7 raco decompile: Decompiling Bytecode

▼ |  | raco: Racket Command-Line Tools

1 | raco make: Compiling Source to Bytecode
2 | raco exe: Creating Stand-Alone Executables
3 | raco distribute: Sharing Stand-Alone Executables
4 | raco planet: Automatic Package Distribution
5 | raco pkg: Package Management
6 | raco setup: Installation Management
7 | raco decompile: Decompiling Bytecode
8 | raco demod: Demodularizing Programs
9 | raco link: Library Collection Links
10 | raco pack: Packing Library Collections
11 | raco unpack: Unpacking Library Collections
12 | raco ctool: Working with C Code
13 | raco test: Run tests
14 | raco docs: Documentation Search
15 | raco expand: Macro Expansion
16 | raco read: Reading and Pretty-Printing
17 | raco scribble: Building Documentation
18 | Adding a raco Command
19 | Installation Configuration and Search Paths

► | 7 | raco decompile: Decompiling Bytecode

7.1 | API for Decompiling
7.2 | API for Parsing Bytecode
7.3 | API for Marshaling Bytecode
7.4 | Bytecode Representation

On this page:

7.1`` API for Decompiling
decompile
7.2`` API for Parsing Bytecode
zo-parse
7.3`` API for Marshaling Bytecode
zo-marshal-to
zo-marshal
7.4`` Bytecode Representation
zo
7.4.1`` Prefix
linkl-directory
linkl-bundle
linkl
function-shape
struct-shape
struct-type-shape
constructor-shape
predicate-shape
accessor-shape
mutator-shape
struct-type-property-shape
property-predicate-shape
property-accessor-shape
struct-other-shape
7.4.2`` Forms and Inline Variants
form
def-values
inline-variant
7.4.3`` Expressions
expr
lam
closure
case-lam
let-one
let-void
install-value
let-rec
boxenv
localref
toplevel
application
branch
with-cont-mark
seq
beg0
varref
assign
apply-values
with-immed-mark
primval

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### 7`` raco decompile: Decompiling Bytecode

The raco decompile command takes the path of a bytecode file (which usually has the file extension ".zo") or a source file with an associated bytecode file (usually created with raco make) and converts the bytecode file’s content back to an approximation of Racket code. Decompiled bytecode is mostly useful for checking the compiler’s transformation and optimization of the source program.

The raco decompile command accepts the following command-line flags:

- --force — skip modification-date comparison on the given file’s path and an associated ".zo" file (if any)

- -n ‹n› or --columns ‹n› — format output for a display with ‹n› columns

Many forms in the decompiled code, such as module, define, and lambda, have the same meanings as always. Other forms and transformations are specific to the rendering of bytecode, and they reflect a specific execution model:

- Top-level variables, variables defined within the module, and variables imported from other modules are prefixed with _, which helps expose the difference between uses of local variables versus other variables. Variables imported from other modules, moreover, have a suffix starting with @ that indicates the source module. Finally, imported variables with constantness have a midfix: :c to indicate constant shape across all instantiations, :f to indicate a fixed value after initialization, :p to indicate a procedure, :P to indicate a procedure that preserves continuation marks on return, :t to indicate a structure type, :mk to indicate a structure constructor, :? to indicate a structure predicate, :ref to indicate a structure accessor, or :set! to indicate a structure mutator.

  Non-local variables are always accessed indirectly though an implicit #%globals or #%modvars variable that resides on the value stack (which otherwise contains local variables). Variable accesses are further wrapped with #%checked when the compiler cannot prove that the variable will be defined before the access.

  Uses of core primitives are shown without a leading _, and they are never wrapped with #%checked.

- Local-variable access may be wrapped with #%sfs-clear, which indicates that the variable-stack location holding the variable will be cleared to prevent the variable’s value from being retained by the garbage collector. Variables whose name starts with unused are never actually stored on the stack, and so they never have #%sfs-clear annotations. (The bytecode compiler normally eliminates such bindings, but sometimes it cannot, either because it cannot prove that the right-hand side produces the right number of values, or the discovery that the variable is unused happens too late with the compiler.)

  Mutable variables are converted to explicitly boxed values using #%box, #%unbox, and #%set-boxes! (which works on multiple boxes at once). A set!-rec-values operation constructs mutually-recursive closures and simultaneously updates the corresponding variable-stack locations that bind the closures. A set!, set!-values, or set!-rec-values form is always used on a local variable before it is captured by a closure; that ordering reflects how closures capture values in variable-stack locations, as opposed to stack locations.

- In a lambda form, if the procedure produced by the lambda has a name (accessible via object-name) and/or source-location information, then it is shown as a quoted constant at the start of the procedure’s body. Afterward, if the lambda form captures any bindings from its context, those bindings are also shown in a quoted constant. Neither constant corresponds to a computation when the closure is called, though the list of captured bindings corresponds to a closure allocation when the lambda form itself is evaluated.

  A lambda form that closes over no bindings is wrapped with #%closed plus an identifier that is bound to the closure. The binding’s scope covers the entire decompiled output, and it may be referenced directly in other parts of the program; the binding corresponds to a constant closure value that is shared, and it may even contain cyclic references to itself or other constant closures.

- A form (#%apply-values proc expr) is equivalent to (call-with-values (lambda () expr) proc), but the run-time system avoids allocating a closure for expr. Similarly, a #%call-with-immediate-continuation-mark call is equivalent to a call-with-immediate-continuation-mark call, but avoiding a closure allocation.

- A define-values form may have (begin '%%inline-variant%% expr1 expr2) for its expression, in which case expr2 is the normal result, but expr1 may be inlined for calls to the definition from other modules. Definitions of functions without an '%%inline-variant%% are never inlined across modules.

- Function arguments and local bindings that are known to have a particular type have names that embed the known type. For example, an argument might have a name that starts argflonum or a local binding might have a name that starts flonum to indicate a flonum value.

- A #%decode-syntax form corresponds to a syntax object.

#### 7.1`` API for Decompiling

(require compiler/decompile) | package: compiler-lib

> procedure (decompile top) → any/c
> top : (or/c linkl-directory? linkl-bundle? linkl?)

Consumes the result of parsing bytecode and returns an S-expression (as described above) that represents the compiled code.

#### 7.2`` API for Parsing Bytecode

(require compiler/zo-parse) | package: zo-lib

The compiler/zo-parse module re-exports compiler/zo-structs in addition to zo-parse.

> procedure (zo-parse [in]) → (or/c linkl-directory? linkl-bundle?)
> in : input-port? = (current-input-port)

Parses a port (typically the result of opening a ".zo" file) containing bytecode. Beware that the structure types used to represent the bytecode are subject to frequent changes across Racket versons.

The parsed bytecode is returned in a linkl-directory or linkl-bundle structure—the latter only for the compilation of a module that contains no submodules.

Within a linklet, the bytecode representation of an expression is closer to an S-expression than a traditional, flat control string. For example, an if form is represented by a branch structure that has three fields: a test expression, a “then” expression, and an “else” expression. Similarly, a function call is represented by an application structure that has a list of argument expressions.

Storage for local variables or intermediate values (such as the arguments for a function call) is explicitly specified in terms of a stack. For example, execution of an application structure reserves space on the stack for each argument result. Similarly, when a let-one structure (for a simple let) is executed, the value obtained by evaluating the right-hand side expression is pushed onto the stack, and then the body is evaluated. Local variables are always accessed as offsets from the current stack position. When a function is called, its arguments are passed on the stack. A closure is created by transferring values from the stack to a flat closure record, and when a closure is applied, the saved values are restored on the stack (though possibly in a different order and likely in a more compact layout than when they were captured).

When a sub-expression produces a value, then the stack pointer is restored to its location from before evaluating the sub-expression. For example, evaluating the right-hand size for a let-one structure may temporarily push values onto the stack, but the stack is restored to its pre-let-one position before pushing the resulting value and continuing with the body. In addition, a tail call resets the stack pointer to the position that follows the enclosing function’s arguments, and then the tail call continues by pushing onto the stack the arguments for the tail-called function.

Values for global and module-level variables are not put directly on the stack, but instead stored in “buckets,” and an array of accessible buckets is kept on the stack. When a closure body needs to access a global variable, the closure captures and later restores the bucket array in the same way that it captured and restores a local variable. Mutable local variables are boxed similarly to global variables, but individual boxes are referenced from the stack and closures.

#### 7.3`` API for Marshaling Bytecode

(require compiler/zo-marshal) | package: zo-lib

> procedure (zo-marshal-to top out) → void?
> top : (or/c linkl-directory? linkl-bundle?)
> out : output-port?

Consumes a representation of bytecode and writes it to out.

> procedure (zo-marshal top) → bytes?
> top : (or/c linkl-directory? linkl-bundle?)

Consumes a representation of bytecode and generates a byte string for the marshaled bytecode.

#### 7.4`` Bytecode Representation

(require compiler/zo-structs) | package: zo-lib

The compiler/zo-structs library defines the bytecode structures that are produced by zo-parse and consumed by decompile and zo-marshal.

> Warning: The compiler/zo-structs library exposes internals of the Racket bytecode abstraction. Unlike other Racket libraries, compiler/zo-structs is subject to incompatible changes across Racket versions.

> > struct
> > (struct |  | zo ()
> #:extra-constructor-name make-zo
> #:prefab)

A supertype for all forms that can appear in compiled code.

##### 7.4.1`` Prefix

> > struct
> > (struct |  | linkl-directory zo (table)
> #:extra-constructor-name make-linkl-directory
> #:prefab)
> table : (hash/c (listof symbol?) linkl-bundle?)
> > struct
> > (struct |  | linkl-bundle zo (table)
> #:extra-constructor-name make-linkl-bundle
> #:prefab)
> table : (hash/c (or/c symbol? fixnum?) (or linkl? any/c))

Wraps compiled code.

Module and top-level compilation produce one or more linklets that represent independent evaluation in a specific phase. Even a single top-level expression or a module with only run-time code will generate multiple linklets to implement metadata and syntax data. A module with no submodules is represented directly by a linkl-bundle, while any other compiled form is represented by a linkl-directory.

A linklet bundle maps an integer to a linklet representing forms to evaluate at the integer-indicated phase. Symbols are mapped to metadata, such as a module’s name as compiled or a linklet implementing literal syntax objects. A linklet directory normally maps '() to the main linklet bundle for a module or a single top-level form; for a linklet directory that corresponds to a sequence of top-level forms, however, there is no “main” linklet bundle, and symbol forms of integers are used to order the linkets.

For a module with submodules, the linklet directory maps submodule paths (as lists of symbols) to linklet bundles for the corresponding submodules.

> > struct
> > (struct |  | linkl zo | ( | name
> > |  |  |  | importss
> > |  |  |  | import-shapess
> > |  |  |  | exports
> > |  |  |  | internals
> > |  |  |  | lifts
> > |  |  |  | source-names
> > |  |  |  | body
> > |  |  |  | max-let-depth
> > |  |  |  | need-instance-access?)
> #:extra-constructor-name make-linkl
> #:prefab)
> name : symbol?
> importss : (listof (listof symbol?))
> | import-shapess |  | : |  |
> (listof (listof (or/c #f 'constant 'fixed
> function-shape?
> struct-shape?)))
> exports : (listof symbol?)
> internals : (listof (or/c symbol? #f))
> lifts : (listof symbol?)
> source-names : (hash/c symbol? symbol?)
> body : (listof (or/c form? any/c))
> max-let-depth : exact-nonnegative-integer?
> need-instance-access? : boolean?

Represents a linklet, which corresponds to a module body or a top-level sequence at a single phase.

The name of a linklet is for debugging purposes, similar to the inferred name of a lambda form.

The importss list of lists describes the linklet’s imports. Each of the elements of the out list corresponds to an import source, and each element of an inner list is the symbolic name of an export from that source. The import-shapess list is in parallel to imports; it reflects optimization assumptions by the compiler that are used by the bytecode validator and checked when the linklet is instantiated.

The exports list describes the linklet’s defined names that are exported. The internals list describes additional definitions within the linket, but they are not accessible from the outside of a linklet or one of its instances; a #f can appear in place of an unreferenced internal definition that has been removed. The lifts list is an extension of internals for procedures that are lifted by the compiler; these procedures have certain properties that can be checked by the bytecode validator.

Each symbol in exports, internals, and lifts must be distinct from any other symbol in those lists. The source-names table maps symbols in exports, internals, and lifts to other symbols, potentially not distinct, that correspond to original source names for the definition. The source-names table is used only for debugging.

When a linklet is instantiated, variables correponding to the flattening of the lists importss, exports, internals, and lifts are placed in an array (in that order) for access via toplevel references. The initial slot is reserved for a variable-like reference that strongly retains a connection to an instance of its enclosing linklet.

The bodys list is the executable content of the linklet. The value of the last element in bodys may be returned when the linklet is instantiated, depending on the way that it’s instantiated.

The max-let-depth field indicates the maximum size of the stack that will be created by any body.

The need-instance-access? boolean indicates whether the linklet contains a toplevel for position 0. A #t is allowed (but suboptimal) if not such reference is present in the linklet body.

> > struct
> > (struct |  | function-shape (arity preserves-marks?)
> #:extra-constructor-name make-function-shape
> #:prefab)
> arity : procedure-arity?
> preserves-marks? : boolean?

Represents the shape of an expected import, which should be a function having the arity specified by arity. The preserves-marks? field is true if calling the function is expected to leave continuation marks unchanged by the time it returns.

> > struct
> > (struct |  | struct-shape ()
> #:extra-constructor-name make-struct-shape
> #:prefab)
> > struct
> > (struct |  | struct-type-shape struct-shape (field-count authentic?)
> #:extra-constructor-name make-struct-type-shape
> #:prefab)
> field-count : exact-nonnegative-integer?
> authentic? : boolean?
> > struct
> > (struct |  | constructor-shape struct-shape (arity)
> #:extra-constructor-name make-constructor-shape
> #:prefab)
> arity : exact-nonnegative-integer?
> > struct
> > (struct |  | predicate-shape struct-shape (authentic?)
> #:extra-constructor-name make-predicate-shape
> #:prefab)
> authentic? : boolean?
> > struct
> > (struct |  | accessor-shape struct-shape (field-count authentic?)
> #:extra-constructor-name make-accessor-shape
> #:prefab)
> field-count : exact-nonnegative-integer?
> authentic? : boolean?
> > struct
> > (struct |  | mutator-shape struct-shape (field-count authentic?)
> #:extra-constructor-name make-mutator-shape
> #:prefab)
> field-count : exact-nonnegative-integer?
> authentic? : boolean?
> > struct
> > (struct |  | struct-type-property-shape struct-shape (has-guard?)
> #:extra-constructor-name make-struct-type-property-shape
> #:prefab)
> has-guard? : boolean?
> > struct
> > (struct |  | property-predicate-shape struct-shape ()
> #:extra-constructor-name make-property-predicate-shape
> #:prefab)
> > struct
> > (struct |  | property-accessor-shape struct-shape ()
> #:extra-constructor-name make-property-accessor-shape
> #:prefab)
> > struct
> > (struct |  | struct-other-shape struct-shape ()
> #:extra-constructor-name make-struct-other-shape
> #:prefab)

Represents the shape of an expected import as a structure-type binding, constructor, etc.

##### 7.4.2`` Forms and Inline Variants

> > struct
> > (struct |  | form zo ()
> #:extra-constructor-name make-form
> #:prefab)

A supertype for all forms that can appear in a linklet body (including exprs), except for literals that are represented as themselves.

> > struct
> > (struct |  | def-values form (ids rhs)
> #:extra-constructor-name make-def-values
> #:prefab)
> ids : (listof toplevel?)
> rhs : (or/c expr? seq? inline-variant? any/c)

Represents a define-values form. Each element of ids references a defined variable in the enclosing linklet.

After rhs is evaluated, the stack is restored to its depth from before evaluating rhs.

> > struct
> > (struct |  | inline-variant zo (direct inline)
> #:extra-constructor-name make-inline-variant
> #:prefab)
> direct : expr?
> inline : expr?

Represents a function that is bound by define-values, where the function has two variants. The first variant is used for normal calls to the function. The second may be used for cross-module inlining of the function.

##### 7.4.3`` Expressions

> > struct
> > (struct |  | expr form ()
> #:extra-constructor-name make-expr
> #:prefab)

A supertype for all expression forms that can appear in compiled code, except for literals that are represented as themselves.

> > struct
> > (struct |  | lam expr | ( | name
> > |  |  |  | flags
> > |  |  |  | num-params
> > |  |  |  | param-types
> > |  |  |  | rest?
> > |  |  |  | closure-map
> > |  |  |  | closure-types
> > |  |  |  | toplevel-map
> > |  |  |  | max-let-depth
> > |  |  |  | body)
> #:extra-constructor-name make-lam
> #:prefab)
> name : (or/c symbol? vector?)
> | flags |  | : |  |
> (listof (or/c 'preserves-marks 'is-method 'single-result
> 'only-rest-arg-not-used 'sfs-clear-rest-args))
> num-params : exact-nonnegative-integer?
> param-types : (listof (or/c 'val 'ref 'flonum 'fixnum 'extflonum))
> rest? : boolean?
> closure-map : (vectorof exact-nonnegative-integer?)
> closure-types : (listof (or/c 'val/ref 'flonum 'fixnum 'extflonum))
> toplevel-map : (or/c #f (set/c exact-nonnegative-integer?))
> max-let-depth : exact-nonnegative-integer?
> body : (or/c expr? seq? any/c)

Represents a lambda form. The name field is a name for debugging purposes. The num-params field indicates the number of arguments accepted by the procedure, not counting a rest argument; the rest? field indicates whether extra arguments are accepted and collected into a “rest” variable. The param-types list contains num-params symbols indicating the type of each argumet, either 'val for a normal argument, 'ref for a boxed argument (representing a mutable local variable), 'flonum for a flonum argument, or 'extflonum for an extflonum argument.

The closure-map field is a vector of stack positions that are captured when evaluating the lambda form to create a closure. The closure-types field provides a corresponding list of types, but no distinction is made between normal values and boxed values; also, this information is redundant, since it can be inferred by the bindings referenced though closure-map.

When a closure captures top-level or module-level variables or refers to a syntax-object constant, the variables and constants are represented in the closure by capturing a prefix (in the sense of prefix). The toplevel-map field indicates which top-level variables (i.e., linklet imports and definitions) are actually used by the closure (so that variables in a prefix can be pruned by the run-time system if they become unused) and whether any syntax objects are used (so that the syntax objects as a group can be similarly pruned). A #f value indicates either that no prefix is captured or all variables and syntax objects in the prefix should be considered used. Otherwise, numbers in the set indicate which variables and lifted variables are used. Variables are numbered consecutively by position in the prefix starting from 0, but the number equal to the number of non-lifted variables corresponds to syntax objects (i.e., the number is include if any syntax-object constant is used). Lifted variables are numbered immediately afterward—which means that, if the prefix contains any syntax objects, lifted-variable numbers are shifted down relative to a toplevel by the number of syntax object in the prefix (which makes the toplevel-map set more compact).

When the function is called, the rest-argument list (if any) is pushed onto the stack, then the normal arguments in reverse order, then the closure-captured values in reverse order. Thus, when body is run, the first value on the stack is the first value captured by the closure-map array, and so on.

The max-let-depth field indicates the maximum stack depth created by body plus the arguments and closure-captured values pushed onto the stack. The body field is the expression for the closure’s body.

Changed in version 6.1.1.8 of package zo-lib: Added a number to toplevel-map to indicate whether any syntax object is used, shifting numbers for lifted variables up by one if any syntax object is in the prefix.

> > struct
> > (struct |  | closure expr (code gen-id)
> #:extra-constructor-name make-closure
> #:prefab)
> code : lam?
> gen-id : symbol?

A lambda form with an empty closure, which is a procedure constant. The procedure constant can appear multiple times in the graph of expressions for bytecode, and the code field can be a cycle for a recursive constant procedure; the gen-id is different for each such constant.

> > struct
> > (struct |  | case-lam expr (name clauses)
> #:extra-constructor-name make-case-lam
> #:prefab)
> name : (or/c symbol? vector?)
> clauses : (listof lam?)

Represents a case-lambda form as a combination of lambda forms that are tried (in order) based on the number of arguments given.

> > struct
> > (struct |  | let-one expr (rhs body type unused?)
> #:extra-constructor-name make-let-one
> #:prefab)
> rhs : (or/c expr? seq? any/c)
> body : (or/c expr? seq? any/c)
> type : (or/c #f 'flonum 'fixnum 'extflonum)
> unused? : boolean?

Pushes an uninitialized slot onto the stack, evaluates rhs and puts its value into the slot, and then runs body. If type is not #f, then rhs must produce a value of the corresponding type, and the slot must be accessed by localrefs that expect the type. If unused? is #t, then the slot must not be used, and the value of rhs is not actually pushed onto the stack (but rhs is constrained to produce a single value).

After rhs is evaluated, the stack is restored to its depth from before evaluating rhs. Note that the new slot is created before evaluating rhs.

> > struct
> > (struct |  | let-void expr (count boxes? body)
> #:extra-constructor-name make-let-void
> #:prefab)
> count : exact-nonnegative-integer?
> boxes? : boolean?
> body : (or/c expr? seq? any/c)

Pushes count uninitialized slots onto the stack and then runs body. If boxes? is #t, then the slots are filled with boxes that contain #<undefined>.

> > struct
> > (struct |  | install-value expr (count pos boxes? rhs body)
> #:extra-constructor-name make-install-value
> #:prefab)
> count : exact-nonnegative-integer?
> pos : exact-nonnegative-integer?
> boxes? : boolean?
> rhs : (or/c expr? seq? any/c)
> body : (or/c expr? seq? any/c)

Runs rhs to obtain count results, and installs them into existing slots on the stack in order, skipping the first pos stack positions. If boxes? is #t, then the values are put into existing boxes in the stack slots.

After rhs is evaluated, the stack is restored to its depth from before evaluating rhs.

> > struct
> > (struct |  | let-rec expr (procs body)
> #:extra-constructor-name make-let-rec
> #:prefab)
> procs : (listof lam?)
> body : (or/c expr? seq? any/c)

Represents a letrec form with lambda bindings. It allocates a closure shell for each lambda form in procs, installs each onto the stack in previously allocated slots in reverse order (so that the closure shell for the last element of procs is installed at stack position 0), fills out each shell’s closure (where each closure normally references some other just-created closures, which is possible because the shells have been installed on the stack), and then evaluates body.

> > struct
> > (struct |  | boxenv expr (pos body)
> #:extra-constructor-name make-boxenv
> #:prefab)
> pos : exact-nonnegative-integer?
> body : (or/c expr? seq? any/c)

Skips pos elements of the stack, setting the slot afterward to a new box containing the slot’s old value, and then runs body. This form appears when a lambda argument is mutated using set! within its body; calling the function initially pushes the value directly on the stack, and this form boxes the value so that it can be mutated later.

> > struct
> > (struct |  | localref expr (unbox? pos clear? other-clears? type)
> #:extra-constructor-name make-localref
> #:prefab)
> unbox? : boolean?
> pos : exact-nonnegative-integer?
> clear? : boolean?
> other-clears? : boolean?
> type : (or/c #f 'flonum 'fixnum 'extflonum)

Represents a local-variable reference; it accesses the value in the stack slot after the first pos slots. If unbox? is #t, the stack slot contains a box, and a value is extracted from the box. If clear? is #t, then after the value is obtained, the stack slot is cleared (to avoid retaining a reference that can prevent reclamation of the value as garbage). If other-clears? is #t, then some later reference to the same stack slot may clear after reading. If type is not #f, the slot is known to hold a specific type of value.

> > struct
> > (struct |  | toplevel expr (depth pos const? ready?)
> #:extra-constructor-name make-toplevel
> #:prefab)
> depth : exact-nonnegative-integer?
> pos : exact-nonnegative-integer?
> const? : boolean?
> ready? : boolean?

Represents a reference to an imported or defined variable within a linklet. The depth field indicates the number of stack slots to skip to reach the prefix array, and pos is the offset into the array.

When the toplevel is an expression, if both const? and ready? are #t, then the variable definitely will be defined, its value stays constant, and the constant is effectively the same for every module instantiation. If only const? is #t, then the value is constant, but it may vary across instantiations. If only ready? is #t, then the variable definitely will be defined, but its value may change. If const? and ready? are both #f, then a check is needed to determine whether the variable is defined.

When the toplevel is the left-hand side for def-values, then const? is #f. If ready? is #t, the variable is marked as immutable after it is defined.

> > struct
> > (struct |  | application expr (rator rands)
> #:extra-constructor-name make-application
> #:prefab)
> rator : (or/c expr? seq? any/c)
> rands : (listof (or/c expr? seq? any/c))

Represents a function call. The rator field is the expression for the function, and rands are the argument expressions. Before any of the expressions are evaluated, (length rands) uninitialized stack slots are created (to be used as temporary space).

> > struct
> > (struct |  | branch expr (test then else)
> #:extra-constructor-name make-branch
> #:prefab)
> test : (or/c expr? seq? any/c)
> then : (or/c expr? seq? any/c)
> else : (or/c expr? seq? any/c)

Represents an if form.

After test is evaluated, the stack is restored to its depth from before evaluating test.

> > struct
> > (struct |  | with-cont-mark expr (key val body)
> #:extra-constructor-name make-with-cont-mark
> #:prefab)
> key : (or/c expr? seq? any/c)
> val : (or/c expr? seq? any/c)
> body : (or/c expr? seq? any/c)

Represents a with-continuation-mark expression.

After each of key and val is evaluated, the stack is restored to its depth from before evaluating key or val.

> > struct
> > (struct |  | seq expr (forms)
> #:extra-constructor-name make-seq
> #:prefab)
> forms : (listof (or/c expr? any/c))

Represents a begin form.

After each form in forms is evaluated, the stack is restored to its depth from before evaluating the form.

> > struct
> > (struct |  | beg0 expr (seq)
> #:extra-constructor-name make-beg0
> #:prefab)
> seq : (listof (or/c expr? seq? any/c))

Represents a begin0 expression.

After each expression in seq is evaluated, the stack is restored to its depth from before evaluating the expression.

Unlike the begin0 source form, the first expression in seq is never in tail position, even if it is the only expression in the list.

> > struct
> > (struct |  | varref expr (toplevel dummy constant? from-unsafe?)
> #:extra-constructor-name make-varref
> #:prefab)
> toplevel : (or/c toplevel? #t #f symbol?)
> dummy : (or/c toplevel? #f)
> constant? : boolean?
> from-unsafe? : boolean?

Represents a #%variable-reference form. The toplevel field is #t if the original reference was to a constant local binding, #f if the variable reference is for (#%variable-reference) and does not refer to a specific variable, or a symbol if the variable reference refers to a primitive variable. The dummy field accesses a variable bucket that strongly references its namespace (as opposed to a normal variable bucket, which only weakly references its namespace); it can be #f.

The value of constant? is true when the toplevel field is not #t but the referenced variable is known to be constant. The value of from-unsafe? is true when the module that created the reference was compiled in unsafe mode.

> > struct
> > (struct |  | assign expr (id rhs undef-ok?)
> #:extra-constructor-name make-assign
> #:prefab)
> id : toplevel?
> rhs : (or/c expr? seq? any/c)
> undef-ok? : boolean?

Represents a set! expression that assigns to a top-level or module-level variable. (Assignments to local variables are represented by install-value expressions.) If undef-ok? is true, the assignment to id succeeds even if id was not previously defined (see also compile-allow-set!-undefined).

After rhs is evaluated, the stack is restored to its depth from before evaluating rhs.

> > struct
> > (struct |  | apply-values expr (proc args-expr)
> #:extra-constructor-name make-apply-values
> #:prefab)
> proc : (or/c expr? seq? any/c)
> args-expr : (or/c expr? seq? any/c)

Represents (call-with-values (lambda () args-expr) proc), which is handled specially by the run-time system.

> > struct
> > (struct |  | with-immed-mark expr (key def-val body)
> #:extra-constructor-name make-with-immed-mark
> #:prefab)
> key : (or/c expr? seq? any/c)
> def-val : (or/c expr? seq? any/c)
> body : (or/c expr? seq? any/c)

Represents a (call-with-immediate-continuation-mark key (lambda (arg) body) val) expression that is handled specially by the run-time system to avoid a closure allocation. One initialized slot is pushed onto the stack after expr and val are evaluated and before body is evaluated.

After each of key and val is evaluated, the stack is restored to its depth from before evaluating key or val.

> > struct
> > (struct |  | primval expr (id)
> #:extra-constructor-name make-primval
> #:prefab)
> id : exact-nonnegative-integer?

Represents a direct reference to a variable imported from the run-time kernel.
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