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ubfa: readubf, writeubf, UValue - read, write and represent values in a UBF(A) data transport encoding


include "ubfa.m";
ubfa := load UBFa UBFa->PATH;

UValue: adt {
    Atom =>
        name:  string;
    Int =>
        value: int;
    String =>
        s:     string;
    Binary =>
        a:     array of byte;
    Tuple =>
        a:     cyclic array of ref UValue;  # tree
    List =>
        l:     cyclic list of ref UValue;   # tree
    Tag =>
        name:  string;
        o:     cyclic ref UValue;
    isatom:    fn(o: self ref UValue): int;
    isstring:  fn(o: self ref UValue): int;
    isint:     fn(o: self ref UValue): int;
    istuple:   fn(o: self ref UValue): int;
    isop:      fn(o: self ref UValue, op: string, arity: int): int;
    islist:    fn(o: self ref UValue): int;
    isbinary:  fn(o: self ref UValue): int;
    istag:     fn(o: self ref UValue): int;
    eq:        fn(o: self ref UValue, v: ref UValue): int;
    op:        fn(o: self ref UValue, arity: int): string;
    args:      fn(o: self ref UValue, arity: int):
                array of ref UValue;
    els:       fn(o: self ref UValue): list of ref UValue;
    val:       fn(o: self ref UValue): int;
    binary:    fn(o: self ref UValue): array of byte;
    objtag:    fn(o: self ref UValue): string;
    obj:       fn(o: self ref UValue): ref UValue;
    text:      fn(o: self ref UValue): string;

init:     fn(bufio: Bufio);
readubf:  fn(input: ref Iobuf): (ref UValue, string);
writeubf: fn(output: ref Iobuf, v: ref UValue): int;
uniq:     fn(s: string): string;

uvatom:   fn(s: string): ref UValue.Atom;
uvint:    fn(i: int): ref UValue.Int;
uvstring: fn(s: string): ref UValue.String;
uvbinary: fn(a: array of byte): ref UValue.Binary;
uvtuple:  fn(a: array of ref UValue): ref UValue.Tuple;
uvlist:   fn(l: list of ref UValue): ref UValue.List;
uvtag:    fn(name: string, o: ref UValue): ref UValue.Tag;


UBFa provides value representations, and encoding and decoding operations for Armstrong's UBF(A) data transport format, defined by ubfa(6).

Init must be called before invoking any other operation of the module. The bufio parameter must refer to the instance of bufio(2) that provides the Iobuf parameters used for input and output.

UValue is the internal representation of values that can be transmitted by the UBF(A) encoding. The various sorts of values are distinguished in a pick adt:

Represents an atom: a symbolic constant, for example the name of an operation or an enumeration literal. The string name gives the spelling of the constant's name.
Represents an integer value (eg, a Limbo int) with the given value.
Represents a character string (eg, a Limbo string) with the value s.
Represents binary data as a sequence of bytes in the array a.
Represents a compound value that contains a fixed number of component values, given by successive elements of the array a. UBF tuples correspond to tuples or non-pick adt values in Limbo.
Represents a compound value containing a variable number of component values, given by successive elements of the list l.
Associates an application-specific tag with another UValue referenced by o.

Readubf reads a single value in ubfa(6) format from the input stream and returns a tuple (val, err). On success, val is a UValue that represents that value. If an error occurs, val is nil and err contains a diagnostic.

Writeubf writes a ubfa(6) representation of the value v to the output stream. It returns 0 on success and -1 on error (setting the system error string).

The easiest way to create a new UValue for subsequent output is with one of the module-level functions uvatom, uvint, uvstring, and so on. As values of a pick adt, a UValue can be inspected using Limbo's tagof operator and the appropriate variant accessed using a pick statement. UValue also supports several groups of common operations, for smaller, tidier code. First, the set of enquiry functions u.isX() return true if the value u is an instance of the UBF type X (atom, int, string, binary, tuple, etc). The other operations are:

Return true if the values of u and v are equal, including the values of corresponding subcomponents, recursively
u.isop(op, n)
Return true if u is a tuple having n components, and its first component is an atom or string with the value op.
If u is a tuple with n components, and the first component is an atom or string, return its value. Otherwise, return nil.
If u is a tuple with n components, return an array containing the values of all but the first component. Otherwise, return nil.
If u is a list, return a Limbo list of its elements (ie, u.l). Otherwise, return nil.
If u is an integer, return its value. Otherwise return zero.
If u is a binary value, return the corresponding array of bytes; if u is an atom or string, return an array of bytes containing its value; otherwise, return nil.
If u is a tag, return the name of the tag. Otherwise, return nil.
If u is a tag, return the tagged value. Otherwise, return u itself.
Return a printable representation of the value u, mainly intended for debugging and tracing.

One difference between atoms and strings is that all atoms with identical spellings refer to the same string in the implementation's storage. Given an atom name, uniq returns the corresponding string, stored in an internal dictionary. It is used by UBFa to create the strings UValue.Atom.s, and can be put to similar use directly by applications. It should only be applied to values that are small in number (as with symbolic constants).




sexprs(2), ubfa(6)
J L Armstrong, ``Getting Erlang to talk to the outside world'', ACM SIGPLAN Erlang workshop 2002 , Pittsburg, PA USA

UBFA(2 ) Rev:  Tue Mar 31 02:42:39 GMT 2015