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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.

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.

Raises an exception, where

represents the exception being raised. The

argument can be anything; it is passed to the current

.

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:

Raises the exception

, which contains an error string. The different forms produce the error string in different ways:

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

• (error message-str v ...) creates a message string by concatenating message-str 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 who-sym format-str v ...) creates a message string equivalent to the string created by

  (format (string-append "~s: " format-str) who-sym 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

Creates an

value and

s it as an exception. The

argument is used as the source procedure’s name in the error message. The

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:

> (feed-machine 'turkey)

feed-machine: contract violation

  expected: integer?

  given: 'turkey

> (feed-cow 'turkey)

feed-cow: contract violation

  expected: 'cow

  given: 'turkey

> (feed-animals 'cow 'sheep 'dog 'cat)

feed-animals: contract violation

  expected: 'goose

  given: 'dog

  argument position: 3rd

  other arguments...:

   'cow

   'sheep

   'cat

Added in version 8.4.0.2 of package base.

Like

, but the error message describe

as a “result” instead of an “argument.”

Added in version 8.4.0.2 of package base.

Creates an

value and

s it as an exception. The

is used as the source procedure’s name in the error message. The

is the error message; if

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

must have a corresponding

, and the two are rendered on their own line in the error message; each

is formatted using the error value conversion handler (see

), unless

is a

, in which case the string content is

ed without using the error value conversion handler. When a string produced by the error value conversion handler or in an unquoted-printing string contains a newline but does not start with a newline, then the string is started on its own line with extra spaces added before each line to indent the string content.

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

Examples:

eat: fish is smaller than its given meal

  fish: 12

  meal: 13

eat: not edible

  candidate:

   a banana made

   of wax

Changed in version 8.15.0.2 of package base: Added indentation for v strings that contain newlines.

Added in version 8.4.0.2 of package base.
Changed in version 8.15.0.2: Added indentation for v strings that contain newlines.

Creates an

value and

s it as an exception to report an out-of-range error. The

string describes the value for which the index is meant to select an element, and

is a prefix for the word “index.” The

argument is the rejected index. The

argument is the value for which the index was meant. The

and

arguments specify the valid range of indices, inclusive; if

is below

, the value is characterized as “empty.” If

is not

, and if

is between

and

, then the error is report as

being less than the “starting” index

.

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)

Added in version 8.4.0.2 of package base.

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

Similar to

, but using Racket’s old formatting conventions, with a required

immediately after the first

string, and with further

strings that are spliced into the message without line breaks or space. Use

, instead.

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

Changed in version 8.15.0.2 of package base: Added indentation for v strings that contain newlines.

Creates an

value and

s it as an exception. The

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"

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

Added in version 8.4.0.2 of package base.

Added in version 7.0.0.11 of package base.

Added in version 8.4.0.2 of package base.

Like

, but reports a “result” mismatch instead of an “argument” mismatch. The

argument can be

to omit an initial source for the error. The

argument, if non-

, 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.

Added in version 8.4.0.2 of package base.

Creates an

value and

s 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:

• 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).

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

Examples:

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.

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.

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.

A

that determines an

used by

when the relevant continuation has no exception handler installed with

or

. Unlike exception handlers installed with

, the uncaught-exception handler must not return a value when called by

; 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.

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:

got a syntax error

got a syntax error

Like

, but if a

procedure is called, breaks are not explicitly disabled, and the handler call is in tail position with respect to the

form.

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).

A parameter for the

, 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.

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

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

lines. A

value for

disables context printing entirely.

A

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

,

, or

structure is affected by the parameter. The default is

.

A

that determines the

, 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).

If the string produced by error-value->string-handler contains a newline but does not start with a newline, then a context that uses the string will add spaces or indentation after each newline as needed. For example, raise-argument-error adds a three-space indentation to the start of each line.

Changed in version 8.15.0.2 of package base: Added indentation convention for string results that contain newlines.

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.

A

that determines the

, which is used to extract the name of a syntactic form when

is called with

as its first argument and a syntax object as its third argument.

The argument to the handler is a the syntax object provided to raise-syntax-error as its third argument. The result must be a symbol if a name can be extracted from the syntax object, #f otherwise.

Added in version 8.15.0.2 of package base.

Added in version 8.16.0.3 of package base.

Exceptions raised by Racket form a hierarchy under exn:

Raised for exceptions that represent errors, as opposed to

.

Raised for errors from the inappropriate run-time use of a function or syntactic form.

Raised when a procedure is applied to the wrong number of arguments.

Raised for division by exact zero.

Raised by functions like

when the result would not be a fixnum.

Raised when a continuation is applied where the jump would cross a continuation barrier.

Raised for a syntax error that is not a

error. The

indicate the relevant source expressions, least-specific to most-specific.

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

Raised by

or

for an unbound identifier within a module.

Raised by the default

or default

to report a module path—

a reported in the

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.

Raised for a

error. The

indicate the relevant source expressions.

Raised for a

error, specifically when the error is due to an unexpected end-of-file.

Raised for a

error, specifically when the error is due to an unexpected non-character (i.e., “special”) element in the input stream.

Raised for an error related to the filesystem (such as a file not found).

Raised for an error when attempting to create a file that exists already.

Raised for a version-mismatch error when loading an extension.

Raised for a filesystem error for which a system error code is available. The symbol part of an

field indicates the category of the error code:

indicates a C/Posix

value,

indicates a Windows system error code (under Windows, only), and

indicates a

error code (which shows up only in

exceptions for operations that resolve hostnames, but is allowed in

instances for consistency).

Raised by the default

or default

to report a module path—

a reported in the

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.

Raised for TCP and UDP errors.

Raised for a TCP or UDP error for which a system error code is available, where the

field is as for

.

Raised for an error due to insufficient memory, in cases where sufficient memory is at least available for raising the exception.

Raised for an error due to an unsupported feature on the current platform or configuration.

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.

A property that identifies structure types that provide a list of

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,

Returns the

-getting procedure associated with

.

A

is most frequently represented by a

structure. More generally, a source location has the same information as a

structure, but potentially represented or accessed differently. For example, source-location information is accessed from a

with functions like

and

, while

accepts a source location as a list, vector, or another syntax object. For ports, a combination of

and

provides location information, especially in a port for which counting has been enabled through

.

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.

Formats

as a string suitable for error reporting. A path source in

is shown relative to the value of

. The result is

if

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.

Returns the

-getting procedure associated with

.

Added in version 6.3 of package base.

Formats

as a string. If

is an

, collects and returns the output from the current

; otherwise, simply converts

to a string using

.

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).

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.

Analogous to

, 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.

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:

• 'name: the procedure receives two arguments, a name symbol and a realm symbol; it returns an adjusted name symbol and an adjusted realm symbol.

• 'message: the procedure receives four arguments: a name symbol or #f (which means that no name will be prefixed on the message), a name-realm symbol, an message string, and a message-realm symbol; it returns four adjusted values.

• 'contract: the procedure receives two arguments, a contract string and a realm symbol; it returns an adjusted contract string and an adjusted realm symbol.

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.

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.

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