Revised^6 Report on the Algorithmic Language Scheme

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Chapter 5
Semantic concepts

## 5.1 Programs and libraries

A Scheme program consists of a top-level program together with a set of libraries, each of which defines a part of the program connected to the others through explicitly specified exports and imports. A library consists of a set of export and import specifications and a body, which consists of definitions, and expressions. A top-level program is similar to a library, but has no export specifications. Chapters 7 and 8 describe the syntax and semantics of libraries and top-level programs, respectively. Chapter 11 describes a base library that defines many of the constructs traditionally associated with Scheme. A separate report [24] describes the various standard librariesprovided by a Scheme system.

The division between the base library and the other standard libraries is based on use, not on construction. In particular, some facilities that are typically implemented as “primitives” by a compiler or the run-time system rather than in terms of other standard procedures or syntactic forms are not part of the base library, but are defined in separate libraries. Examples include the fixnums and flonums libraries, the exceptions and conditions libraries, and the libraries for records.

## 5.2 Variables, keywords, and regions

Within the body of a library or top-level program, an identifiermay name a kind of syntax, or it may name a location where a value can be stored. An identifier that names a kind of syntax is called a keyword, or syntactic keyword, and is said to be bound to that kind of syntax (or, in the case of a syntactic abstraction, a transformer that translates the syntax into more primitive forms; see section 9.2). An identifier that names a location is called a variableand is said to be bound to that location. At each point within a top-level program or a library, a specific, fixed set of identifiers is bound. The set of these identifiers, the set of visible bindings, is known as the environment in effect at that point.

Certain forms are used to create syntactic abstractions and to bind keywords to transformers for those new syntactic abstractions, while other forms create new locations and bind variables to those locations. Collectively, these forms are called binding constructs.Some binding constructs take the form of definitions, while others are expressions. With the exception of exported library bindings, a binding created by a definition is visible only within the body in which the definition appears, e.g., the body of a library, top-level program, or `lambda` expression. Exported library bindings are also visible within the bodies of the libraries and top-level programs that import them (see chapter 7).

Expressions that bind variables include the `lambda`, `let`, `let*`, `letrec`, `letrec*`, `let-values`, and `let*-values` forms from the base library (see sections 11.4.2, 11.4.6). Of these, `lambda` is the most fundamental. Variable definitions appearing within the body of such an expression, or within the bodies of a library or top-level program, are treated as a set of `letrec*` bindings. In addition, for library bodies, the variables exported from the library can be referenced by importing libraries and top-level programs.

Expressions that bind keywords include the `let-syntax` and `letrec-syntax` forms (see section 11.18). A `define` form (see section 11.2.1) is a definition that creates a variable binding (see section 11.2), and a `define-syntax` form is a definition that creates a keyword binding (see section 11.2.2).

Scheme is a statically scoped language with block structure. To each place in a top-level program or library body where an identifier is bound there corresponds a region of code within which the binding is visible. The region is determined by the particular binding construct that establishes the binding; if the binding is established by a `lambda` expression, for example, then its region is the entire `lambda` expression. Every mention of an identifier refers to the binding of the identifier that establishes the innermost of the regions containing the use. If a use of an identifier appears in a place where none of the surrounding expressions contains a binding for the identifier, the use may refer to a binding established by a definition or import at the top of the enclosing library or top-level program (see chapter 7). If there is no binding for the identifier, it is said to be unbound.

## 5.3 Exceptional situations

A variety of exceptional situations are distinguished in this report, among them violations of syntax, violations of a procedure’s specification, violations of implementation restrictions, and exceptional situations in the environment. When an exceptional situation is detected by the implementation, an exception is raised, which means that a special procedure called the current exception handler is called. A program can also raise an exception, and override the current exception handler; see library section on “Exceptions”.

When an exception is raised, an object is provided that describes the nature of the exceptional situation. The report uses the condition system described in library section on “Conditions” to describe exceptional situations, classifying them by condition types.

Some exceptional situations allow continuing the program if the exception handler takes appropriate action. The corresponding exceptions are called continuable. For most of the exceptional situations described in this report, portable programs cannot rely upon the exception being continuable at the place where the situation was detected. For those exceptions, the exception handler that is invoked by the exception should not return. In some cases, however, continuing is permissible, and the handler may return. See library section on “Exceptions”.

Implementations must raise an exception when they are unable to continue correct execution of a correct program due to some implementation restriction. For example, an implementation that does not support infinities must raise an exception with condition type `&implementation-restriction` when it evaluates an expression whose result would be an infinity.

Some possible implementation restrictions such as the lack of representations for NaNs and infinities (see section 11.7.2) are anticipated by this report, and implementations typically must raise an exception of the appropriate condition type if they encounter such a situation.

This report uses the phrase “an exception is raised” synonymously with “an exception must be raised”. This report uses the phrase “an exception with condition type t” to indicate that the object provided with the exception is a condition object of the specified type. The phrase “a continuable exception is raised” indicates an exceptional situation that permits the exception handler to return.

## 5.4 Argument checking

Many procedures specified in this report or as part of a standard library restrict the arguments they accept. Typically, a procedure accepts only specific numbers and types of arguments. Many syntactic forms similarly restrict the values to which one or more of their subforms can evaluate. These restrictions imply responsibilitiesfor both the programmer and the implementation. Specifically, the programmer is responsible for ensuring that the values indeed adhere to the restrictions described in the specification. The implementation must check that the restrictions in the specification are indeed met, to the extent that it is reasonable, possible, and necessary to allow the specified operation to complete successfully. The implementation’s responsibilities are specified in more detail in chapter 6 and throughout the report.

Note that it is not always possible for an implementation to completely check the restrictions set forth in a specification. For example, if an operation is specified to accept a procedure with specific properties, checking of these properties is undecidable in general. Similarly, some operations accept both lists and procedures that are called by these operations. Since lists can be mutated by the procedures through the `(rnrs mutable-pairs (6))` library (see library chapter on “Mutable pairs”), an argument that is a list when the operation starts may become a non-list during the execution of the operation. Also, the procedure might escape to a different continuation, preventing the operation from performing more checks. Requiring the operation to check that the argument is a list after each call to such a procedure would be impractical. Furthermore, some operations that accept lists only need to traverse these lists partially to perform their function; requiring the implementation to traverse the remainder of the list to verify that all specified restrictions have been met might violate reasonable performance assumptions. For these reasons, the programmer’s obligations may exceed the checking obligations of the implementation.

When an implementation detects a violation of a restriction for an argument, it must raise an exception with condition type `&assertion` in a way consistent with the safety of execution as described in section 5.6.

## 5.5 Syntax violations

The subforms of a special form usually need to obey certain syntactic restrictions. As forms may be subject to macro expansion, which may not terminate, the question of whether they obey the specified restrictions is undecidable in general.

When macro expansion terminates, however, implementations must detect violations of the syntax. A syntax violation is an error with respect to the syntax of library bodies, top-level bodies, or the “syntax” entries in the specification of the base library or the standard libraries. Moreover, attempting to assign to an immutable variable (i.e., the variables exported by a library; see section 7.1) is also considered a syntax violation.

If a top-level or library form in a program is not syntactically correct, then the implementation must raise an exception with condition type `&syntax`, and execution of that top-level program or library must not be allowed to begin.

## 5.6 Safety

The standard libraries whose exports are described by this document are said to be safe libraries. Libraries and top-level programs that import only from safe libraries are also said to be safe.

As defined by this document, the Scheme programming language is safe in the following sense: The execution of a safe top-level program cannot go so badly wrong as to crash or to continue to execute while behaving in ways that are inconsistent with the semantics described in this document, unless an exception is raised.

Violations of an implementation restriction must raise an exception with condition type `&implementation-restriction`, as must all violations and errors that would otherwise threaten system integrity in ways that might result in execution that is inconsistent with the semantics described in this document.

The above safety properties are guaranteed only for top-level programs and libraries that are said to be safe. In particular, implementations may provide access to unsafe libraries in ways that cannot guarantee safety.

## 5.7 Boolean values

Although there is a separate boolean type, any Scheme value can be used as a boolean value for the purpose of a conditional test. In a conditional test, all values count as true in such a test except for `#f`. This report uses the word “true” to refer to any Scheme value except `#f`, and the word “false” to refer to `#f`.

## 5.8 Multiple return values

A Scheme expression can evaluate to an arbitrary finite number of values. These values are passed to the expression’s continuation.

Not all continuations accept any number of values. For example, a continuation that accepts the argument to a procedure call is guaranteed to accept exactly one value. The effect of passing some other number of values to such a continuation is unspecified. The `call-with-values` procedure described in section 11.15 makes it possible to create continuations that accept specified numbers of return values. If the number of return values passed to a continuation created by a call to `call-with-values` is not accepted by its consumer that was passed in that call, then an exception is raised. A more complete description of the number of values accepted by different continuations and the consequences of passing an unexpected number of values is given in the description of the `values` procedure in section 11.15.

A number of forms in the base library have sequences of expressions as subforms that are evaluated sequentially, with the return values of all but the last expression being discarded. The continuations discarding these values accept any number of values.

## 5.9 Unspecified behavior

If an expression is said to “return unspecified values”, then the expression must evaluate without raising an exception, but the values returned depend on the implementation; this report explicitly does not say how many or what values should be returned. Programmers should not rely on a specific number of return values or the specific values themselves.

## 5.10 Storage model

Variables and objects such as pairs, vectors, bytevectors, strings, hashtables, and records implicitly refer to locationsor sequences of locations. A string, for example, contains as many locations as there are characters in the string. (These locations need not correspond to a full machine word.) A new value may be stored into one of these locations using the `string-set!` procedure, but the string contains the same locations as before.

An object fetched from a location, by a variable reference or by a procedure such as `car`, `vector-ref`, or `string-ref`, is equivalent in the sense of `eqv?` (section 11.5) to the object last stored in the location before the fetch.

Every location is marked to show whether it is in use. No variable or object ever refers to a location that is not in use. Whenever this report speaks of storage being allocated for a variable or object, what is meant is that an appropriate number of locations are chosen from the set of locations that are not in use, and the chosen locations are marked to indicate that they are now in use before the variable or object is made to refer to them.

It is desirable for constants(i.e. the values of literal expressions) to reside in read-only memory. To express this, it is convenient to imagine that every object that refers to locations is associated with a flag telling whether that object is mutableor immutable. Literal constants, the strings returned by `symbol->string`, records with no mutable fields, and other values explicitly designated as immutable are immutable objects, while all objects created by the other procedures listed in this report are mutable. An attempt to store a new value into a location referred to by an immutable object should raise an exception with condition type `&assertion`.

## 5.11 Proper tail recursion

Implementations of Scheme must be properly tail-recursive. Procedure calls that occur in certain syntactic contexts called tail contextsare tail calls. A Scheme implementation is properly tail-recursive if it supports an unbounded number of active tail calls. A call is active if the called procedure may still return. Note that this includes regular returns as well as returns through continuations captured earlier by `call-with-current-continuation` that are later invoked. In the absence of captured continuations, calls could return at most once and the active calls would be those that had not yet returned. A formal definition of proper tail recursion can be found in Clinger’s paper [5]. The rules for identifying tail calls in constructs from the `(rnrs base (6))` library are described in section 11.20.

## 5.12 Dynamic extent and the dynamic environment

For a procedure call, the time between when it is initiated and when it returns is called its dynamic extent. In Scheme, `call-with-current-continuation` (section 11.15) allows reentering a dynamic extent after its procedure call has returned. Thus, the dynamic extent of a call may not be a single, connected time period.

Some operations described in the report acquire information in addition to their explicit arguments from the dynamic environment. For example, `call-with-current-continuation` accesses an implicit context established by `dynamic-wind` (section 11.15), and the `raise` procedure (library section on “Exceptions”) accesses the current exception handler. The operations that modify the dynamic environment do so dynamically, for the dynamic extent of a call to a procedure like `dynamic-wind` or `with-exception-handler`. When such a call returns, the previous dynamic environment is restored. The dynamic environment can be thought of as part of the dynamic extent of a call. Consequently, it is captured by `call-with-current-continuation`, and restored by invoking the escape procedure it creates.