SEXPRS(2) SEXPRS(2)
NAME
Sexprs: Sexp - S-expressions
SYNOPSIS
include "bufio.m";
include "sexprs.m";
sexprs := load Sexprs Sexprs->PATH;
Sexp: adt {
pick {
String =>
s: string;
hint: string;
Binary =>
data: array of byte;
hint: string;
List =>
l: cyclic list of ref Sexp;
}
read: fn(b: ref Bufio->Iobuf): (ref Sexp, string);
parse: fn(s: string): (ref Sexp, string, string);
pack: fn(e: self ref Sexp): array of byte;
packedsize: fn(e: self ref Sexp): int;
text: fn(e: self ref Sexp): string;
b64text: fn(e: self ref Sexp): string;
unpack: fn(a: array of byte): (ref Sexp, array of byte, string);
eq: fn(e: self ref Sexp, t: ref Sexp): int;
copy: fn(e: self ref Sexp): ref Sexp;
astext: fn(e: self ref Sexp): string;
asdata: fn(e: self ref Sexp): array of byte;
islist: fn(e: self ref Sexp): int;
els: fn(e: self ref Sexp): list of ref Sexp;
op: fn(e: self ref Sexp): string;
args: fn(e: self ref Sexp): list of ref Sexp;
};
init: fn();
DESCRIPTION
Sexprs provides a data type and I/O for S-expressions, or
`symbolic expressions', which represent complex data as
trees. This implementation provides the variant defined by
Rivest in Internet Draft `draft-rivest-sexp-00.txt' (4 May
1997), as used for instance by the Simple Public Key Infras-
tructure (SPKI). It offers a basic set of operations on the
internal representation, and input and output in both
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SEXPRS(2) SEXPRS(2)
canonical and advanced transport encodings. Canonical form
conveys binary data directly and efficiently (unlike some
other schemes such as XML). Canonical encoding must be used
when exchanging S-expressions between computers, and when
digitally signing an expression. Advanced encoding is a
more elaborate form similar to that used by Lisp inter-
preters, typically using only printable characters: repre-
senting any binary data in hexadecimal or base 64 encodings,
and quoting strings containing special characters, using
escape sequences as required. Unquoted text is called a
token, restricted by the standard to a specific alphabet: it
must start with a letter or a character from the set
`-./_:*+=', and contain only letters, digits and characters
from that set. Upper- and lower-case letters are distinct.
See sexprs(6) for a precise description.
Init must be called before invoking any other operation of
the module.
Sexp is the internal representation of S-expression data, as
lists and non-list values (atoms) that in general can form a
tree structure; that is, a list may contain not just atoms
but other lists as its elements, and so on recursively. The
atoms are strings of text or binary. A well-formed S-
expression might be a tree, but cannot contain cycles.
For convenience in processing, Sexp distinguishes three
variants represented in a pick adt:
Sexp.String
An atom that can be represented as a textual string s,
including all tokens but also any other data that con-
tains no characters outside the 7-bit ASCII set and no
control-characters other than space. Hint is the `dis-
play hint', typically nil (see the Internet Draft for
its intended use).
Sexp.Binary
An atom that must be represented as an array of bytes
data (typically because it is purely binary data or
contains non-space control-characters). Hint again is
the display hint.
Sexp.List
A list of S-expression values, l.
Sexp provides the following operations for input and output,
using bufio(2)'s buffered channels (directly or indirectly):
read(b)
Read one S-expression (a list or a single token) from
Iobuf b. Return a tuple of the form (e,err), where e is
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the Sexp representing the data just read, and err is
nil on success; b is positioned at the first character
after the end of the S-expression. On an error, e is
nil, and err contains the diagnostic string. On end-
of-file, both e and err are nil.
parse(s)
Parse the first S-expression in string s, and return a
tuple (e,t,err where e is the Sexp representating that
expression, t is the unparsed tail of string s, and err
is a diagnostic string that is nil on success. On an
error, e is nil, t is as before, and err contains the
diagnostic.
e.pack()
Return an array of byte that represents Sexp e as an
S-expression in canonical transport form.
e.packedsize()
Return the size in bytes of the canonical transport
representation of e.
e.b64text()
Return a string that contains the base-64 representa-
tion of the canonical representation of e, surrounded
by braces.
e.text()
Return a string that represents e as an S-expression in
advanced (`human-readable') transport form containing
no newlines. The result of text can always be inter-
preted by Sexp.read and Sexp.parse, and furthermore
Sexp.parse(e.text()) yields the same tree value as e
(similarly for Sexp.read).
unpack(a)
Parse the first S-expression in array of byte a, and
return a tuple (e,r,err where e is the Sexp represent-
ing the S-expression, r is a slice of a giving the por-
tion of a after the S-expression, and err is nil on
success. On error, e is nil, r is as before, and err
contains a diagnostic string. The data in a is typi-
cally in canonical transport form, read from a file or
network connection.
All input functions accept S-expression in either canonical
or advanced form, or any legal mixture of forms. Expres-
sions can cross line boundaries. For output in canonical
form, use pack; for output in advanced form (similar to
Lisp's S-expressions), use text.
Sexp provides a further small collection of operations:
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e1.eq(e2)
Return non-zero if expression e1 and e2 are identical
(isomorphic in tree structure and atoms in correspond-
ing positions in e1 and e2 equal); return 0 otherwise.
e.copy()
Return a new Sexp value equal to e, but sharing no
storage with it. (In other words, it returns a copy of
the whole tree e).
e.islist()
Return true iff e is a list (ie, a value of type
Sexp.List).
Two operations provide a shorthand for fetching the value of
an atom, returning nil if applied to a list:
e.astext()
Return the value of e as a string; binary data is
assumed to be a string in utf(6) representation.
e.asdata()
Return the value of e as an array of bytes. A String
value will be converted to an array of bytes giving its
utf(6).
The remaining operations extract values from lists, and
return nil if applied to an atom:
e.els()
Return the elements of list e; return nil if e is not a
list.
e.op()
Return the first token of list e, if it is a string;
return nil if it is not a string or e is not a list.
The first token of a list often gives an operation
name.
e.args()
Return a list containing the second and subsequent val-
ues in list e; useful when the first value is an opera-
tion name and the rest represent parameters (arguments)
to that operation.
EXAMPLES
The following S-expression is in advanced transport form:
(snicker "abc" (#03# |YWJj|))
It represents a list of three elements: the token `snicker',
the token `abc', and a sub-list with two elements (a
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hexadecimal constant representing the byte `03', and a
base-64 constant `YWjj' that represents the bytes `abc').
Here is another in advanced form with two sublists:
(certificate
(issuer bob)
(subject "alice b"))
Its equivalent in canonical form (as produced by pack) is
shown below:
(11:certificate(6:issuer3:bob)(7:subject7:alice b))
Nesting parentheses still mark the start and end of lists,
but there is no other punctuation or white space, and the
byte sequence representing each atom is preceded by a deci-
mal count, so that binary values appear unencoded, and for
instance the space in the last string is not a delimiter but
part of the token.
SOURCE
/appl/lib/sexprs.b
SEE ALSO
bufio(2), xml(2), sexprs(6)
R. Rivest, ``S-expressions'', Network Working Group Internet
Draft, `http://theory.lcs.mit.edu/~rivest/sexp.txt' (4 May
1997), reproduced in /lib/sexp.
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