#lang scribble/doc
@(require "mz.rkt"
          (for-label racket/linklet
                     racket/unsafe/ops))

@title[#:tag "linklets"]{Linklets and the Core Compiler}

@defmodule[racket/linklet]

A @deftech{linklet} is a primitive element of compilation, bytecode
marshaling, and evaluation. Racket's implementations of modules,
macros, and top-level evaluation are all built on linklets. Racket
programmers generally do not encounter linklets directly, but the
@racketmodname[racket/linklet] library provides access to linklet
facilities.

A single Racket module (or collection of top-level forms) is typically
implemented by multiple linklets. For example, each phase of
evaluation that exists in a module is implemented in a separate
linklet. A linklet is also used for metadata such as the @tech{module
path index}es for a module's @racket[require]s. These linklets, plus
some other metadata, are combined to form a @deftech{linklet bundle}.
Information in a @tech{linklet bundle} is keyed by either a symbol or
a @tech{fixnum}. A @tech{linklet bundle} containing
@tech{linklet}s can be marshaled to and from a byte stream by
@racket[write] and (with @racket[read-accept-compiled] is enabled)
@racket[read].

When a Racket module has submodules, the @tech{linklet bundles} for
the module and the submodules are grouped together in a
@deftech{linklet directory}. A @tech{linklet directory} can have
nested linklet directories. Information in a linklet directory is
keyed by @racket[#f] or a symbol, where @racket[#f] must be mapped to
a @tech{linklet bundle} (if anything) and each symbol must be mapped
to a @tech{linklet directory}. A @tech{linklet directory} can be
equivalently viewed as a mapping from a lists of symbols to a
@tech{linklet bundle}. Like @tech{linklet bundles}, a @tech{linklet
directory} can be marshaled to and from a byte stream by
@racket[write] and @racket[read]; the marshaled form allows individual
@tech{linklet bundles} to be loaded independently.

A linklet consists of a set of variable definitions and expressions,
an exported subset of the defined variable names, a set of variables to export
from the linklet despite having no corresponding definition, and a set
of imports that provide other variables for the linklet to use. To run
a linklet, it is instantiated as as @deftech{linklet instance} (or
just @defterm{instance}, for short). When a linklet is instantiated,
it receives other @tech{linklet instances} for its imports, and it
extracts a specified set of variables that are exported from each of
the given instances. The newly created @tech{linklet instance}
provides its exported variables for use by other linklets or for
direct access via @racket[instance-variable-value]. A @tech{linklet
instance} can be synthesized directly with @racket[make-instance].

A linklet is created by compiling an enriched S-expression
representation of its source. Since linklets exist below the layer of
macros and syntax objects, linklet compilation does not use
@tech{syntax objects}. Instead, linklet compilation uses
@deftech{correlated objects}, which are like @tech{syntax objects}
without lexical-context information and without the constraint that
content is coerced to correlated objects. Using an S-expression or
@tech{correlated object}, the grammar of a linklet as recognized by
@racket[compile-linklet] is

@specform[(linklet [[imported-id/renamed ...] ...]
                   [exported-id/renamed ...]
            defn-or-expr ...)
          #:grammar
          ([imported-id/renamed imported-id
                                (external-imported-id internal-imported-id)]
           [exported-id/renamed exported-id
                                (internal-exported-id external-exported-id)])]

Each import set @racket[[_imported-id/renamed ...]] refers to a single
imported instance, and each @racket[_import-id/renamed] corresponds to
a variable from that instance. If separate
@racket[_external-imported-id] and @racket[_internal-imported-id] are
specified, then @racket[_external-imported-id] is the name of the
variable as exported by the instance, and
@racket[_internal-imported-id] is the name used to refer to the
variable in the @racket[_defn-or-expr]s. For exports, separate
@racket[_internal-exported-id] and @racket[_external-exported-id]
names corresponds to the variable name as exported as referenced 
in the @racket[_defn-or-expr]s, respectively.

The grammar of an @racket[_defn-or-expr] is similar to the expander's
grammar of fully expanded expressions (see @secref["fully-expanded"])
with some exceptions: @racket[quote-syntax] and @racket[#%top] are not allowed;
@racket[#%plain-lambda] is spelled @racket[lambda];
@racket[#%plain-app] is omitted (i.e., application is implicit);
@racket[lambda], @racket[case-lambda], @racket[let-values], and
@racket[letrec-values] can have only a single body expression;
@racket[begin-unsafe] is like @racket[begin] in an expression position,
but its body is compiled in @tech{unsafe mode}; and
numbers, booleans, strings, and byte strings are self-quoting.
Primitives are accessed directly by name, and shadowing is not allowed
within a @racketidfont{linklet} form for primitive names (see
@racket[linklet-body-reserved-symbol?]), imported variables, defined
variables, or local variables.

When an @racket[_exported-id/renamed] has no corresponding definition
among the @racket[_defn-or-expr]s, then the variable is effectively
defined as uninitialized; referencing the variable will trigger
@racket[exn:fail:contract:variable], the same as referencing a
variable before it is defined. When a target instance is provided to
@racket[instantiate-linklet], any existing variable with the same name
will be left as-is, instead of set to undefined. This treatment of
uninitialized variables provides core support for top-level evaluation
where variables may be referenced and then defined in a separate
element of compilation.

@history[#:added "6.90.0.1"]

@; --------------------------------------------------

@defproc[(linklet? [v any/c]) boolean?]{

Returns @racket[#t] if @racket[v] is a @tech{linklet}, @racket[#f]
otherwise.}

@defproc*[([(compile-linklet [form (or/c correlated? any/c)]
                             [info (or/c hash? any/c) #f]
                             [import-keys #f #f]
                             [get-import #f #f]
                             [options (listof (or/c 'serializable 'unsafe 'static 'quick
                                                     'use-prompt 'unlimited-compile
                                                     'uninterned-literal))
                                      '(serializable)])
            linklet?]
           [(compile-linklet [form (or/c correlated? any/c)]
                             [info (or/c hash? any/c)]
                             [import-keys vector?]
                             [get-import (or/c #f (any/c . -> . (values (or/c linklet? instance? #f)
                                                                        (or/c vector? #f))))
                                         #f]
                             [options (listof (or/c 'serializable 'unsafe 'static 'quick
                                                    'use-prompt 'unlimited-compile
                                                    'uninterned-literal))
                                      '(serializable)])
            (values linklet? vector?)])]{

Takes an S-expression or @tech{correlated object} for a
@schemeidfont{linklet} form and produces a @tech{linklet}.
As long as @racket['serializable] included in @racket[options], the
resulting linklet can be marshaled to and from a byte stream when it is
part of a @tech{linklet bundle} (possibly in a @tech{linklet directory}).

The optional @racket[info] hash provides various debugging details
about the linklet, such as the module name the linklet is part of,
the linklet name, and the phase for body linklets. If a @racket['name]
value is present in the hash, it is associated to the linklet for
debugging purposes and as the default name of the linklet's instance.
If @racket[info] is not a hash, it is assumed to be a name value
directly for backward compatibility.

The optional @racket[import-keys] and @racket[get-import] arguments
support cross-linklet optimization. If @racket[import-keys] is a
vector, it must have as many elements as sets of imports in
@racket[form]. If the compiler becomes interested in optimizing a
reference to an imported variable, it passes back to
@racket[get-import] (if non-@racket[#f]) the element of @racket[import-keys] that
corresponds to the variable's import set. The @racket[get-import]
function can then return a linklet or instance that represents an instance to be
provided to the compiled linklet when it is eventually instantiated;
ensuring consistency between reported linklet or instance and the eventual
instance is up to the caller of @racket[compile-linklet], but see also
@racket[linklet-add-target-machine-info]. If
@racket[get-import] returns @racket[#f] as its first value, the
compiler will be prevented from making any assumptions about the
imported instance. The second result from @racket[get-import] is an
optional vector of keys to provide transitive information on a
returned linklet's imports (and is not allowed for a returned instance);
the returned vector must have the same
number of elements as the linklet has imports. When vector elements
are @racket[eq?] and non-@racket[#f], the compiler can assume that
they correspond to the same run-time instance. A @racket[#f]
value for @racket[get-import] is equivalent to a function that
always returns two @racket[#f] results.

When @racket[import-keys] is not @racket[#f], then the compiler is
allowed to grow or shrink the set of imported instances for the
linklet. The result vector specifies the keys of the imports for the
returned linklet. Any key that is @racket[#f] or a @tech{linklet instance}
must be preserved intact, however.

If @racket['unsafe] is included in @racket[options], then the linklet
is compiled in @deftech{unsafe mode}: uses of safe operations within
the linklet can be converted to unsafe operations on the assumption
that the relevant contracts are satisfied. For example, @racket[car]
is converted to @racket[unsafe-car]. Some substituted unsafe
operations may not have directly accessible names, such as the unsafe
variant of @racket[in-list] that can be substituted in @tech{unsafe
mode}. An unsafe operation is substituted only if its (unchecked)
contract is subsumed by the safe operation's contract. The fact that
the linklet is compiled in @tech{unsafe mode} can be exposed through
@racket[variable-reference-from-unsafe?] using a variable reference
produced by a @racket[#%variable-reference] form within the module
body. Within a linklet an individual expression can be compiled in
unsafe mode by wrapping it in @racket[begin-unsafe]; when a whole
linklet is compiled in unsafe mode, @racket[begin-unsafe] is redundant
and ignored.

If @racket['static] is included in @racket[options], then the linklet
must be instantiated only once; if the linklet is serialized, then any
individual instance read from the serialized form must be instantiated
at most once. Compilation with @racket['static] is intended to improve
the performance of references within the linklet to defined and
imported variables.

If @racket['quick] is included in @racket[options], then linklet
compilation may trade run-time performance for compile-time
performance---that is, spend less time compiling the linklet, but the
resulting linklet may run more slowly.

If @racket['use-prompt] is included in @racket[options], then
instantiating resulting linklet always wraps a prompt around each
definition and immediate expression in the linklet. Otherwise,
supplying @racket[#t] as the @racket[_use-prompt?] argument to
@racket[instantiate-linklet] may only wrap a prompt around the entire
instantiation.

If @racket['unlimited-compile] is included in @racket[options], then
compilation never falls back to interpreted mode for an especially
large linklet. See also @secref["cs-compiler-modes"].

If @racket['uninterned-literal] is included in @racket[options], then
literals in @racket[form] will not necessarily be interned via
@racket[datum-intern-literal] when compiling or loading the linklet.
Disabling the use of @racket[datum-intern-literal] can be especially
useful of the linklet includes a large string or byte string constant
that is not meant to be shared.

The symbols in @racket[options] must be distinct, otherwise
@exnraise[exn:fail:contract].

@history[#:changed "7.1.0.8" @elem{Added the @racket['use-prompt] option.}
         #:changed "7.1.0.10" @elem{Added the @racket['uninterned-literal] option.}
         #:changed "7.5.0.14" @elem{Added the @racket['quick] option.}
         #:changed "8.11.1.2" @elem{Changed @racket[info] to a hash.}
         #:changed "8.13.0.9" @elem{Added the @racket['unlimited-compile] option.}]}


@defproc*[([(recompile-linklet [linklet linklet?]
                               [info (or/c hash? any/c) #f]
                               [import-keys #f #f]
                               [get-import #f #f]
                               [options (listof (or/c 'serializable 'unsafe 'static 'quick
                                                      'use-prompt 'uninterned-literal))
                                        '(serializable)])
            linklet?]
           [(recompile-linklet [linklet linklet?]
                               [info (or/c hash? any/c)]
                               [import-keys vector?]
                               [get-import (or/c (any/c . -> . (values (or/c linklet? #f)
                                                                       (or/c vector? #f)))
                                                 #f)
                                           (lambda (import-key) (values #f #f))]
                               [options (listof (or/c 'serializable 'unsafe 'static 'quick
                                                      'use-prompt 'uninterned-literal))
                                        '(serializable)])
             (values linklet? vector?)])]{

Like @racket[compile-linklet], but takes an already-compiled linklet
and potentially optimizes it further.

@history[#:changed "7.1.0.6" @elem{Added the @racket[options] argument.}
         #:changed "7.1.0.8" @elem{Added the @racket['use-prompt] option.}
         #:changed "7.1.0.10" @elem{Added the @racket['uninterned-literal] option.}
         #:changed "7.5.0.14" @elem{Added the @racket['quick] option.}
         #:changed "8.11.1.2" @elem{Changed @racket[info] to a hash.}]}


@defproc[(eval-linklet [linklet linklet?]) linklet?]{

Returns a variant of a @racket[linklet] that is prepared for JIT
compilation such that every later use of the result linklet with
@racket[instantiate-linklet] shares the JIT-generated code. However,
the result of @racket[eval-linklet] cannot be marshaled to a byte
stream as part of a @tech{linklet bundle}, and it cannot be used with
@racket[recompile-linklet].}



@defproc*[([(instantiate-linklet [linklet linklet?]
                                 [import-instances (listof instance?)]
                                 [target-instance? #f #f]
                                 [use-prompt? any/c #t])
            instance?]
           [(instantiate-linklet [linklet linklet?]
                                 [import-instances (listof instance?)]
                                 [target-instance instance?]
                                 [use-prompt? any/c #t])
            any])]{

Instantiates @racket[linklet] by running its definitions and
expressions, using the given @racket[import-instances] for its
imports. The number of instances in @racket[import-instances] must
match the number of import sets in @racket[linklet].

If @racket[target-instance] is @racket[#f] or not provided, the result
is a fresh instance for the linklet. If @racket[target-instance] is an
instance, then the instance is used and modified for the linklet
definitions and expressions, and the result is the value of the last
expression in the linklet.

The linklet's exported variables are accessible in the result instance
or in @racket[target-instance] using the linklet's external name for
each export. If @racket[target-instance] is provided as
non-@racket[#f], its existing variables remain intact if they are not
modified by a linklet definition.

If @racket[use-prompt?] is true, then a a @tech{prompt} is wrapped
around the linklet instantiation in same ways as an expression in a
module body. If the linklet contains multiple definitions or immediate
expressions, then a prompt may or may not be wrapped around each
definition or expression; supply @racket['use-prompt] to
@racket[compile-linklet] to ensure that a prompt is used around each
definition and expression.}


@defproc[(linklet-import-variables [linklet linklet?])
         (listof (listof symbol?))]{

Returns a description of a linklet's imports. Each element of the
result list corresponds to an import set as satisfied by a single
instance on instantiation, and each member of the set is a variable
name that is used from the corresponding imported instance.}

@defproc[(linklet-export-variables [linklet linklet?])
         (listof symbol?)]{

Returns a description of a linklet's exports. Each element of the list
corresponds to a variable that is made available by the linklet in its
instance.}

@defproc[(linklet-add-target-machine-info [linklet linklet?]
                                          [from-linklet (or linklet? hash?)])
         linklet?]{

When @racket[compile-linklet] or @racket[recompile-linklet] requests a
linklet via @racket[_get-import] for cross-module information, the
linklet is expected to have information compatible with the current
compilation target as determined by
@racket[current-compile-target-machine]. To simplify the management of
linklets to both run and use for cross-compilation, a linklet
implementation may support information for multiple target machines
within a linklet, in which case
@racket[linklet-add-target-machine-info] returns a linklet like
@racket[linklet] but with target-specific information added from
@racket[from-linklet]. The two linklets must be from compatible
sources, but @racket[linklet-add-target-machine-info] might perform
only a sanity check for compatibility.

The @racket[from-linklet] can be a linklet or a summary of a linklet's
information as produced by @racket[linklet-summarize-target-machine-info].

@history[#:added "8.12.0.3"
         #:changed "8.17.0.3" @elem{Added support for @racket[from-linklet]
                                    as a summary.}]}


@defproc[(linklet-summarize-target-machine-info [from-linklet linklet?])
         hash?]{

Returns a value that has the same information as @racket[from-linklet]
for @racket[linklet-add-target-machine-info], but in a form that can be
portably serialized via @racketmodname[racket/fasl].

@history[#:added "8.17.0.3"]}


@defproc[(linklet-directory? [v any/c]) boolean?]{

Returns @racket[#t] if @racket[v] is a @tech{linklet directory},
@racket[#f] otherwise.}


@defproc[(hash->linklet-directory [content (and/c hash? hash-eq? immutable? (not/c impersonator?))])
         linklet-directory?]{

Constructs a @tech{linklet directory} given mappings in the form of a
@tech{hash table}. Each key of @racket[content] must be either a
symbol or @racket[#f], each symbol must be mapped to a @tech{linklet
directory}, and @racket[#f] must be mapped to a @tech{linklet bundle}
or not mapped.}


@defproc[(linklet-directory->hash [linklet-directory linklet-directory?])
         (and/c hash? hash-eq? immutable? (not/c impersonator?))]{

Extracts the content of a @tech{linklet directory} into a @tech{hash
table}.}

         
@defproc[(linklet-bundle? [v any/c]) boolean?]{

Returns @racket[#t] if @racket[v] is a @tech{linklet bundle},
@racket[#f] otherwise.}


@defproc[(hash->linklet-bundle [content (and/c hash? hash-eq? immutable? (not/c impersonator?))])
         linklet-bundle?]{

Constructs a @tech{linklet bundle} given mappings in the form of a
@tech{hash table}. Each key of @racket[content] must be either a
symbol or a @tech{fixnum}. Values in the hash table are unconstrained,
but the intent is that they are all @tech{linklets} or values that can
be recovered from @racket[write] output by @racket[read].}


@defproc[(linklet-bundle->hash [linklet-bundle linklet-bundle?])
         (and/c hash? hash-eq? immutable? (not/c impersonator?))]{

Extracts the content of a @tech{linklet bundle} into a @tech{hash
table}.}


@defproc[(linklet-body-reserved-symbol? [sym symbol?]) boolean?]{

Return @racket[#t] if @racket[sym] is a primitive name or other
identifier that is not allowed as a binding within a linklet,
@racket[#f] otherwise.

@history[#:added "8.2.0.1"]}
         

@defproc[(instance? [v any/c]) boolean?]{

Returns @racket[#t] if @racket[v] is a @tech{linklet instance},
@racket[#f] otherwise.}


@defproc[(make-instance [name any/c]
                        [data any/c #f]
                        [mode (or/c #f 'constant 'consistent) #f]
                        [variable-name symbol?]
                        [variable-value any/c] ... ...)
         instance?]{

Constructs a @tech{linklet instance} directly. Besides associating an
arbitrary @racket[name] and @racket[data] value to the instance, the
instance is populated with variables as specified by
@racket[variable-name] and @racket[variable-value].

The optional @racket[data] and @racket[mode] arguments must be
provided if any @racket[variable-name] and @racket[variable-value]
arguments are provided. The @racket[mode] argument is used as in
@racket[instance-set-variable-value!] for every
@racket[variable-name].}


@defproc[(instance-name [instance instance?]) any/c]{

Returns the value associated to @racket[instance] as its name---either
the first value provided to @racket[make-instance] or the name of a
linklet that was instantiated to create the instance.}


@defproc[(instance-data [instance instance?]) any/c]{

Returns the value associated to @racket[instance] as its data---either
the second value provided to @racket[make-instance] or the default
@racket[#f].}


@defproc[(instance-variable-names [instance instance?]) (list symbol?)]{

Returns a list of all names for all variables accessible from
@racket[instance].}


@defproc[(instance-variable-value [instance instance?]
                                  [name symbol?]
                                  [fail-k any/c (lambda () (error ....))])
         any]{

Returns the value of the variable exported as @racket[name] from
@racket[instance]. If no such variable is exported, then
@racket[fail-k] is used in the same way as by @racket[hash-ref].}


@defproc[(instance-set-variable-value! [instance instance?]
                                       [name symbol?]
                                       [v any/c]
                                       [mode (or/c #f 'constant 'consistent) #f])
          void?]{

Sets or creates the variable exported as @racket[name] in
@racket[instance] so that its value is @racket[v], as long as the
variable does not exist already as constant. If a variable for
@racket[name] exists as constant, the @exnraise[exn:fail:contract].

If @racket[mode] is @racket['constant] or @racket['consistent], then
the variable is created or changed to be constant. Furthermore, when
the instance is reported for a linklet's import though a
@racket[_get-import] callback to @racket[compile-linklet], the
compiler can assume that the variable will be constant in all future
instances that are used to satisfy a linklet's imports.

If @racket[mode] is @racket['consistent], when the instance is
reported though a callback to @racket[compile-linklet], the compiler
can further assume that the variable's value will be the same for
future instances. For compilation purposes, ``the same'' can mean that
a procedure value will have the same arity and implementation details,
a @tech{structure type} value will have the same configuration, a
marshalable constant will be @racket[equal?] to the current value, and
so on.}


@defproc[(instance-unset-variable! [instance instance?]
                                   [name symbol?])
          void?]{

Changes @racket[instance] so that it does not export a variable as
@racket[name], as long as @racket[name] does not exist as a constant
variable. If a variable for @racket[name] exists as constant, the
@exnraise[exn:fail:contract].}


@defproc[(instance-describe-variable! [instance instance?]
                                      [name symbol?]
                                      [desc-v any/c])
          void?]{

Registers information about @racket[name] in @racket[instance] that
may be useful for compiling linklets where the instance is return via
the @racket[_get-import] callback to @racket[compile-linklet]. The
@racket[desc-v] description can be any value; the recognized
descriptions depend on virtual machine, but may include the following:

@itemlist[

 @item{@racket[`(procedure ,arity-mask)] --- the value is always a
       procedure that is not impersonated and not a structure, and its
       arity in the style of @racket[procedure-arity-mask] is
       @racket[arity-mask].}

 @item{@racket[`(procedure/succeeds ,arity-mask)] --- like
       @racket[`(procedure ,arity-mask)], but for a procedure that
       never raises an exception of otherwise captures or escapes the
       calling context.}

 @item{@racket[`(procedure/pure ,arity-mask)] --- like
       @racket[`(procedure/succeeds ,arity-mask)], but with no
       observable side effects, so a call to the procedure can be
       reordered.}

]

@history[#:added "7.1.0.8"]}

@defproc[(variable-reference->instance [varref variable-reference?]
                                       [ref-site? any/c #f])
         (if ref-site? (or/c instance? #f symbol?) instance?)]{

Extracts the instance where the variable of @racket[varref] is defined
if @var[ref-site?] is @racket[#f], and returns the instance where
@racket[varref] itself resides if @racket[ref-site?] is true. This
notion of @tech{variable reference} is the same as at the module level
and can reflect the linklet instance that implements a particular
phase of a module instance.

When @var[ref-site?] is @racket[#f], the result is @racket[#f] when
@racket[varref] is from @racket[(#%variable-reference)] with no
identifier. The result is a symbol if @racket[varref] refers to a
primitive.}

@deftogether[(
@defproc[(correlated? [v any/c]) boolean?]
@defproc[(correlated-source [crlt correlated?]) any]
@defproc[(correlated-line [crlt correlated?])
         (or/c exact-positive-integer? #f)]
@defproc[(correlated-column [crlt correlated?])
         (or/c exact-nonnegative-integer? #f)]
@defproc[(correlated-position [crlt correlated?])
         (or/c exact-positive-integer? #f)]
@defproc[(correlated-span [crlt correlated?])
         (or/c exact-nonnegative-integer? #f)]
@defproc[(correlated-e [crlt correlated?]) any]
@defproc[(correlated->datum [crlt (or/c correlated? any/c)]) any]
@defproc[(datum->correlated [v any/c]
                        [srcloc (or/c correlated? #f
                                      (list/c any/c
                                              (or/c exact-positive-integer? #f)
                                              (or/c exact-nonnegative-integer? #f)
                                              (or/c exact-positive-integer? #f)
                                              (or/c exact-nonnegative-integer? #f))
                                      (vector/c any/c
                                               (or/c exact-positive-integer? #f)
                                               (or/c exact-nonnegative-integer? #f)
                                               (or/c exact-positive-integer? #f)
                                               (or/c exact-nonnegative-integer? #f)))
                                #f]
                         [prop (or/c correlated? #f) #f])
          correlated?]
@defproc*[([(correlated-property [crlt correlated?]
                                 [key any/c]
                                 [val any/c])
             correlated?]
           [(correlated-property [crlt correlated?] [key any/c]) any/c])]
@defproc[(correlated-property-symbol-keys [crlt correlated?]) list?]
)]{

Like @racket[syntax?], @racket[syntax-source], @racket[syntax-line],
@racket[syntax-column], @racket[syntax-position],
@racket[syntax-span], @racket[syntax-e], @racket[syntax->datum],
@racket[datum->syntax], @racket[syntax-property], and
@racket[syntax-property-symbol-keys], but for @tech{correlated
objects}.

Unlike @racket[datum->syntax], @racket[datum->correlated] does not
recur through the given S-expression and convert pieces to
@tech{correlated objects}. Instead, a @tech{correlated object} is
simply wrapped around the immediate value. In contrast,
@racket[correlated->datum] recurs through its argument (which is not
necessarily a @tech{correlated object}) to discover any
@tech{correlated objects} and convert them to plain S-expressions.

@history[#:changed "7.6.0.6" @elem{Added the @racket[prop] argument
                                   to @racket[datum->correlated].}]}
