► |  | The Racket Reference

1 | Language Model
2 | Notation for Documentation
3 | Syntactic Forms
4 | Datatypes
5 | Structures
6 | Classes and Objects
7 | Units
8 | Contracts
9 | Pattern Matching
10 | Control Flow
11 | Concurrency and Parallelism
12 | Macros
13 | Input and Output
14 | Reflection and Security
15 | Operating System
16 | Memory Management
17 | Unsafe Operations
18 | Running Racket
| Bibliography
| Index

▼ | 10 | Control Flow

10.1 | Multiple Values
10.2 | Exceptions
10.3 | Delayed Evaluation
10.4 | Continuations
10.5 | Continuation Marks
10.6 | Breaks
10.7 | Exiting

► | 10.2 | Exceptions

10.2.1 | Error Message Conventions
10.2.2 | Raising Exceptions
10.2.3 | Handling Exceptions
10.2.4 | Configuring Default Handling
10.2.5 | Built-in Exception Types
10.2.6 | Additional Exception Functions

On this page:

10.2.1`` Error Message Conventions
10.2.2`` Raising Exceptions
raise
error
raise-user-error
raise-argument-error
raise-result-error
raise-arguments-error
raise-range-error
raise-type-error
raise-mismatch-error
raise-arity-error
raise-arity-mask-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-value->string-handler
error-print-source-location
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

top ← prev up next →

#### 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
> > (define-struct (my-exception exn:fail:user) ())
> > (with-handlers ([my-exception? (lambda (e)
> #f)])
> (+ 5 (raise (make-my-exception
> "failed"
> (current-continuation-marks)))))
> #f
> > (raise 'failed #t)
> uncaught exception: failed

> procedure (error sym) → any
> sym : symbol?
> (error msg v ...) → any
> msg : string?
> v : any/c
> (error src format v ...) → any
> src : symbol?
> format : string?
> v : any/c

Raises the exception exn:fail, which contains an error string. The different forms produce the error string in different ways:

- (error sym) creates a message string by concatenating "error: " with the string form of sym. Use this form sparingly.

- (error msg v ...) creates a message string by concatenating msg with string versions of the vs (as produced by the current error value conversion handler; see error-value->string-handler). A space is inserted before each v. Use this form sparingly, because it does not conform well to Racket’s error message conventions; consider raise-arguments-error, instead.

- (error src frmat v ...) creates a message string equivalent to the string created by

  > (format (string-append "~s: " frmat) src v ...)

  When possible, use functions such as raise-argument-error, instead, which construct messages that follow Racket’s error message conventions.

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 sym) → any
> sym : symbol?
> (raise-user-error msg v ...) → any
> msg : string?
> v : any/c
> (raise-user-error src format v ...) → any
> src : symbol?
> format : 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.

Examples:

> > (define (feed-machine bits)
> (if (not (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)
> (if (not (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)
> (if (not (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-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-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.

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-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?)

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.

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-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.

> > 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.

> 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.

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-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-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.

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
> values...:
> 'a
> 2.0
> "three"

Added in version 6.90.0.26 of package base.

> > procedure
> > (raise-syntax-error |  | name |  |  |  |
> > |  | message |  |  |  |
> > | [ | expr |  |  |  |
> > |  | sub-expr |  |  |  |
> > |  | extra-sources |  |  |  |
> > |  | message-suffix]) |  | → |  | 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? = ""

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

- When name is #f, and when expr is either an identifier or a syntax pair containing an identifier as its first element, then the form name from the error message is the identifier’s symbol.

- When name is #f and when expr is not an identifier or a syntax pair containing an identifier as its first element, then the form name in the error message is "?".

- When name is a symbol, then the symbol is used as the form name in the generated error message.

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).

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

> procedure (unquoted-printing-string? v) → boolean?
> v : any/c
> procedure (unquoted-printing-string s) → unquoted-printing-string?
> s : string?
> procedure (unquoted-printing-string-value ups) → string?
> ups : unquoted-printing-string?

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.

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.

> parameter (error-print-context-length) → exact-nonnegative-integer?
> (error-print-context-length cnt) → void?
> cnt : exact-nonnegative-integer?

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-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 “...”.

If the string returned by an error value conversion handler is longer than requested, the string is destructively “truncated” by setting the first extra position in the string to the null character. If a non-string 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-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.

##### 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.

> value prop:exn:srclocs : struct-type-property?

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.
> (define-struct (exn:fail:he-who-shall-not-be-named
> exn:fail)
> (a-srcloc)
> #:property prop:exn:srclocs
> (lambda (a-struct)
> (match a-struct
> [(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 (make-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 the one-who-shall-not-be-named
> ;; 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)

The fields of a srcloc instance are as follows:

- source — An arbitrary value identifying the source, often a path (see Paths).

- line — The line number (counts from 1) or #f (unknown).

- column — The column number (counts from 0) or #f (unknown).

- position — The starting position (counts from 1) or #f (unknown).

- span — The number of covered positions (counts from 0) or #f (unknown).

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.

> value prop:exn:missing-module : struct-type-property?

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.

> procedure (exn:missing-module-accessor v)
> → (exn:missing-module? . -> . module-path?)
> v : exn:srclocs?

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).
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