UBFA(2) UBFA(2)
NAME
ubfa: readubf, writeubf, UValue - read, write and represent
values in a UBF(A) data transport encoding
SYNOPSIS
include "ubfa.m";
ubfa := load UBFa UBFa->PATH;
UValue: adt {
pick{
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;
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UBFA(2) UBFA(2)
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;
DESCRIPTION
UBFa provides value representations, and encoding and decod-
ing 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:
UValue.Atom
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.
UValue.Int
Represents an integer value (eg, a Limbo int) with the
given value.
UValue.String
Represents a character string (eg, a Limbo string) with
the value s.
UValue.Binary
Represents binary data as a sequence of bytes in the
array a.
UValue.Tuple
Represents a compound value that contains a fixed num-
ber of component values, given by successive elements
of the array a. UBF tuples correspond to tuples or
non-pick adt values in Limbo.
UValue.List
Represents a compound value containing a variable num-
ber of component values, given by successive elements
of the list l.
UValue.Tag
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UBFA(2) UBFA(2)
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 appropri-
ate 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 opera-
tions are:
u.eq(v)
Return true if the values of u and v are equal, includ-
ing the values of corresponding subcomponents, recur-
sively
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.
u.op(n)
If u is a tuple with n components, and the first compo-
nent is an atom or string, return its value. Other-
wise, return nil.
u.args(n)
If u is a tuple with n components, return an array con-
taining the values of all but the first component.
Otherwise, return nil.
u.els()
If u is a list, return a Limbo list of its elements
(ie, u.l). Otherwise, return nil.
u.val()
If u is an integer, return its value. Otherwise return
zero.
u.binary()
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UBFA(2) UBFA(2)
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.
u.objtag()
If u is a tag, return the name of the tag. Otherwise,
return nil.
u.obj()
If u is a tag, return the tagged value. Otherwise,
return u itself.
u.text()
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).
SOURCE
/appl/lib/ubfa.b
SEE ALSO
sexprs(2), ubfa(6)
J L Armstrong, ``Getting Erlang to talk to the outside
world'', ACM SIGPLAN Erlang workshop 2002 , Pittsburg, PA
USA
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