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Extensions

Multiple values

The multiple-value feature will be in R5RS.

Function: values object ...
Delivers all of its arguments to its continuation.

Function: call-with-values thunk receiver
Call its thunk argument with a continuation that, when passed some values, calls the receiver procedure with those values as arguments.

Special named constants

Constant: #!optional
Special self-evaluating literal used in lambda parameter lists before optional parameters.

Constant: #!rest
Special self-evaluating literal used in lambda parameter lists before the rest parameter.

Constant: #!key
Special self-evaluating literal used in lambda parameter lists before keyword parameters.

Constant: #!eof
The end-of-file object.

Note that if the Scheme reader sees this literal at top-level, it is returned literally. This is indistinguishable from coming to the end of the input file. If you do not want to end reading, but want the actual value of #!eof, you should quote it.

Constant: #!void
The void value. Same as (values). If this is the value of an expression in a read-eval-print loop, nothing is printed.

Constant: #!null
The Java null value. This is not really a Scheme value, but is useful when interfacing to low-level Java code.

Keywords

Keywords are similar to symbols. The main difference is that keywords are self-evaluating and therefore do not need to be quoted in expressions. They are used mainly for specifying keyword arguments.

keyword = identifier:

A keyword is a single token; therefore no whitespace is allowed between the identifier and the colon (which is not considered part of the name of the keyword).

Function: keyword? obj
Return #t if obj is a keyword, and otherwise returns #f.

Function: keyword->string keyword
Returns the name of keyword as a string. The name does not include the final #\:.

Function: string->keyword string
Returns the keyword whose name is string. (The string does not include a final #\:.)

Optional and keyword lambda parameters

Kawa borrows the extended formal argument list of DSSSL:

lambda-expression = (lambda (formal-arguments) body)

You can of course also use the extended format in a define:

(define (name formal-arguments) body)
formal-arguments =
req-opt-args . rest-arg or:
req-opt-args rest-key-args
req-opt-args = req-arg* (#!optional opt-arg*)?
rest-key-args = (#!rest rest-arg)? (#!key key-arg*)?
opt-arg = variable | (variable initializer)
req-arg = variable
key-arg = variable | (variable initializer)
rest-arg = variable

When the procedure is applied to a list of actual arguments, the formal and actual arguments are processed from left to right as follows:

Logical Number Operations

These functions operate on the 2's complement binary representation of an exact integer.

Function: logand i ...
Returns the bit-wise logical "and" of the arguments. If no argument is given, the result is -1.

Function: logior i ...
Returns the bit-wise logical "(inclusive) or" of the arguments. If no argument is given, the result is 0.

Function: logxor i ...
Returns the bit-wise logical "exclusive or" of the arguments. If no argument is given, the result is 0.

Function: lognot i
Returns the bit-wise logical inverse of the argument.

Function: logop op x y
Perform one of the 16 bitwise operations of x and y, depending on op.

Function: bittest i j
Returns true if the arguments have any bits in common. Same as (not (zero? (logand i j))), but is more efficient.

Function: logbit? i pos
Returns #t iff the bit numbered pos in i is one.

Function: arithmetic-shift i j
Shifts i by j. It is a "left" shift if j>0, and a "right" shift if j<0.

The result is equal to (floor (* i (expt 2 j))).

Function: ash i j
Alias for arithmetic-shift.

Function: logcount i
Count the number of 1-bits in i, if it is non-negative. If i is negative, count number of 0-bits.

Function: integer-length i
Return number of bits needed to represent i in an unsigned field. Regardless of the sign of i, return one less than the number of bits needed for a field that can represent i as a two's complement integer.

Function: bit-extract n start end
Return the integer formed from the (unsigned) bit-field starting at start and ending just before end. Same as (arithmetic-shift (bitand n (bitnot (arithmetic-shift -1 end))) (- start)).

Records

The Record package provides a facility for user to define their own record data types. A record type is implemented as Java Class object, and records are extensions of the class Record. These procedures use the Java 1.1 reflection facility.

Function: make-record-type type-name field-names
Returns a record-type descriptor, a value representing a new data type disjoint from all others. The type-name argument must be a string, but is only used for debugging purposes (such as the printed representation of a record of the new type). The field-names argument is a list of symbols naming the fields of a record of the new type. It is an error if the list contains any duplicates.

In Kawa, returns a newly-created Class object that extends the Record class. Each record field is implemented as a public Java instance field.

Function: record-constructor rtd [field-names]
Returns a procedure for constructing new members of the type represented by rtd. The returned procedure accepts exactly as many arguments as there are symbols in the given list, field-names; these are used, in order, as the initial values of those fields in a new record, which is returned by the constructor procedure. The values of any fields not named in that list are unspecified. The field-names argument defaults to the list of field names in the call to make-record-type that created the type represented by rtd; if the field-names argument is provided, it is an error if it contains any duplicates or any symbols not in the default list.

In Kawa, rtd may be any Class that has a public default constructor, as long as the field-names are public instance fields. (The fields should have type Object -- unless you know what you are doing!)

Function: record-predicate rtd
Returns a procedure for testing membership in the type represented by rtd. The returned procedure accepts exactly one argument and returns a true value if the argument is a member of the indicated record type; it returns a false value otherwise.

In Kawa, the returned procedure checks if the argument is an instance of rtd or one of its sub-classes.

Function: record-accessor rtd field-name
Returns a procedure for reading the value of a particular field of a member of the type represented by rtd. The returned procedure accepts exactly one argument which must be a record of the appropriate type; it returns the current value of the field named by the symbol field-name in that record. The symbol field-name must be a member of the list of field-names in the call to make-record-type that created the type represented by rtd. (In Kawa, the field-name can be any public non-final Object field of the Class rtd.)

Function: record-modifier rtd field-name
Returns a procedure for writing the value of a particular field of a member of the type represented by rtd. The returned procedure accepts exactly two arguments: first, a record of the appropriate type, and second, an arbitrary Scheme value; it modifies the field named by the symbol field-name in that record to contain the given value. The returned value of the modifier procedure is unspecified. The symbol field-name must be a member of the list of field-names in the call to make-record-type that created the type represented by rtd. (In Kawa, the field-name can be any public non-final Object field of the Class rtd.)

Function: record? obj
Returns a true value if obj is a record of any type and a false value otherwise.

In Kawa, this is true if obj is an instance of kawa.lang.Record.

Function: record-type-descriptor record
Returns a record-type descriptor representing the type of the given record. That is, for example, if the returned descriptor were passed to record-predicate, the resulting predicate would return a true value when passed the given record. In Kawa, record may be any object, and the value returned is the class of the object.

Function: record-type-name rtd
Returns the type-name associated with the type represented by rtd. The returned value is eqv? to the type-name argument given in the call to make-record-type that created the type represented by rtd.

Function: record-type-field-names rtd
Returns a list of the symbols naming the fields in members of the type represented by rtd. The returned value is equal? to the field-names argument given in the call to make-record-type that created the type represented by rtd.

File System Interface

Function: file-exists? filename
Returns true iff the file named filename actually exists.

Function: file-directory? filename
Returns true iff the file named filename actually exists and is a directory.

Function: file-readable? filename
Returns true iff the file named filename actually exists and can be read from.

Function: file-writable? filename
Returns true iff the file named filename actually exists and can be writen to. (Undefined if the filename does not exist, but the file can be created in the directory.)

Function: delete-file filename
Delete the file named filename.

Function: rename-file oldname newname
Renames the file named oldname to newname.

Function: copy-file oldname newname-from path-to
Copy the file named oldname to newname. The return value is unspecified.

Function: create-directory dirname
Create a new directory named dirname. Unspecified what happens on error (such as exiting file with the same name). (Currently returns #f on error, but may change to be more compatible with scsh.)

Ports

Function: call-with-input-string string proc
Create an input port that gets its data from string, call proc with that port as its one argument, and return the result from the call of proc

Function: call-with-output-string proc
Create an output port that writes its data to a string, and call proc with that port as its one argument. Return a string consisting of the data written to the port.

Function: force-output [port]
Forces any pending output on port to be delivered to the output device and returns an unspecified value. If the port argument is omitted it defaults to the value returned by (current-output-port).

An interactive input port has a prompt procedure associated with it. The prompt procedure is called before a new line is read. It is passed the port as an argument, and returns a string, which gets printed as a prompt.

Function: input-port-prompter port
Get the prompt procedure associated with port.

Function: set-input-port-prompter! port prompter
Set the prompt procedure associated with port to prompter, which must be a one-argument procedure taking an input port, and returning a string.

Function: default-prompter port
The default prompt procedure. It returns "#|kawa:L|# ", where L is the current line number of port. When reading a continuation line, the result is "#|C---:L|# ", where C is the character returned by (input-port-read-state port). The prompt has the form of a comment to make it easier to cut-and-paste.

Function: input-port-line-number port
Get the line number of the current line of port, which must be a (non-binary) input port. The initial line is line 1.

Function: set-input-port-line-number! port num
Set line number of the current line of port to num.

Function: input-port-column-number port
Get the column number of the current line of port, which must be a (non-binary) input port. The initial column is column 1.

Function: input-port-read-state port
Returns a character indicating the current read state of the port. Returns #\Return if not current doing a read, #\" if reading a string; #\| if reading a comment; #\( if inside a list; and #\Space when otherwise in a read. The result is intended for use by prompt prcedures, and is not necessarily correct except when reading a new-line.

Variable: symbol-read-case
A symbol that controls how read handles letters when reading a symbol. If the first letter is `U', then letters in symbols are upper-cased. If the first letter is `D' or `L', then letters in symbols are down-cased. If the first letter is `I', then the case of letters in symbols is inverted. Otherwise (the default), the letter is not changed. (Letters following a `\' are always unchanged.)

Variable: port-char-encoding
Controls how bytes in external files are converted to/from internal Unicode characters. Can be either a symbol or a boolean. If port-char-encoding is #f, the file is assumed to be a binary file and no conversion is done. Otherwise, the file is a text file. The default is #t, which uses a locale-dependent conversion. If port-char-encoding is a symbol, it must be the name of a character encoding known to Java. For all text files (that is if port-char-encoding is not #f), on input a #\Return character or a #\Return followed by #\Newline are converted into plain #\Newline.

This variable is checked when the file is opened; not when actually reading or writing. Here is an example of how you can safely change the encoding temporarily:

(define (open-binary-input-file name)
  (fluid-let ((port-char-encoding #f)) (open-input-file name)))

Signalling and recovering from exceptions

Function: catch key thunk handler
Invoke thunk in the dynamic context of handler for exceptions matching key. If thunk throws to the symbol key, then handler is invoked this way:

(handler key args ...)

key may be a symbol. The thunk takes no arguments. If thunk returns normally, that is the return value of catch.

Handler is invoked outside the scope of its own catch. If handler again throws to the same key, a new handler from further up the call chain is invoked.

If the key is #t, then a throw to any symbol will match this call to catch.

Function: throw key &rest args ...
Invoke the catch form matching key, passing args to the handler.

If the key is a symbol it will match catches of the same symbol or of #t.

If there is no handler at all, an error is signaled.

procedure: error message args ...
Raise an error with key misc-error and a message constructed by displaying msg and writing args. This normally prints a stack trace, and brings you back to the top level, or exits kawa if you are not running interactively.

Function: primitive-throw exception
Throws the exception, which must be an instance of a sub-class of <java.lang.Throwable>.

Syntax: try-finally body handler
Evaluate body, and return its result. However, before it returns, evaluate handler. Even if body returns abnormally (by throwing an exception), handler is evaluated.

(This is implemented just like Java's try-finally.)

Syntax: try-catch body handler ...
Evaluate body, in the conect of the given handler-specs. Each handler has the form:
var type exp ...

If an exception is thrown in body, the first handle-spec is selected such that the thrown exception is an instance of the handler's type. If no handler is selected, the exception is propagated through the dynamic execution context until a matching handler is found. (If no matching handler is found, then an error message is printed, and the computation terminated.)

Once a handler is selected, the var is bound to the thrown exception, and the exp in the handler are executed. The result of the try-catch is the result of body if no exception is thrown, or the value of the last exp in the selected handler if an exception is thrown.

(This is implemented just like Java's try-catch.)

Function: dynamic-wind in-guard thunk out-guard
All three arguments must be 0-argument procedures. First calls in-guard, then thunk, then out-guard. The result of the expression is that of thunk. If thunk is exited abnormally (by throwing an exception or invoking a continuation), out-guard is called.

If the continuation of the dynamic-wind is re-entered (which is not yet possible in Kawa), the in-guard is called again.

This function will be in R5RS.

Eval and Environments

Function: eval expression [environment]
eval evaluates expression in the environment indicated by environment.

The default for environment is the result of (interaction-environment).

Function: null-environment
This procedure returns an environment that contains no variable bindings, but contains (syntactic) bindings for all the syntactic keywords.

The effect of assigning to a variable in this environment (such as let) is undefined.

Function: scheme-report-environment version
The version must be an exact non-negative inetger corresponding to a version of one of the Revisedversion Reports on Scheme. The procedure returns an environment that contains exactly the set of bindings specified in the corresponding report.

This implementation supports version that is 4 or 5.

The effect of assigning to a variable in this environment (such as car) is undefined.

Function: interaction-environment
This procedure return an environment that contains implementation-defined bindings, as well as top-level user bindings.

Function: environment-bound? environment symbol
Return true #t if there is a binding for symbol in environment; otherwise returns #f.

Syntax: fluid-let ((variable init) ...) body ...
Evaluate the init expressions. Then modify the dynamic bindings for the variables to the values of the init expressions, and evaluate the body expressions. Return the result of the last expression in body. Before returning, restore the original bindings. The temporary bindings are only visible in the current thread, and its descendent threads.

Quantities

As a super-class of numbers, Kawa also provides quantities. A quantity is a product of a unit and a pure number. The number part can be an arbitrary complex number. The unit is a product of integer powers of base units, such as meter or second.

Kawa quantities are a generalization of the quantities in DSSSL, which only has length-derived quantities.

The precise syntax of quantity literals may change, but some examples are 10pt (10 points), 5s (5 seconds), and 4cm2 (4 square centimeters).

Function: quantity? object
True iff object is a quantity. Note that all numbers are quantities, but not the other way round.

Function: quantity->number q
Returns the pure number part of the quantity q, relative to primitive (base) units. If q is a number, returns q. If q is a unit, yields the magitude of q relative to base units.

Function: quantity->unit q
Returns the unit of the quantity q. If q is a number, returns the empty unit.

Function: make-quantity x unit
Returns the product of x (a pure number) and unit. You can specify a string instead of unit, such as "cm" or "s" (seconds).

Syntax: define-unit unit-name expression
Define unit-name as a unit (that can be used in literals) equal to the quantity expression.

Threads

There is a very preliminary interface to create parallel threads. The interface is similar to the standard delay/force, where a thread is basically the same as a promise, except that evaluation may be in parallel.

So far, little or no effort has been made into making Kawa thread-safe. There are no per-thread bindings, and the current input and output parts are global. That needs to change.

Syntax: future expression
Creates a new thread that evaluates expression.

Function: force thread
The standard force function has generalized to also work on threads. If waits for the thread's expression to finish executing, and returns the result.

Function: sleep time
Suspends the current thread for the specified time. The time can be either a pure number (in secords), or a quantity whose unit is a time unit (such as 10s).

Standard Types

Kawa has first-class types, that you can use in various ways. Currently, these are mainly useful for interfacing with primitive Java methods (such as primitive-virtual-function, but they will be useful for other purposes (such as declarating variables) later.

These types are bound to identifiers having the form <TYPENAME>. (This syntax and most of the names are as in RScheme.)

To find which Java classes these types map into, look in kawa/lang/PrimProcedure.java.

Note that the value of these variables are instances of gnu.bytecode.Type, not (as you might at first expect) java.lang.Class.

Variable: <object>
An arbitrary Scheme value - and hence an arbitrary Java object.

Variable: <number>
The type of Scheme numbers.

Variable: <integer>
The type of Scheme integers.

Variable: <symbol>
The type of Scheme symbols.

Variable: <keyword>
The type of keyword values. See section Keywords.

Variable: <list>
The type of Scheme lists (pure and impure, including the empty list).

Variable: <pair>
The type of Scheme pairs. This is a sub-type of <list>.

Variable: <string>
The type of (mutable) Scheme strings. This is not the same as (non-mutable) Java strings (which happen to be the same as <symbol>).

Variable: <vector>
The type of Scheme vectors.

Variable: <function>
The type of Scheme procedures.

Variable: <input-port>
The type of Scheme input ports.

Variable: <output-port>
The type of Scheme output ports.

More will be added later.

In addition, any Java type can be named using this syntax. For example <java.lang.StringBuffer[]> represents an array of references to java.lang.StringBuffer objects.

Processes

Function: make-process command envp
Creates a <java.lang.Process> object, using the specified command and envp. The command is converted to an array of Java strings (that is an object that has type <java.lang.String[]>. It can be a Scheme vector or list (whose elements should be Java strings or Scheme strings); a Java array of Java strings; or a Scheme string. In the latter case, the command is converted using command-parse. The envp is process environment; it should be either a Java array of Java strings, or the special #!null value.

Function: system command
Runs the specified command, and waits for it to finish. Returns the return code from the command. The return code is an integer, where 0 conventionally means successful completion. The command can be any of the types handled by make-process.

Variable: command-parse
The value of this variable should be a one-argument procedure. It is used to convert a command from a Scheme string to a Java array of the constituent "words". The default binding, on Unix-like systems, returns a new command to invoke "/bin/sh" "-c" concatenated with the command string; on non-Unix-systems, it is bound to tokenize-string-to-string-array.

Function: tokenize-string-to-string-array command
Uses a java.util.StringTokenizer to parse the command string into an array of words. This splits the command using spaces to delimit words; there is no special processing for quotes or other special characters. (This is the same as what java.lang.Runtime.exec(String) does.)

Miscellaneous

Function: scheme-implementation-version
Returns the Kawa version number as a string.

Function: gentemp
Returns a new (interned) symbol each time it is called. The symbol names are implementation-dependent.

Syntax: defmacro name lambda-list form ...
Defines an old-style macro a la Common Lisp, and installs (lambda lambda-list form ...) as the expansion function for name. When the translator sees an application of name, the expansion function is called with the rest of the application as the actual arguments. The resulting object must be a Scheme source form that is futher processed (it may be repeatedly macro-expanded).

If you define a macro with defmacro, you (currently) cannot use the macro in the same compilation as the definition. This restriction does not apply to macros defined by define-syntax.

Variable: command-line-arguments
Any command-line arguments (following flags processed by Kawa itself) are assigned to the global variable `command-line-arguments', which is a vector of strings.

Variable: home-directory
A string containing the home directory of the user.

Function: exit [code]
Exits the Kawa interpreter, and ends the Java session. The integer value code is returned to the operating system. If code is not specified, zero is returned, indicating normal (non-error) termination.

Function: scheme-window [shared]
Create a read-eval-print-loop in a new top-level window. If shared is true, it uses the same environment as the current (interaction-environment); if not (the default), a new top-level environment is created.

You can create multiple top-level window that can co-exist. They run in separate threads.

Function: apply proc [arg1 ...] args
Args must be a sequence (list, vector, or string) or a primitive Java array. (This is an extension over standard Scheme, which requires that args be a list.) Calls the proc (which must be a procedure), using as arguments the arg1... values plus all the elements of args.

Syntax: constant-fold proc arg1 ...
Same as (proc arg1 ...), unless proc and all the following arguments are compile-time constants. (That is: They are either constant, or symbols that have a global binding and no lexical binding.) In that case, proc is applied to the arguments at compile-time, and the result replaces the constant-fold form. If the application raises an exception, a compile-time error is reported. For example:
(constant-fold vector 'a 'b 'c)

is equivalent to (quote #(a b c)), assuming vector has not been re-bound.

Syntax: when condition form...
If condition is true, evaluate each form in order, returning the value of the last one.

Syntax: unless condition form...
If condition is false, evaluate each form in order, returning the value of the last one.

Function: vector-append arg...
Creates a new vector, containing the elements from all the args appended together. Each arg may be a vector or a list.

Function: instance? value type
Returns #t iff value is an instance of type type. (Undefined if type is a primitive type, such as <int>.)

Function: as type value
Converts or coerces value to a value of type type. Throws an exception if that cannot be done. Not supported for type to be a primitive type such as <int>.


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