# (lispkit core) Library `(lispkit core)` defines a foundational API for * [primitive procedures](#primitives), * [definitions](#definitions) (including an [import mechanism](#importing-definitions)), * [procedures](#procedures) (including support for [optional arguments](#procedures-with-optional-arguments) and [tagged procedures](#tagged-procedures)), * [delayed execution](#delayed-execution), * [multiple return values](#multiple-values), * [symbols](#symbols), * [booleans](#booleans), * [environments](#environments), * [syntax errors](#syntax-errors), and * [loading source files](#loading-source-files) and support for [conditional compilation](#conditional-and-inclusion-compilation). ## Primitives **(eval *****expr*****)** \ \&#xNAN;**(eval *****expr env*****)** If *expr* is an expression, it is evaluated in the specified environment *env* and its values are returned. If it is a definition, the specified identifiers are defined in the specified environment, provided the environment is not immutable. Should *env* not be provided, the global interaction environment is used. **(apply *****proc arg1 ... args*****)** The `apply` procedure calls *proc* with the elements of the list `(append (list arg1 ...) args)` as the actual arguments. **(equal? *****obj1 obj2*****)** The `equal?` procedure, when applied to pairs, vectors, strings and bytevectors, recursively compares them, returning `#t` when the unfoldings of its arguments into possibly infinite trees are equal (in the sense of `equal?`) as ordered trees, and `#f` otherwise. It returns the same as `eqv?` when applied to booleans, symbols, numbers, characters, ports, procedures, and the empty list. If two objects are `eqv?`, they must be `equal?` as well. In all other cases, `equal?` may return either `#t` or `#f`. Even if its arguments are circular data structures, `equal?` must always terminate. As a rule of thumb, objects are generally `equal?` if they print the same. **(eqv? *****obj1 obj2*****)** The `eqv?` procedure defines a useful equivalence relation on objects. It returns `#t` if *obj1* and *obj2* are regarded as the same object. The `eqv?` procedure returns `#t` if: * *obj1* and *obj2* are both `#t` or both `#f` * *obj1* and *obj2* are both symbols and are the same symbol according to the `symbol=?` procedure * *obj1* and *obj2* are both exact numbers and are numerically equal in the sense of `=` * *obj1* and *obj2* are both inexact numbers such that they are numerically equal in the sense of `=`, and they yield the same results in the sense of `eqv?` when passed as arguments to any other procedure that can be defined as a finite composition of Scheme’s standard arithmetic procedures, provided it does not result in a NaN value * *obj1* and *obj2* are both characters and are the same character according to the `char=?` procedure * *obj1* and *obj2* are both the empty list * *obj1* and *obj2* are both a pair and `car` and `cdr` of both pairs are the same in the sense of `eqv?` * *obj1* and *obj2* are ports, vectors, hashtables, bytevectors, records, or strings that denote the same location in the store * *obj1* and *obj2* are procedures whose location tags are equal The `eqv?` procedure returns `#f` if: * *obj1* and *obj2* are of different types * one of *obj1* and *obj2* is `#t` but the other is `#f` * *obj1* and *obj2* are symbols but are not the same symbol according to the `symbol=?` procedure * one of *obj1* and *obj2* is an exact number but the other is an inexact number * *obj1* and *obj2* are both exact numbers and are numerically unequal in the sense of `=` * *obj1* and *obj2* are both inexact numbers such that either they are numerically unequal in the sense of `=`, or they do not yield the same results in the sense of `eqv?` when passed as arguments to any other procedure that can be defined as a finite composition of Scheme’s standard arithmetic procedures, provided it does not result in a NaN value. As an exception, the behavior of `eqv?` is unspecified when both *obj1* and *obj2* are NaN. * *obj1* and *obj2* are characters for which the `char=?` procedure returns `#f` * one of *obj1* and *obj2* is the empty list but the other is not * *obj1* and *obj2* are both a pair but either `car` or `cdr` of both pairs are not the same in the sense of `eqv?` * *obj1* and *obj2* are ports, vectors, hashtables, bytevectors, records, or strings that denote distinct locations * *obj1* and *obj2* are procedures that would behave differently (i.e. return different values or have different side effects) for some arguments **(eq? *****obj1 obj2*****)** The `eq?` procedure is similar to `eqv?` except that in some cases it is capable of discerning distinctions finer than those detectable by `eqv?`. It always returns `#f` when `eqv?` also would, but returns `#f` in some cases where `eqv?` would return `#t`. On symbols, booleans, the empty list, pairs, and records, and also on non-empty strings, vectors, and bytevectors, `eq?` and `eqv?` are guaranteed to have the same behavior. **(quote *****datum*****)** `(quote` *datum*`)` evaluates to *datum*. *datum* can be any external representation of a LispKit object. This notation is used to include literal constants in LispKit code. `(quote` *datum*`)` can be abbreviated as ’*datum*. The two notations are equivalent in all respects. Numerical constants, string constants, character constants, vector constants, bytevector constants, and boolean constants evaluate to themselves. They need not be quoted. **(quasiquote *****template*****)** Quasiquote expressions are useful for constructing a list or vector structure when some but not all of the desired structure is known in advance. If no commas appear within *template*, the result of evaluating `(quasiquote` *template*`)` is equivalent to the result of evaluating `(quote` *template*`)`. If a comma appears within *template*, however, the expression following the comma is evaluated ("unquoted") and its result is inserted into the structure instead of the comma and the expression. If a comma appears followed without intervening whitespace by `@`, then it is an error if the following expression does not evaluate to a list; the opening and closing parentheses of the list are then "stripped away" and the elements of the list are inserted in place of the `,@` expression sequence. `,@` normally appears only within a list or vector. Quasiquote expressions can be nested. Substitutions are made only for unquoted components appearing at the same nesting level as the outermost quasiquote. The nesting level increases by one inside each successive quasiquotation, and decreases by one inside each unquotation. Comma corresponds to form `unquote`, `,@` corresponds to form `unquote-splicing`. ## Definitions **(define *****var expr*****)** \ \&#xNAN;**(define *****var expr doc*****)**\ \&#xNAN;**(define (*****f arg ...*****) *****expr ...*****)**\ \&#xNAN;**(define (*****f arg ...*****) *****doc expr ...*****)** `define` is used to define variables. At the outermost level of a program, a definition `(define` *var expr*`)` has essentially the same effect as the assignment expression `(set!` *var expr*`)` if variable *var* is bound to a non-syntax value. However, if *var* is not bound, or is a syntactic keyword, then the definition will bind *var* to a new location before performing the assignment, whereas it would be an error to perform a `set!` on an unbound variable. The form `(define (`*f arg ...*`)` *expr*`)` defines a function *f* with arguments *arg ...* and body *expr*. It is equivalent to `(define` *f* `(lambda (`*arg ...*`)` *expr*`))`. The parameter *doc* is a string literal defining documentation for variable *var*. It can be accessed, for instance, by using the procedure `environment-documentation` on the environment in which the variable was bound. ```scheme (define pi 3.141 "documentation for `pi`") (environment-documentation (interaction-environment) 'pi) ⇒ "documentation for `pi`" ``` **(define-values *****var expr*****)** \ \&#xNAN;**(define-values (*****var ...*****) *****expr*****)**\ \&#xNAN;**(define-values (*****var doc ...*****) *****expr*****)** `define-values` creates multiple definitions *var ...* from a single expression *expr* returning multiple values. It is allowed wherever `define` is allowed. *expr* is evaluated, and the variables *var ...* are bound to the return values in the same way that the formal arguments in a `lambda` expression are matched to the actual arguments in a procedure call. It is an error if a variable *var* appears more than once in *var ...*. ```scheme (define-values (x y) (integer-sqrt 17)) (list x y) ⇒ (4 1) (define-values vs (values 'a 'b 'c)) vs ⇒ (a b c) ``` The parameter *doc* is an optional string literal defining documentation for variable *var*. It directly follows the identifier name. **(define-syntax *****keyword transformer*****)** \ \&#xNAN;**(define-syntax *****keyword doc transformer*****)** Syntax definitions have the form `(define-syntax` *keyword transformer*`)`. *keyword* is an identifier, and *transformer* is an instance of `syntax-rules`. Like variable definitions, syntax definitions can appear at the outermost level or nested within a body. If the `define-syntax` occurs at the outermost level, then the global syntactic environment is extended by binding the *keyword* to the specified *transformer*, but previous expansions of any global binding for *keyword* remain unchanged. Otherwise, it is an internal syntax definition, and is local to the "body" in which it is defined. Any use of a syntax keyword before its corresponding definition is an error. Macros can expand into definitions in any context that permits them. However, it is an error for a definition to define an identifier whose binding has to be known in order to determine the meaning of the definition itself, or of any preceding definition that belongs to the same group of internal definitions. Similarly, it is an error for an internal definition to define an identifier whose binding has to be known in order to determine the boundary between the internal definitions and the expressions of the body it belongs to. Here is an example defining syntax for `while` loops. `while` evaluates the body of the loop as long as the predicate is true. ```scheme (define-syntax while (syntax-rules () ((_ pred body ...) (let loop () (when pred body ... (loop)))))) ``` The parameter *doc* is an optional string literal defining documentation for the keyword *var*: ```scheme (define-syntax kwote "alternative to quote" (syntax-rules () ((kwote exp) (quote exp)))) ``` **(syntax-rules (*****literal ...*****) *****rule ...*****)** \ \&#xNAN;**(syntax-rules *****ellipsis***** (*****literal ...*****) *****rule ...*****)** A *transformer spec* has one of the two forms listed above. It is an error if any of the *literal ...*, or the *ellipsis* symbol in the second form, is not an identifier. It is also an error if syntax rules *rule* are not of the form * `(`*pattern template*`)`. The *pattern* in a *rule* is a list pattern whose first element is an identifier. In general, a *pattern* is either an identifier, a constant, or one of the following: * `(`*pattern ...*`)` * `(`*pattern pattern ... . pattern*`)` * `(`*pattern ... pattern ellipsis pattern ...*`) (`*pattern ... pattern ellipsis pattern ... . pattern*`)` * `#(`*pattern ...*`)` * `#(`*pattern ... pattern ellipsis pattern ...*`)` A *template* is either an identifier, a constant, or one of the following: * `(`*element ...*`)` * `(`*element element ... . template*`) (`*ellipsis template*`)` * `#(`*element ...*`)` where an *element* is a *template* optionally followed by an *ellipsis*. An *ellipsis* is the identifier specified in the second form of `syntax-rules`, or the default identifier `...` (three consecutive periods) otherwise. Here is an example showcasing how `when` can be defined in terms of `if`: ```scheme (define-syntax when (syntax-rules () ((_ c e ...) (if c (begin e ...))))) ``` **(define-library (*****name ...*****) *****declaration ...*****)** A library definition takes the following form: `(define-library (`*name ...*`)` *declaration ...*`)`. `(`*name ...*`)` is a list whose members are identifiers and exact non-negative integers. It is used to identify the library uniquely when importing from other programs or libraries. It is inadvisable, but not an error, for identifiers in library names to contain any of the characters `|, \, ?, *, <, ", :, >, +, [, ], /`. A *declaration* is any of: * `(export` *exportspec ...*`)` * `(export-mutable` *exportspec ...*`)` * `(import` *importset ...*`)` * `(begin` *statement ...*`)` * `(include` *filename ...*`)` * `(include-ci` *filename ...*`)` * `(include-library-declarations` *filename ...*`)` * `(cond-expand` *clause ...*`)` An export declaration specifies a list of identifiers which can be made visible to other libraries or programs. An *exportspec* takes one of the following forms: * *ident* * `(rename` *ident1 ident2*`)` In an *exportspec*, an identifier *ident* names a single binding defined within or imported into the library, where the external name for the export is the same as the name of the binding within the library. A `rename` spec exports the binding defined within or imported into the library and named by *ident1* in each `(`*ident1 ident2*`)` pairing, using *ident2* as the external name. A regular `export` statement exports bindings in a immutable fashion, not allowing binding changes outside of the library. `export-mutable` is a LispKit-specific variant which allows library-external mutations of the exported bindings. An `import` declaration provides a way to import the identifiers exported by another library. It has the same syntax and semantics as an `import` declaration used in a program or at the read-eval-print loop. The `begin`, `include`, and `include-ci` declarations are used to specify the body of the library. They have the same syntax and semantics as the corresponding expression types. The `include-library-declarations` declaration is similar to `include` except that the contents of the file are spliced directly into the current library definition. This can be used, for example, to share the same `export` declaration among multiple libraries as a simple form of library interface. The `cond-expand` declaration has the same syntax and semantics as the `cond-expand` expression type, except that it expands to spliced-in library declarations rather than expressions enclosed in `begin`. **(set! *****var expr*****)** Procedure `set!` is used to assign values to variables. *expr* is evaluated, and the resulting value is stored in the location to which variable *var* is bound. It is an error if *var* is not bound either in some region enclosing the `set!` expression or else globally. The result of the `set!` expression is unspecified. ## Importing definitions **(import *****importset ...*****)** An `import` declaration provides a way to import identifiers exported by a library. Each *importset* names a set of bindings from a library and possibly specifies local names for the imported bindings. It takes one of the following forms: * *libraryname* * `(only` *importset identifier ...*`)` * `(except` *importset identifier ...*`)` * `(prefix` *importset identifier*`)` * `(rename` *importset* `(`*ifrom ito*`)` ...`)` In the first form, all of the identifiers in the named library’s export clauses are imported with the same names (or the exported names if exported with `rename`). The additional *importset* forms modify this set as follows: * `only` produces a subset of the given *importset* including only the listed identifiers (after any renaming). It is an error if any of the listed identifiers are not found in the original set. * `except` produces a subset of the given *importset*, excluding the listed identifiers (after any renaming). It is an error if any of the listed identifiers are not found in the original set. * `rename` modifies the given *importset*, replacing each instance of *ifrom* with *ito*. It is an error if any of the listed identifiers are not found in the original set. * `prefix` automatically renames all identifiers in the given *importset*, prefixing each with the specified identifier. In a program or library declaration, it is an error to import the same identifier more than once with different bindings, or to redefine or mutate an imported binding with a definition or with `set!`, or to refer to an identifier before it is imported. However, a read-eval-print loop will permit these actions. ## Procedures **(procedure? *****obj*****)** Returns `#t` if *obj* is a procedure; otherwise, it returns `#f`. **(thunk? *****obj*****)** Returns `#t` if *obj* is a procedure which accepts zero arguments; otherwise, it returns `#f`. **(procedure-of-arity? *****obj n*****)** Returns `#t` if *obj* is a procedure that accepts *n* arguments; otherwise, it returns `#f`. This is equivalent to: ```scheme (and (procedure? obj) (procedure-arity-includes? obj n)) ``` **(lambda *****(arg1 ...) expr ...*****)** \ \&#xNAN;**(lambda *****(arg1 ... . rest) expr ...*****)**\ \&#xNAN;**(lambda *****rest expr ...*****)**\ \&#xNAN;**(λ *****(arg1 ...) expr ...*****)**\ \&#xNAN;**(λ *****(arg1 ... . rest) expr ...*****)**\ \&#xNAN;**(λ *****rest expr ...*****)** A lambda expression evaluates to a procedure. The environment in effect when the lambda expression was evaluated is remembered as part of the procedure. When the procedure is later called with some actual arguments, the environment in which the lambda expression was evaluated will be extended by binding the variables in the formal argument list *arg1 ...* to fresh locations, and the corresponding actual argument values will be stored in those locations. Next, the expressions in the body of the lambda expression will be evaluated sequentially in the extended environment. The results of the last expression in the body will be returned as the results of the procedure call. **(case-lambda *****(formals expr ...) ...*****)** \ \&#xNAN;**(case-λ *****(formals expr ...) ...*****)** A case-lambda expression evaluates to a procedure that accepts a variable number of arguments and is lexically scoped in the same manner as a procedure resulting from a lambda expression. When the procedure is called, the first clause for which the arguments agree with *formals* is selected, where agreement is specified as for *formals* of a lambda expression. The variables of *formals* are bound to fresh locations, the values of the arguments are stored in those locations, the expressions in the body are evaluated in the extended environment, and the results of the last expression in the body is returned as the results of the procedure call. It is an error for the arguments not to agree with *formals* of any clause. Here is an example showing how to use `case-lambda` for defining a simple accumulator: ```scheme (define (make-accumulator n) (case-lambda (() n) ((m) (set! n (+ n m)) n))) (define a (make-accumulator 1)) (a) ⇒ 1 (a 5) ⇒ 6 (a) ⇒ 6 ``` **(thunk *****expr ...*****)** Returns a procedure accepting no arguments and evaluating *expr* ..., returning the result of the last expression being evaluated as the result of a procedure call. `(thunk expr ...)` is equivalent to `(lambda () expr ...)`. **(thunk\* *****expr ...*****)** Returns a procedure accepting an arbitrary amount of arguments and evaluating *expr* ..., returning the result of the last expression being evaluated as the result of a procedure call. `(thunk* expr ...)` is equivalent to `(lambda args expr ...)`. **(procedure-name *****proc*****)** Returns the name of procedure *proc* as a string, or `#f` if *proc* does not have a name. **(procedure-rename *****proc name*****)** Creates a copy of procedure *proc* with *name* as name. *name* is either a string or a symbol. If it is not possible to create a renamed procedure, `procedure-rename` returns `#f`. **(procedure-arity *****proc*****)** Returns a value representing the arity of procedure *proc*, or returns `#f` if no arity information is available for *proc*. If `procedure-arity` returns a fixnum *k*, then procedure *proc* accepts exactly *k* arguments and applying *proc* to some number of arguments other than *k* will result in an arity error. If `procedure-arity` returns an "arity-at-least" object *a*, then procedure *proc* accepts `(arity-at-least-value a)` or more arguments and applying *proc* to some number of arguments less than `(arity-at-least-value a)` will result in an arity error. If `procedure-arity` returns a list, then procedure *proc* accepts any of the arities described by the elements of the list. Applying *proc* to some number of arguments not described by an element of the list will result in an arity error. **(procedure-arity-range *****proc*****)** Returns the smallest arity range in form of a pair *(min . max)* such that if *proc* is provided *n* arguments with *n < min* or *n > max*, an arity error gets raised. ```scheme (procedure-arity-range (lambda () 3)) ⇒ (0 . 0) (procedure-arity-range (lambda (x) x)) ⇒ (1 . 1) (procedure-arity-range (lambda x x)) ⇒ (0 . #f) (procedure-arity-range (lambda (x . y) x)) ⇒ (1 . #f) ``` **(procedure-arity-includes? *****proc k*****)** Returns `#t` if procedure *proc* can accept *k* arguments and `#f` otherwise. If this procedure returns `#f`, applying *proc* to *k* arguments will result in an arity error. **(arity-at-least? *****obj*****)** Returns `#t` if *obj* is an "arity-at-least" object and `#f` otherwise. **(arity-at-least-value *****obj*****)** Returns a fixnum denoting the minimum number of arguments required by the given "arity-at-least" object *obj*. ## Procedures with optional arguments **(opt-lambda *****(arg1 ... arg1 bind1 ... bindm) expr ...*****)** \ \&#xNAN;**(opt-lambda *****(arg1 ... argn bind1 ... bindm . rest) expr ...*****)**\ \&#xNAN;**(opt-lambda *****rest expr ...*****)** An `opt-lambda` expression evaluates to a procedure and is lexically scoped in the same manner as a procedure resulting from a `lambda` expression. When the procedure is later called with actual arguments, the variables are bound to fresh locations, the values of the corresponding arguments are stored in those locations, the body *expr ...* is evaluated in the extended environment, and the result of the last body expression is returned as the result of the procedure call. Formal arguments *argi* are required arguments. Arguments *bindi* are optional. They are of the form `(var init)`, with *var* being a symbol and *init* an initialization expression which gets evaluated and assigned to *var* if the argument is not provided when the procedure is called. It is a syntax violation if the same variable appears more than once among the variables. A procedure created with the first syntax of `opt-formals` takes at least *n* arguments and at most *n + m* arguments. A procedure created with the second syntax of `opt-formals` takes *n* or more arguments. If the procedure is called with fewer than *n* + *m* (but at least *n* arguments), the missing actual arguments are substituted by the values resulting from evaluating the corresponding \_init\_s. The corresponding \_init\_s are evaluated in an unspecified order in the lexical environment of the `opt-lambda` expression when the procedure is called. **(opt\*-lambda *****(arg1 ... arg1 bind1 ... bindm) expr ...*****)** \ \&#xNAN;**(opt\*-lambda *****(arg1 ... argn bind1 ... bindm . rest) expr ...*****)**\ \&#xNAN;**(opt\*-lambda *****rest expr ...*****)** Similar to syntax `opt-lambda` except that the initializers of optional arguments *bindi* corresponding to missing actual arguments are evaluated sequentially from left to right. The region of the binding of a variable is that part of the `opt*-lambda` expression to the right of it or its binding. **(define-optionals (*****f arg ... bind ...*****) *****expr ...*****)** \ \&#xNAN;**(define-optionals (*****f arg ... bind ... . rest*****) *****expr ...*****)** `define-optionals` is operationally equivalent to `(define f (opt-lambda (arg ... bind ...) expr ...))` or `(define f (opt-lambda (arg ... bind ... . rest) expr ...))` if the second syntactical form is used. **(define-optionals\* (*****f arg ... bind ...*****) *****expr ...*****)** \ \&#xNAN;**(define-optionals\* (*****f arg ... bind ... . rest*****) *****expr ...*****)** `define-optionals*` is operationally equivalent to `(define f (opt*-lambda (arg ... bind ...) expr ...))` or `(define f (opt*-lambda (arg ... bind ... . rest) expr ...))` if the second syntactical form is used. ## Tagged procedures LispKit allows a data object to be associated with a procedure. Such data objects are called *tags*, procedures with an associated tag are called *tagged procedures*. The tag of a procedure is immutable. It is defined at procedure creation time and can later be retrieved without calling the procedure. **(procedure/tag? *****obj*****)** Returns `#t` if *obj* is a tagged procedure and `#f` otherwise. Procedures are tagged procedures if they were created either via `lambda/tag` or `case-lambda/tag`. **(procedure-tag *****proc*****)** Returns the tag of the tagged procedure *proc*. It is an error if *proc* is not a tagged procedure. **(lambda/tag *****tag (arg1 ...) expr ...*****)** \ \&#xNAN;**(lambda/tag *****tag (arg1 ... . rest) expr ...*****)**\ \&#xNAN;**(lambda/tag *****tag rest expr ...*****)** A `lambda/tag` expression evaluates to a tagged procedure. First, *tag* is evaluated to obtain the tag value for the procedure. Then, the tagged procedure itself gets created for the given formal arguments. The procedure is lexically scoped in the same manner as a procedure resulting from a `lambda` expression. When the procedure is called, it behaves as if constructed by a `lambda` expression with the same formal arguments and body. **(case-lambda/tag *****tag (formals expr ...) ...*****)** A `case-lambda/tag` expression evaluates to a tagged procedure. First, *tag* is evaluated to obtain the tag value for the procedure. Then, the tagged procedure itself gets created, accepting a variable number of arguments. It is lexically scoped in the same manner as a procedure resulting from a `lambda` expression. When the procedure is called, it behaves as if it was constructed by a `case-lambda` expression with the same formal arguments and bodies. ## Delayed execution LispKit provides *promises* to delay the execution of code. Built on top of *promises* are *streams*. Streams are similar to lists, except that the tail of a stream is not computed until it is de-referenced. This allows streams to be used to represent infinitely long lists. Library `(lispkit core)` only defines procedures for *streams* equivalent to *promises*. Library `(lispkit stream)` provides all the list-like functionality. **(promise? *****obj*****)** The `promise?` procedure returns `#t` if argument *obj* is a promise, and `#f` otherwise. **(make-promise *****obj*****)** \ \&#xNAN;**(eager *****obj*****)** The `make-promise` procedure returns a promise which, when forced, will return *obj*. It is similar to `delay`, but does not delay its argument: it is a procedure rather than syntax. If *obj* is already a promise, it is returned. `eager` represents the same procedure like `make-promise`. **(delay *****expr*****)** The `delay` construct is used together with the procedure `force` to implement lazy evaluation or "call by need". `(delay expr)` returns an object called a promise which, at some point in the future, can be asked (by the `force` procedure) to evaluate *expr*, and deliver the resulting value. **(delay-force *****expr*****)** \ \&#xNAN;**(lazy *****expr*****)** The expression `(delay-force expr)` is conceptually similar to `(delay (force expr))`, with the difference that forcing the result of `delay-force` will in effect result in a tail call to `(force expr)`, while forcing the result of `(delay (force expr))` might not. Thus iterative lazy algorithms that might result in a long series of chains of delay and force can be rewritten using `delay-force` to prevent consuming unbounded space during evaluation. `lazy` represents the same procedure like `delay-force`. **(force *****promise*****)** The `force` procedure forces the value of a promise created by `delay`, `delay-force`, or `make-promise`. If no value has been computed for the promise, then a value is computed and returned. The value of the promise must be cached (or "memoized") so that if it is forced a second time, the previously computed value is returned. Consequently, a delayed expression is evaluated using the parameter values and exception handler of the call to `force` which first requested its value. If *promise* is not a promise, it may be returned unchanged. ```scheme (force (delay (+ 1 2))) ⇒ 3 (let ((p (delay (+ 1 2)))) (list (force p) (force p))) ⇒ (3 3) (define integers (letrec ((next (lambda (n) (delay (cons n (next (+ n 1))))))) (next 0))) (define head (lambda (stream) (car (force stream)))) (define tail (lambda (stream) (cdr (force stream)))) (head (tail (tail integers))) ⇒ 2 ``` The following example is a mechanical transformation of a lazy stream-filtering algorithm into Scheme. Each call to a constructor is wrapped in `delay`, and each argument passed to a deconstructor is wrapped in `force`. The use of `(delay-force ...)` instead of `(delay (force ...))` around the body of the procedure ensures that an ever-growing sequence of pending promises does not exhaust available storage, because `force` will, in effect, force such sequences iteratively. ```scheme (define (stream-filter p? s) (delay-force (if (null? (force s)) (delay ’()) (let ((h (car (force s))) (t (cdr (force s)))) (if (p? h) (delay (cons h (stream-filter p? t))) (stream-filter p? t)))))) (head (tail (tail (stream-filter odd? integers)))) ⇒ 5 ``` The following examples are not intended to illustrate good programming style, as `delay`, `force`, and `delay-force` are mainly intended for programs written in the functional style. However, they do illustrate the property that only one value is computed for a promise, no matter how many times it is forced. ```scheme (define count 0) (define p (delay (begin (set! count (+ count 1)) (if (> count x) count (force p))))) (define x 5) p ⇒ a promise (force p) ⇒ 6 p ⇒ a promise (begin (set! x 10) (force p)) ⇒ 6 ``` **(stream? *****obj*****)** The `stream?` procedure returns `#t` if argument *obj* is a stream, and `#f` otherwise. If *obj* is a stream, `stream?` does not force its promise. If `(stream? obj)` is `#t`, then one of `(stream-null? obj)` and `(stream-pair? obj)` will be `#t` and the other will be `#f`; if `(stream? obj)` is `#f`, both `(stream-null? obj)` and `(stream-pair? obj)` will be `#f`. **(make-stream *****obj*****)** \ \&#xNAN;**(stream-eager *****obj*****)** The `make-stream` procedure returns a stream which, when forced, will return *obj*. It is similar to `stream-delay`, but does not delay its argument: it is a procedure rather than syntax. If *obj* is already a stream, it is returned. `stream-eager` represents the same procedure like `make-stream`. **(stream-delay *****expr*****)** The `stream-delay` syntax is used together with procedure `stream-force` to implement lazy evaluation or "call by need". `(stream-delay expr)` returns an object called a stream which, at some point in the future, can be asked (by the `stream-force` procedure) to evaluate *expr*, and deliver the resulting value. **(stream-delay-force *****expr*****)** \ \&#xNAN;**(stream-lazy *****expr*****)** The expression `(stream-delay-force expr)` is conceptually similar to `(stream-delay (stream-force expr))`, with the difference that forcing the result of `stream-delay-force` will in effect result in a tail call to `(stream-force expr)`, while forcing the result of `(stream-delay (stream-force expr))` might not. Thus iterative lazy algorithms that might result in a long series of chains of delay and force can be rewritten using `stream-delay-force` to prevent consuming unbounded space during evaluation. `stream-lazy` represents the same procedure like `stream-delay-force`. ## Symbols **(symbol? *****obj*****)** Returns `#t` if *obj* is a symbol, otherwise returns `#f`. **(symbol-interned? *****obj*****)** Returns `#t` if *obj* is an interned symbol, otherwise returns `#f`. **(gensym)** \ \&#xNAN;**(gensym *****str*****)** Returns a new (fresh) symbol whose name consists of prefix *str* followed by a number. If *str* is not provided, "g" is used as a prefix. **(generate-uninterned-symbol)** \ \&#xNAN;**(generate-uninterned-symbol *****str*****)** Returns a new uninterned symbol whose name consists of prefix *str* followed by a number. *str* is either a symbol or a string. If *str* is not provided or set to `#f`, "g" is used as a prefix. This procedure is similar to `gensym` but always generates uninterned symbols. **(symbol=? *****sym ...*****)** Returns `#t` if all the arguments are symbols and all have the same names in the sense of `string=?`. **(symbol\ Returns `#t` if the identifiers of the symbols *sym ...* are monotonically increasing in lexicographic order (according to `stringsymbol *****str*****)** Returns the symbol whose name is string *str*. This procedure can create symbols with names containing special characters that would require escaping when written, but does not interpret escapes in its input. **(string->uninterned-symbol *****str*****)** Returns a new uninterned symbol whose name is *str*. This procedure can create symbols with names containing special characters that would require escaping when written, but does not interpret escapes in its input. **(symbol->string *****sym*****)** Returns the name of symbol *sym* as a string, but without adding escapes. ## Booleans The standard boolean objects for true and false are written as `#t` and `#f`. Alternatively, they can be written `#true` and `#false`, respectively. What really matters, though, are the objects that the Scheme conditional expressions (`if`, `cond`, `and`, `or`, `when`, `unless`, `do`) treat as true or false. The phrase a "true value" (or sometimes just "true") means any object treated as true by the conditional expressions, and the phrase "a false value" (or "false") means any object treated as false by the conditional expressions. Of all the Scheme values, only `#f` counts as false in conditional expressions. All other Scheme values, including `#t`, count as true. Boolean literals evaluate to themselves, so they do not need to be quoted in programs. **(boolean? *****obj*****)** The `boolean?` predicate returns `#t` if *obj* is either `#t` or `#f` and returns `#f` otherwise. ```scheme (boolean? #f) ⇒ #t (boolean? 0) ⇒ #f (boolean? '()) ⇒ #f ``` **(boolean=? *****obj1 obj2 ...*****)** Returns `#t` if all the arguments are booleans and all are `#t` or all are `#f`. **(and *****test ...*****)** The *test ...* expressions are evaluated from left to right, and if any expression evaluates to `#f`, then `#f` is returned. Any remaining expressions are not evaluated. If all the expressions evaluate to true values, the values of the last expression are returned. If there are no expressions, then `#t` is returned. ```scheme (and (= 2 2) (> 2 1)) ⇒ #t (and (= 2 2) (< 2 1)) ⇒ #f (and 12 'c '(f g)) ⇒ (f g) (and) ⇒ #t ``` **(or *****test ...*****)** The *test ...* expressions are evaluated from left to right, and the value of the first expression that evaluates to a true value is returned. Any remaining expressions are not evaluated. If all expressions evaluate to `#f` or if there are no expressions, then `#f` is returned. ```scheme (or (= 2 2) (> 2 1)) ⇒ #t (or (= 2 2) (< 2 1)) ⇒ #t (or #f #f #f) ⇒ #f (or (memq 'b '(a b c)) (/ 3 0)) ⇒ (b c) ``` **(not *****obj*****)** The `not` procedure returns `#t` if *obj* is false, and returns `#f` otherwise. ```scheme (not #t) ⇒ #f (not 3) ⇒ #f (not (list 3)) ⇒ #f (not #f) ⇒ #t (not '()) ⇒ #f (not (list)) ⇒ #f (not 'nil) ⇒ #f ``` **(opt *****pred obj*****)** \ \&#xNAN;**(opt *****pred obj failval*****)** The `opt` procedure returns *failval* if *obj* is `#f`. If *obj* is not `#f`, `opt` applies predicate *pred* to *obj* and returns the result of this function application. If *failval* is not provided, `#t` is used as a default. This function is useful to verify a given predicate *pred* for an optional value *obj*. ## Conditional and inclusion compilation **(cond-expand *****ce-clause1 ce-clause2 ...*****)** The `cond-expand` expression type provides a way to statically expand different expressions depending on the implementation. A *ce-clause* takes the following form: (*featurerequirement* *expression ...*) The last clause can be an “else clause,” which has the form: (else *expression ...*) A *featurerequirement* takes one of the following forms: * *featureidentifier* * `(library` *name*`)` * `(and` *featurerequirement ...*`)` * `(or` *featurerequirement ...*`)` * `(not` *featurerequirement*`)` LispKit maintains a list of feature identifiers which are present, as well as a list of libraries which can be imported. The value of a *featurerequirement* is determined by replacing each *featureidentifier* and `(library` *name*`)` with `#t`, and all other feature identifiers and library names with `#f`, then evaluating the resulting expression as a Scheme boolean expression under the normal interpretation of `and`, `or`, and `not`. A `cond-expand` is then expanded by evaluating the *featurerequirements* of successive *ce-clause* in order until one of them returns `#t`. When a true clause is found, the corresponding *expression ...* are expanded to a `begin`, and the remaining clauses are ignored. If none of the listed *featurerequirement* evaluates to `#t`, then if there is an "else" clause, its *expression ...* are included. Otherwise, the behavior of the `cond-expand` is unspecified. Unlike `cond`, `cond-expand` does not depend on the value of any variables. The exact features provided are defined by the implementation, its environment and host platform. LispKit supports the following *featureidentifier*: * `lispkit` * `r7rs` * `ratios` * `complex` * `syntax-rules` * `little-endian` * `big-endian` * `dynamic-loading` * `modules` * `32bit` * `64bit` * `macos` * `macosx` * `ios` * `linux` * `i386` * `x86-64` * `arm64` * `arm` **(include *****str1 str2 ...*****)** \ \&#xNAN;**(include-ci *****str1 str2 ...*****)** Both `include` and `include-ci` take one or more filenames expressed as string literals, apply an implementation-specific algorithm to find corresponding files, read the contents of the files in the specified order as if by repeated applications of read, and effectively replace the `include` or `include-ci` expression with a `begin` expression containing what was read from the files. The difference between the two is that `include-ci` reads each file as if it began with the `#!fold-case` directive, while `include` does not. ## Multiple values **(values *****obj ...*****)** Delivers all of its arguments to its continuation. The `values` procedure might be defined as follows: ```scheme (define (values . things) (call-with-current-continuation (lambda (cont) (apply cont things)))) ``` **(call-with-values *****producer consumer*****)** Calls its *producer* argument with no arguments and a continuation that, when passed some values, calls the *consumer* procedure with those values as arguments. The continuation for the call to *consumer* is the continuation of the call to `call-with-values`. ```scheme (call-with-values (lambda () (values 4 5)) (lambda (a b) b)) ⇒ 5 (call-with-values * -) ⇒ -1 ``` **(apply-with-values *****proc vals*****)** `apply-with-values` calls procedure *proc* with *vals* as its arguments and returns the corresponding result. *vals* might refer to multiple values created via procedure `values`. This is a LispKit-specific procedure that relies on multiple return values being represented by a container object. ## Environments Environments are first-class objects which associate identifiers (symbols) with values. Environments are used implicitly by the LispKit compiler and runtime system, but library `(lispkit core)` also provides an API allowing systems to manipulate and use environments programmatically. For instance, when a top-level variable gets created with `define`, the name/value association for that variable is added to the "top-level" environment. The LispKit compiler implicitly creates environments other than the top-level environment, for example, when compiling and executing libraries. There are several types of bindings that can occur within an environment. A *variable binding* associates a value with an identifier. This is the most common type of binding. In addition to variable bindings, environments can have *keyword bindings*. A keyword binding associates an identifier with a macro transformer (usually via `syntax-rules`). There are also *unassigned* bindings referring to bindings without a known value. **(environment? *****obj*****)** Returns `#t` if *obj* is an environment. Otherwise, it returns `#f`. **(interaction-environment? *****obj*****)** Returns `#t` if *obj* is an interaction environment, i.e. a mutable environment in which expressions entered by the user into a read-eval-print loop are being evaluated. Otherwise, procedure `interaction-environment?` returns `#f`. **(custom-environment? *****obj*****)** Returns `#t` if *obj* is a custom environment, i.e. an environment that was programmatically constructed. Otherwise, predicate `custom-environment?` returns `#f`. **(the-environment)** Special form `the-environment` returns the current top-level environment. If there is none, `the-environment` returns `#f`. Here is an example how one can print the names bound at compile-time: ```scheme (define-library (foo) (import (only (lispkit core) the-environment environment-bound-names) (only (lispkit port) display newline)) (begin (display "bound = ") (display (environment-bound-names (the-environment))) (newline))) (import (foo)) ⇒ bound = (display the-environment newline environment-bound-names) ``` **(environment *****list1 ...*****)** This procedure returns an environment that results by starting with an empty environment and then importing each list, considered as an import set, into it. The bindings of the environment represented by the specifier are immutable, as is the environment itself. **(environment-bound-names *****env*****)** Returns a list of the symbols that are bound by environment *env*. **(environment-bindings *****env*****)** Returns a list of the bindings of environment *env*. Each element of this list takes one of two forms: the form *(name)* indicates that *name* is bound but unassigned, while *(name obj)* indicates that *name* is bound to value *obj*. **(environment-bound? *****env ident*****)** Returns `#t` if symbol *ident* is bound in environment *env*; otherwise returns `#f`. **(environment-lookup *****env ident*****)** Returns the value to which symbol *ident* is bound in environment *env*. This procedure throws an error if *ident* is not bound to a value in *env*. **(environment-assignable? *****env ident*****)** Symbol *ident* must be bound in environment *env*. Procedure `environment-assignable?` returns `#t` if the binding of *ident* may be modified. **(environment-assign! *****env ident obj*****)** Symbol *ident* must be bound in environment *env* and must be assignable. Procedure `environment-assign!` modifies the binding to have *obj* as its value. **(environment-definable? *****env ident*****)** Predicate `environment-definable?` returns `#t` if symbol *ident* is definable in environment *env*, and `#f` otherwise. Currently, interaction environments and custom environments allow for identifiers to be defined. For all other types of environments, this predicate returns `#f` independent of *ident*. **(environment-define *****env ident obj*****)** Defines *ident* to be bound to *obj* in environment *env*. This procedure signals an error if *ident* is not definable in *env*. **(environment-define-syntax *****env ident transf*****)** Defines *ident* to be a keyword bound to macro transformer *transf* (typically expressed in terms of `syntax-rules`) in environment *env*. This procedure signals an error if *ident* is not definable in environment *env*. **(environment-import *****env ident importset*****)** Imports the identifiers exported by a library and specified via an import set *importset* into the environment *env*. The procedure fails if the type of environment does not allow identifiers to be defined programmatically. **(environment-documentation *****env ident*****)** Returns the documentation associated with the identifier *ident* in environment *env* as a string. This procedure returns `#f` if *ident* is not associated with any documentation. **(environment-assign-documentation! *****env ident str*****)** Assigns the documentation string *str* to identifier *ident* in environment *env*. **(scheme-report-environment *****version*****)** If *version* is equal to 5, corresponding to R5RS, *scheme-report-environment* returns an environment that contains only the bindings defined in the R5RS library. **(null-environment *****version*****)** If *version* is equal to 5, corresponding to R5RS, the *null-environment* procedure returns an environment that contains only the bindings for all syntactic keywords defined in the R5RS library. **(interaction-environment)** This procedure returns a mutable environment which is the environment in which expressions entered by the user into a read-eval-print loop are evaluated. This is typically a superset of bindings from *(lispkit base)*. ## Source files **(load *****filename*****)** \ \&#xNAN;**(load *****filename*** ***environment*****)** `load` reads a source file specified by *filename* and executes it in the given *environment*. If no environment is specified, the current *interaction environment* is used, which can be accessed via `(interaction-environment)`. Execution of the file consists of reading expressions and definitions from the file, compiling them, and evaluating them sequentially in the environment. `load` returns the result of evaluating the last expression or definition from the file. During compilation, the special form `source-directory` can be used to access the directory in which the executed file is located. It is an error if *filename* is not a string. If *filename* is not an absolute file path, LispKit will try to find the file in a predefined set of directories, such as the default search path. If no file name suffix, also called *path extension*, is provided, the system will try to determine the right suffix. For instance, `(load "Prelude")` will find the prelude file, determine its suffix and load and execute the file. **(load-program *****filename*****)** `load-program` reads a source file specified by *filename* and executes it in a new empty *environment*. Execution of the file consists of reading expressions and definitions from the file, compiling them, and evaluating them sequentially. `load-program` returns the evaluation result of the last expression. It is an error if *filename* is not a string. If *filename* is not an absolute file path, LispKit will try to find the file in a predefined set of directories, such as the default search path. If no file name suffix is provided, the system will try to determine the right suffix. ## Syntax errors **(syntax-error *****message args ...*****)** `syntax-error` behaves similarly to `error` except that implementations with an expansion pass separate from evaluation should signal an error as soon as `syntax-error` is expanded. This can be used as a `syntax-rules` *template* for a *pattern* that is an invalid use of the macro, which can provide more descriptive error messages. *message* is a string literal, and *args ...* are arbitrary expressions providing additional information. Applications cannot count on being able to catch syntax errors with exception handlers or guards. ```scheme (define-syntax simple-let (syntax-rules () ((_ (head ... ((x . y) val) . tail) body1 body2 ...) (syntax-error "expected an identifier but got" (x . y))) ((_ ((name val) ...) body1 body2 ...) ((lambda (name ...) body1 body2 ...) val ...)))) ``` ## Utilities **(void)** Performs no operation and returns nothing. This is often useful as a placeholder or whenever a no-op statement is needed. **(void? *****obj*****)** Returns `#t` if *obj* is the "void" value (i.e. no value); returns `#f` otherwise. **(identity *****obj*****)** The identity function is always returning its argument *obj*. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://www.lisppad.app/libraries/lispkit/lispkit-core.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. 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