntax error"))

got a syntax error

> (with-handlers ([exn:fail:syntax?
                   (λ (e) (displayln "got a syntax error"))]
                  [exn:fail?
                   (λ (e) (displayln "fallback clause"))])
    (raise-syntax-error #f "a syntax error"))

got a syntax error

syntax

(with-handlers* ([pred-expr handler-expr] ...)
  body ...+)
Like with-handlers, but if a handler-expr procedure is called, breaks are not explicitly disabled, and the handler call is in tail position with respect to the with-handlers* form.

10.2.4 Configuring Default Handling🔗

parameter

(error-escape-handler)  (-> any)

(error-escape-handler proc)  void?
  proc : (-> any)
A parameter for the error escape handler, which takes no arguments and escapes from the dynamic context of an exception. The default error escape handler escapes using (abort-current-continuation (default-continuation-prompt-tag) void).

The error escape handler is normally called directly by an exception handler, in a parameterization that sets the error display handler and error escape handler to the default handlers, and it is normally parameterize-breaked to disable breaks. To escape from a run-time error in a different context, use raise or error.

Due to a continuation barrier around exception-handling calls, an error escape handler cannot invoke a full continuation that was created prior to the exception, but it can abort to a prompt (see call-with-continuation-prompt) or invoke an escape continuation (see call-with-escape-continuation).

parameter

(error-display-handler)  (string? any/c . -> . any)

(error-display-handler proc)  void?
  proc : (string? any/c . -> . any)
A parameter for the error display handler, which is called by the default exception handler with an error message and the exception value. More generally, the handler’s first argument is a string to print as an error message, and the second is a value representing a raised exception. An error display handler can print errors in different ways, but it should always print to the current error port.

The default error display handler displays its first argument to the current error port (determined by the current-error-port parameter) and extracts a stack trace (see continuation-mark-set->context) to display from the second argument if it is an exn value but not an exn:fail:user value.

The default error display handler in DrRacket also uses the second argument to highlight source locations.

To report a run-time error, use raise or procedures like error, instead of calling the error display handler directly.

parameter

(error-print-width)  (and/c exact-integer? (>=/c 3))

(error-print-width width)  void?
  width : (and/c exact-integer? (>=/c 3))
A parameter whose value is used as the maximum number of characters used to print a Racket value that is embedded in a primitive error message.

A parameter whose value is used by the default error display handler as the maximum number of lines of context (or “stack trace”) to print; a single “...” line is printed if more lines are available after the first cnt lines. A 0 value for cnt disables context printing entirely.

parameter

(error-print-source-location)  boolean?

(error-print-source-location include?)  void?
  include? : any/c
A parameter that controls whether read and syntax error messages include source information, such as the source line and column or the expression. This parameter also controls the error message when a module-defined variable is accessed before its definition is executed; the parameter determines whether the message includes a module name. Only the message field of an exn:fail:read, exn:fail:syntax, or exn:fail:contract:variable structure is affected by the parameter. The default is #t.

parameter

(error-value->string-handler)

  
(any/c exact-nonnegative-integer?
       . -> .
       string?)
(error-value->string-handler proc)  void?
  proc : 
(any/c exact-nonnegative-integer?
       . -> .
       string?)
A parameter that determines the error value conversion handler, which is used to print a Racket value that is embedded in a primitive error message.

The integer argument to the handler specifies the maximum number of characters that should be used to represent the value in the resulting string. The default error value conversion handler prints the value into a string (using the current global port print handler; see global-port-print-handler). If the printed form is too long, the printed form is truncated and the last three characters of the return string are set to “...”.

When called by function like error, if the string returned by an error value conversion handler is longer than requested, the string is truncated to the requested length. If a byte string is returned instead of a string, it is converted using bytes->string/utf-8. If any other non-string value is returned, then the string "..." is used. If a primitive error string needs to be generated before the handler has returned, the default error value conversion handler is used.

Calls to an error value conversion handler are parameterized to re-install the default error value conversion handler, and to enable printing of unreadable values (see print-unreadable).

parameter

(error-syntax->string-handler)

  
(any/c (or/c exact-nonnegative-integer? #f)
      . -> .
      string?)
(error-syntax->string-handler proc)  void?
  proc : 
(any/c (or/c exact-nonnegative-integer? #f)
      . -> .
      string?)
A parameter that determines the error syntax conversion handler, which is used to print a syntax form that is embedded in an error message, such as from raise-syntax-error when error-print-source-location is #t.

The arguments to the handler are analogous to the arguments for a error value conversion handler as configured with error-value->string-handler, except that #f can be provided instead of an integer for the length, meaning that the printed form should not be truncated. The first argument is normally a syntax object, but in the same way that raise-syntax-error accepts other S-expressions, the error syntax conversion handler must also handle representations that are not syntax objects.

Added in version 8.2.0.8 of package base.

10.2.5 Built-in Exception Types🔗

struct

(struct exn (message continuation-marks)
    #:extra-constructor-name make-exn
    #:transparent)
  message : string?
  continuation-marks : continuation-mark-set?
The base structure type for exceptions. The message field contains an error message, and the continuation-marks field contains the value produced by (current-continuation-marks) immediately before the exception was raised.

Exceptions raised by Racket form a hierarchy under exn:

exn
  exn:fail
    exn:fail:contract
      exn:fail:contract:arity
      exn:fail:contract:divide-by-zero
      exn:fail:contract:non-fixnum-result
      exn:fail:contract:continuation
      exn:fail:contract:variable
    exn:fail:syntax
      exn:fail:syntax:unbound
      exn:fail:syntax:missing-module
    exn:fail:read
      exn:fail:read:eof
      exn:fail:read:non-char
    exn:fail:filesystem
      exn:fail:filesystem:exists
      exn:fail:filesystem:version
      exn:fail:filesystem:errno
      exn:fail:filesystem:missing-module
    exn:fail:network
      exn:fail:network:errno
    exn:fail:out-of-memory
    exn:fail:unsupported
    exn:fail:user
  exn:break
    exn:break:hang-up
    exn:break:terminate

struct

(struct exn:fail exn ()
    #:extra-constructor-name make-exn:fail
    #:transparent)
Raised for exceptions that represent errors, as opposed to exn:break.

struct

(struct exn:fail:contract exn:fail ()
    #:extra-constructor-name make-exn:fail:contract
    #:transparent)
Raised for errors from the inappropriate run-time use of a function or syntactic form.

struct

(struct exn:fail:contract:arity exn:fail:contract ()
    #:extra-constructor-name make-exn:fail:contract:arity
    #:transparent)
Raised when a procedure is applied to the wrong number of arguments.

struct

(struct exn:fail:contract:divide-by-zero exn:fail:contract ()
    #:extra-constructor-name
    make-exn:fail:contract:divide-by-zero
    #:transparent)
Raised for division by exact zero.

struct

(struct exn:fail:contract:non-fixnum-result exn:fail:contract ()
    #:extra-constructor-name
    make-exn:fail:contract:non-fixnum-result
    #:transparent)
Raised by functions like fx+ when the result would not be a fixnum.

struct

(struct exn:fail:contract:continuation exn:fail:contract ()
    #:extra-constructor-name make-exn:fail:contract:continuation
    #:transparent)
Raised when a continuation is applied where the jump would cross a continuation barrier.

struct

(struct exn:fail:contract:variable exn:fail:contract (id)
    #:extra-constructor-name make-exn:fail:contract:variable
    #:transparent)
  id : symbol?
Raised for a reference to a not-yet-defined top-level variable or module-level variable.

struct

(struct exn:fail:syntax exn:fail (exprs)
    #:extra-constructor-name make-exn:fail:syntax
    #:transparent)
  exprs : (listof syntax?)
Raised for a syntax error that is not a read error. The exprs indicate the relevant source expressions, least-specific to most-specific.

This structure type implements the prop:exn:srclocs property.

struct

(struct exn:fail:syntax:unbound exn:fail:syntax ()
    #:extra-constructor-name make-exn:fail:syntax:unbound
    #:transparent)
Raised by #%top or set! for an unbound identifier within a module.

struct

(struct exn:fail:syntax:missing-module exn:fail:syntax (path)
    #:extra-constructor-name make-exn:fail:syntax:missing-module
    #:transparent)
  path : module-path?
Raised by the default module name resolver or default load handler to report a module path—a reported in the path field—whose implementation file cannot be found.

The default module name resolver raises this exception only when it is given a syntax object as its second argument, and the default load handler raises this exception only when the value of current-module-path-for-load is a syntax object (in which case both the exprs field and the path field are determined by the syntax object).

This structure type implements the prop:exn:missing-module property.

struct

(struct exn:fail:read exn:fail (srclocs)
    #:extra-constructor-name make-exn:fail:read
    #:transparent)
  srclocs : (listof srcloc?)
Raised for a read error. The srclocs indicate the relevant source expressions.

struct

(struct exn:fail:read:eof exn:fail:read ()
    #:extra-constructor-name make-exn:fail:read:eof
    #:transparent)
Raised for a read error, specifically when the error is due to an unexpected end-of-file.

struct

(struct exn:fail:read:non-char exn:fail:read ()
    #:extra-constructor-name make-exn:fail:read:non-char
    #:transparent)
Raised for a read error, specifically when the error is due to an unexpected non-character (i.e., “special”) element in the input stream.

struct

(struct exn:fail:filesystem exn:fail ()
    #:extra-constructor-name make-exn:fail:filesystem
    #:transparent)
Raised for an error related to the filesystem (such as a file not found).

struct

(struct exn:fail:filesystem:exists exn:fail:filesystem ()
    #:extra-constructor-name make-exn:fail:filesystem:exists
    #:transparent)
Raised for an error when attempting to create a file that exists already.

struct

(struct exn:fail:filesystem:version exn:fail:filesystem ()
    #:extra-constructor-name make-exn:fail:filesystem:version
    #:transparent)
Raised for a version-mismatch error when loading an extension.

struct

(struct exn:fail:filesystem:errno exn:fail:filesystem (errno)
    #:extra-constructor-name make-exn:fail:filesystem:errno
    #:transparent)
  errno : (cons/c exact-integer? (or/c 'posix 'windows 'gai))
Raised for a filesystem error for which a system error code is available. The symbol part of an errno field indicates the category of the error code: 'posix indicates a C/Posix errno value, 'windows indicates a Windows system error code (under Windows, only), and 'gai indicates a getaddrinfo error code (which shows up only in exn:fail:network:errno exceptions for operations that resolve hostnames, but is allowed in exn:fail:filesystem:errno instances for consistency).

struct

(struct exn:fail:filesystem:missing-module exn:fail:filesystem
  (path)
    #:extra-constructor-name
    make-exn:fail:filesystem:missing-module
    #:transparent)
  path : module-path?
Raised by the default module name resolver or default load handler to report a module path—a reported in the path field—whose implementation file cannot be found.

The default module name resolver raises this exception only when it is not given a syntax object as its second argument, and the default load handler raises this exception only when the value of current-module-path-for-load is not a syntax object.

This structure type implements the prop:exn:missing-module property.

struct

(struct exn:fail:network exn:fail ()
    #:extra-constructor-name make-exn:fail:network
    #:transparent)
Raised for TCP and UDP errors.

struct

(struct exn:fail:network:errno exn:fail:network (errno)
    #:extra-constructor-name make-exn:fail:network:errno
    #:transparent)
  errno : (cons/c exact-integer? (or/c 'posix 'windows 'gai))
Raised for a TCP or UDP error for which a system error code is available, where the errno field is as for exn:fail:filesystem:errno.

struct

(struct exn:fail:out-of-memory exn:fail ()
    #:extra-constructor-name make-exn:fail:out-of-memory
    #:transparent)
Raised for an error due to insufficient memory, in cases where sufficient memory is at least available for raising the exception.

struct

(struct exn:fail:unsupported exn:fail ()
    #:extra-constructor-name make-exn:fail:unsupported
    #:transparent)
Raised for an error due to an unsupported feature on the current platform or configuration.

struct

(struct exn:fail:user exn:fail ()
    #:extra-constructor-name make-exn:fail:user
    #:transparent)
Raised for errors that are intended to be seen by end users. In particular, the default error printer does not show the program context when printing the error message.

struct

(struct exn:break exn (continuation)
    #:extra-constructor-name make-exn:break
    #:transparent)
  continuation : continuation?
Raised asynchronously (when enabled) in response to a break request. The continuation field can be used to resume the interrupted computation in the uncaught-exception handler or call-with-exception-handler (but not with-handlers because it escapes from the exception context before evaluating any predicates or handlers).

struct

(struct exn:break:hang-up exn:break ()
    #:extra-constructor-name make-exn:break:hang-up
    #:transparent)
Raised asynchronously for hang-up breaks. The default uncaught-exception handler reacts to this exception type by calling the exit handler.

struct

(struct exn:break:terminate exn:break ()
    #:extra-constructor-name make-exn:break:terminate
    #:transparent)
Raised asynchronously for termination-request breaks. The default uncaught-exception handler reacts to this exception type by calling the exit handler.

A property that identifies structure types that provide a list of srcloc values. The property is normally attached to structure types used to represent exception information.

The property value must be a procedure that accepts a single value—the structure type instance from which to extract source locations—and returns a list of srclocs. Some error display handlers use only the first returned location.

As an example,
#lang racket
 
;; We create a structure that supports the
;; prop:exn:srcloc protocol.  It carries
;; with it the location of the syntax that
;; is guilty.
(struct exn:fail:he-who-shall-not-be-named exn:fail
  (a-srcloc)
  #:property prop:exn:srclocs
  (lambda (a-struct)
    (match a-struct
      [(exn:fail:he-who-shall-not-be-named msg marks a-srcloc)
       (list a-srcloc)])))
 
;; We can play with this by creating a form that
;; looks at identifiers, and only flags specific ones.
(define-syntax (skeeterize stx)
  (syntax-case stx ()
    [(_ expr)
     (cond
       [(and (identifier? #'expr)
             (eq? (syntax-e #'expr) 'voldemort))
        (quasisyntax/loc stx
          (raise (exn:fail:he-who-shall-not-be-named
                  "oh dear don't say his name"
                  (current-continuation-marks)
                  (srcloc '#,(syntax-source #'expr)
                          '#,(syntax-line #'expr)
                          '#,(syntax-column #'expr)
                          '#,(syntax-position #'expr)
                          '#,(syntax-span #'expr)))))]
       [else
        ;; Otherwise, leave the expression alone.
        #'expr])]))
 
(define (f x)
  (* (skeeterize x) x))
 
(define (g voldemort)
  (* (skeeterize voldemort) voldemort))
 
;; Examples:
(f 7)
(g 7)
;; The error should highlight the use
;; of voldemort in g.

procedure

(exn:srclocs? v)  boolean?

  v : any/c
Returns #t if v has the prop:exn:srclocs property, #f otherwise.

procedure

(exn:srclocs-accessor v)

  (exn:srclocs? . -> . (listof srcloc))
  v : exn:srclocs?
Returns the srcloc-getting procedure associated with v.

struct

(struct srcloc (source line column position span)
    #:extra-constructor-name make-srcloc
    #:transparent)
  source : any/c
  line : (or/c exact-positive-integer? #f)
  column : (or/c exact-nonnegative-integer? #f)
  position : (or/c exact-positive-integer? #f)
  span : (or/c exact-nonnegative-integer? #f)
A source location is most frequently represented by a srcloc structure. More generally, a source location has the same information as a srcloc structure, but potentially represented or accessed differently. For example, source-location information is accessed from a syntax object with functions like syntax-source and syntax-line, while datum->syntax accepts a source location as a list, vector, or another syntax object. For ports, a combination of object-name and port-next-location provides location information, especially in a port for which counting has been enabled through port-count-lines!.

The fields of a srcloc instance are as follows:

See Printing Compiled Code for information about the treatment of srcloc values that are embedded in compiled code.

procedure

(srcloc->string srcloc)  (or/c string? #f)

  srcloc : srcloc?
Formats srcloc as a string suitable for error reporting. A path source in srcloc is shown relative to the value of current-directory-for-user. The result is #f if srcloc does not contain enough information to format a string.

A property that identifies structure types that provide a module path for a load that fails because a module is not found.

The property value must be a procedure that accepts a single value—the structure type instance from which to extract source locations—and returns a module path.

procedure

(exn:missing-module? v)  boolean?

  v : any/c
Returns #t if v has the prop:exn:missing-module property, #f otherwise.

Returns the module path-getting procedure associated with v.

10.2.6 Additional Exception Functions🔗

 (require racket/exn) package: base
The bindings documented in this section are provided by the racket/exn library, not racket/base or racket.

Added in version 6.3 of package base.

procedure

(exn->string exn)  string?

  exn : (or/c exn? any/c)
Formats exn as a string. If exn is an exn?, collects and returns the output from the current (error-display-handler); otherwise, simply converts exn to a string using (format "~s\n" exn).

10.2.7 Realms and Error Message Adjusters🔗

A realm identifies a convention for naming functions and specifying contracts for function arguments and results. Realms are intended to help improve layering and interoperability among languages that are implemented on top of Racket.

Realms primarily enable a language to recognize and rewrite error messages that are generated by lower layers of an implementation. For example, a language’s implementation of “arrays” might use Racket vectors directly, but when an object-type or primitive bounds check fails for a vector, the generated error message mentions “vector” and possibly a contract like vector? and a function name like vector-ref. Since these error messages are identified as being from the 'racket/primitive realm, a language implementation can look for 'racket/primitive to detect and rewrite error messages with minimal danger of mangling error messages from other parts of an application (possibly implemented in the new language) that happen to use the word “vector.”

Each procedure and each module also has a realm. A procedure’s realm is relevant, for example, when it is applied to the wrong number of arguments; in that case, the arity-error message itself is from the 'racket/primitive realm, but the error message also should include the name of the procedure, which can be from some different realm. Along similar lines, continuation-mark-set->context can report the realm associated with (the procedure for) each frame in a continuation, which might be useful to identify boundary crossings.

The construction of an error message must cooperate explicitly with error-message adjusting. The most basic may to cooperate is through functions like error-message->adjusted-string and error-contract->adjusted-string, which run error-message adjusters via the current-error-message-adjuster parameter and other adjusters associated with the current continuation using error-message-adjuster-key as a continuation-mark key. Functions like raise-argument-error and raise-arity-error use error-message->adjusted-string and error-contract->adjusted-string with the default realm, 'racket. Functions like raise-argument-error* and raise-arity-error* accept an explicit realm argument.

Not all error functions automatically cooperate with error-message adjusting. For example, the raise-reader-error and raise-syntax-error functions do not call adjusters, because they report errors that are intimately tied to syntax (and, along those lines, errors of a more static nature).

procedure

(error-message->adjusted-string name    
  name-realm    
  message    
  message-realm)  string?
  name : (or/c symbol? #f)
  name-realm : symbol?
  message : string?
  message-realm : symbol?
Combines name (if it is not #f) with ": " and then message to generate an error-message string, but first giving error-message adjusters a chance to adjust name and/or message.

Any adjuster functions associated with the current continuation as a continuation mark with error-message-adjuster-key are run first; the adjusters are run in order from shallowest to deepest. Then, the adjuster value of current-error-message-adjuster is used.

Each adjuster is tried with the 'message protocol, first. If the adjuster responds with #f for 'message, then the 'name protocol is tried. See current-error-message-adjuster for information on the protocols. An adjuster that responds with #f for both is skipped, as is any value associated as continuation mark using error-message-adjuster-key where the value is not a procedure that accepts one argument. In addition, the 'name protocol is skipped if the (possibly adjusted) name is #f.

Added in version 8.4.0.2 of package base.

procedure

(error-contract->adjusted-string contract-str    
  contract-realm)  string?
  contract-str : string?
  contract-realm : symbol?
Analogous to error-message->adjusted-string, but for just the contract part of an error message. The result string is typically incorporated into a larger error message that may then be adjusted further.

Adjustment of contract string uses the 'contract protocol as described for current-error-message-adjuster.

Added in version 8.4.0.2 of package base.

parameter

(current-error-message-adjuster)

  (symbol? . -> . (or/c procedure? #f))
(current-error-message-adjuster proc)  void?
  proc : (symbol? . -> . (or/c procedure? #f))
A parameter that determines an error-message adjuster that is applied after any adjusters associated to the current continuation via error-message-adjuster-key.

An adjuster procedure receives a symbol identifying a protocol, and it must return either #f or a procedure for performing adjustments through that protocol. The following protocols are currently defined, but more may be added in the future:

A new library or language can introduce additional mode symbols, too. To avoid conflicts, prefix the mode symbol with a collection or library name followed by /.

If an adjuster procedure returns #f for a protocol, it’s the same as returning a function that performs no adjustment and returns its arguments. The default value of this parameter returns #f for any symbol argument except the protocols listed above, for which it returns a procedure that checks its arguments and returns them with no adjustment.

Added in version 8.4.0.2 of package base.

An uninterned symbol intended for use as a continuation mark key with an error-adjuster procedure value. An error adjuster associated with the key should follow the same protocol as a value of current-error-message-adjuster.

See error-message->adjusted-string for a description of how marks using this key are can adjust error messages.

Added in version 8.4.0.2 of package base.