vrtcl, by PYK, is an extensible Tcl notation for hierarchical data. It is a work in progress, and its current state is embryonic.
vrtcl was conceived in the course of pondering JSON, which bills itself as a "fat-free alternative to XML". Although JSON is lean, Tcl syntax is leaner and cleaner. Other Tcl formats such as huddle exist, but vrtcl is different in some key characteristics. The goal of vrtcl is to be more readable than other formats without sacrificing expressivity, and also to be extensible.
The path to vertical began with a comparison between a JSON example and one plausible Tcl equivalent:
JSON:
{
"title": "Example Schema",
"type": "object",
"properties": {
"firstName": {
"type": "string"
},
"lastName": {
"type": "string"
},
"age": {
"description": "Age in years",
"type": "integer",
"minimum": 0
}
},
"required": ["firstName", "lastName"]
}Tcl:
title {Example Schema}
type object
properties {
firstName {
type string
}
lastName {
type string
}
age {
description {Age in years}
type integer
minimum 0
}
}
required {firstName lastName}Here is another example from json.org, in XML, JSON, and Tcl:
<!DOCTYPE glossary PUBLIC "-//OASIS//DTD DocBook V3.1//EN">
<glossary><title>example glossary</title>
<GlossDiv><title>S</title>
<GlossList>
<GlossEntry ID="SGML" SortAs="SGML">
<GlossTerm>Standard Generalized Markup Language</GlossTerm>
<Acronym>SGML</Acronym>
<Abbrev>ISO 8879:1986</Abbrev>
<GlossDef>
<para>A meta-markup language, used to create markup
languages such as DocBook.</para>
<GlossSeeAlso OtherTerm="GML">
<GlossSeeAlso OtherTerm="XML">
</GlossDef>
<GlossSee OtherTerm="markup">
</GlossEntry>
</GlossList>
</GlossDiv>
</glossary>{
"glossary": {
"title": "example glossary",
"GlossDiv": {
"title": "S",
"GlossList": {
"GlossEntry": {
"ID": "SGML",
"SortAs": "SGML",
"GlossTerm": "Standard Generalized Markup Language",
"Acronym": "SGML",
"Abbrev": "ISO 8879:1986",
"GlossDef": {
"para": "A meta-markup language, used to create markup languages such as DocBook.",
"GlossSeeAlso": ["GML", "XML"]
},
"GlossSee": "markup"
}
}
}
}
}glossary {
title {example glossary}
GlossDiv {
title S
GlossList {
GlossEntry {
ID SGML
SortAs SGML
GlossTerm {Standard Generalized Markup Language}
Acronym SGML
Abbrev {ISO 8879:1986}
GlossDef {
para {
A meta-markup language, used to create markup
languages such as DocBook.
}
GlossSeeAlso {GML XML}
}
GlossSee markup
}
}
}
}In the Tcl examples above, the format is that of a script where each command takes exactly one argument and knows the type of that argument. Even though JSON expresses types in its syntax, JSON data is usually also consumed by a script which already knows what type to interpret the data as, since it conforms to a scheme that is usually documented as part of a service API. In most cases, the additional type syntax of JSON is not needed.
The first type information that comes to mind for the Tcl format is the ability to distinguish between a word and a list. One approach in the spirit of Tcl would be to interpret all values as lists, making sure that where a single value is intended, it is formatted as a list containing one item:
{
title {{Example Schema}}
type object
properties {
firstName {
type string
}
lastName {
type string
}
age {
description {{Age in years}}
type integer
minimum 0
}
}
required {firstName lastName}
}glossary {
title {{example glossary}}
GlossDiv {
title S
GlossList {
GlossEntry {
ID SGML
SortAs SGML
GlossTerm {{Standard Generalized Markup Language}}
Acronym SGML
Abbrev {{ISO 8879:1986}}
GlossDef {
para {{
A meta-markup language, used to create markup
languages such as DocBook.
}}
GlossSeeAlso {GML XML}
}
GlossSee markup
}
}
}
}This is already a reasonable format to work with. A more full-featured notation would support type information and other metadata in an extensible way. In JSON, it is possible to differentiate between the string "3.14159" and the number 3.14159, but all other numeric types are inferred by the format of the number. This is a hint that explicit typing is often not needed. In JSON, this numeric type inference stems from from the origins of JSON in Javascript. Given that JSON data is typically described in the documentation for an API, the explicit double-quote syntax to differentiate numbers and strings is in practice probably always redundant with the documentation, and therefore not really necessary. Where type information is used by the consuming program, it would probably always be trivial (excepting of course political issues such as backwards compatibility and organizational inertia) to add a field to the API to specify the type of the value, rather than relying on the syntax of the serialization format.
[TODO: Find real-world examples of programs that actually rely on this distinction]
The Tcl examples above are valid scripts in which each "command" takes exactly one argument. Where explicit typing is desired, a variation could be employed in which a command given only one argument treats it as its list value, and when it is given two arguments, treats the first argument as metadata, and the second as the value:
glossary {
title {{example glossary}}
GlossDiv record {
title S
GlossList record {
GlossEntry record {
ID SGML
SortAs SGML
GlossTerm {{Standard Generalized Markup Language}}
Acronym SGML
Abbrev {ISO 8879:1986}
GlossDef record {
para {{
A meta-markup language, used to create markup
languages such as DocBook.
}}
GlossSeeAlso list {GML XML}
}
GlossSee markup
}
}
}
}The vrtcl data format, which is a syntactically-valid Tcl script, is specified as follows:
Unless otherwise noted, terminology defined in the dodekalogue has the syntax described there, but sometimes with redefined semantics.
A document is an independent record. A record is a script composed of fields. Each field is a command whose semantics depend on the number of words in the command:
The type of the metadata is record. If the first field of the metadata contains only one word, the name of the field is type, and the value of the field is the first word. If the second field ontains only one word, the name of the field is itype, the value of the field is the first word.
The built-in metadata fields are:
The built-in types are:
null is written like this:
field1 null {}or, in an array:
null {}When the name of the first field of a document is meta, the field is the metadata for the document.
The built-in document metadata fields are:
The built-in fields of a metadata types record are:
The built-in fields of a metadata elements record are
#! /bin/env tclsh
#this is a vrtcl document
transaction {
location 83391
customer 17611
date {2014 05 13}
time 07:23:00
items list {eggs milk}
#also valid:
#items {{eggs milk}}
amount 8.32
}
transaction {
location 16912
customer 17611
date {2014 07 13}
time 18:47:17
items list {donuts {ice cream}}
#also valid:
#items {{donuts {ice cream}}}
amount 14.71
}#! /bin/env tclsh
#This is a vrtcl document
META {
types {
#change the `itype` of record to `array`
record {
defaults {
itype array
}
}
#change the `itype` for list to command
list {defaults {itype command}}
}
}
#the default itype for a record is now "array", so there are no record names
{one two {{three four}}}
#if the itype of list had not been changed to command, it would have been:
#{one two {three four}}
record {
name Capaneus
son Sthenelus
}
One advantage of vrtcl over huddle is that there is no need for a keyword like HUDDLE in the data. Values that look like vrtcl data can be given the word or list types, and those values will not be misinterpreted as part of the structure of the vrtcl record. vrtcl records can be composed of other vrtcl records.
In huddle the type of each value is explicit in the notation. vrtcl relies on its rules to obviate the need for explicit type notation in the common cases.
Here are some comparisons with huddle:
#huddle
HUDDLE {D {
a {s b} c {s d}}}
#vrtcl
{} {a b; c d}
#huddle
HUDDLE {
L {{s e} {s f} {s g} {s h}}}
#vertcl
{e f g h}
#vertcl, declaring the elements as `type` word
{} {list; word} {e f g h}
#vertcl, when in an array
list {e f g h}
#vertcl, each element having its own type
{} record {
{} word e
{} number 3.14159
{} list {one two {three four}}
#the same as above
{} {one two {three four}}
#the same as a above
{} {list; field} {one two {{three four}}}
}
#vertcl, each element having its own type, `array` syntax
{} array {
word e
number 3.14159
list {one two {{three four}}}
#the same as above
{one two {three four}}
#the same as above
{list; field} {one two {{three four}}}
}
#huddle
HUDDLE {D {bb {D {a {s b} c {s d}}} cc {L {{s e} {s f} {s g} {s h}}}}}
#vertcl
{} {bb {a b; c d}; cc list {e f g h}}
#HUDDLE
{L {
{D {
bb {
D {
a {s b}
c {s d}}}
cc {L {
{s e} {s f} {s g} {s h}}}}}
{s p}
{L {{s q} {s r}}}
{s s}}}
#vertcl
{} array {
record {
bb {
a b
c d
}
cc list {e f g h}
}
p
{q r}
s
}
HUDDLE {D {a {L {{D {c {s 1}}} {D {d {L {{s 2} {s 2} {s 2}}} e {s 3}}}}} b {L {{D {f {s 4} g {s 5}}}}}}}
#vertcl
{} {
a array {
record {
c 1
}
record {
d {2 2 2}
e 3
}
}
b array {
record {
f 4
g 5
}
}
}
#vrtcl, using lists instead of arrays
#vertcl
{} {
a {list; record} {
{c 1}
{d {2 2 2}; {e 3}}
}
b {list; record} {
{ f 4; g 5 }
}
}AMG: Is this called vrtcl or vertcl? Both spellings appear on this page. Do these two terms refer to distinct yet related concepts?
AMG: "Each field is a command whose semantics depend on the number of words in the command:" Customary Tcl terminology counts the name of the command as the first word of the command. Here, you probably mean to say arguments instead of words.
AMG: How does this compare to TDL? See also the configuration file format I derived from TDL and implemented in Config file using slave interp.
I'll answer my own question. TDL and my derived format target XML (including attributes), whereas you target JSON (which lacks XML-like attributes). When attributes are not used, my format matches your basic specification, though not your explicit type tagging and extra list encoding variations.
AMG: I have no JSON experience, so I would very much like to know whether it's possible for a program to make decisions based on the type of a value. This is essentially the same question as whether or not explicit type tagging has any value. You argue that it does not (EIAS and all that), but then proceed to present a way to incorporate type tagging, with only a TODO note to research whether or not there's any reason to do so. Perhaps someone else could fill us in.
Like you, I argue that inline type tagging is not useful because the consumer already knows what types to expect. In other words, the schema is embedded in the program itself.
That's not to say explicit schemas aren't useful. They are good documentation, they make it possible for a generic program to validate a document, and they may assist compression. But merely embedding types in the document does not constitute a true schema. It cannot protect against misspelled field names, and it fails to document data relationships and unused features. So explicit schemas really ought to be external documents. In short, I see no benefit to embedding type tags in this application.
Well, actually, I do see one benefit, and maybe this is what you had in mind. Since other formats require type tags, and since it's generally impossible to unambiguously detect the "type" of a Tcl word, compatibility with other formats requires type tagging. That type tagging can go inline or in an explicit schema.
As you may guess from what I said about schemas, I prefer the latter approach, though I freely admit that the former is much easier to implement. There is a third approach, which is to write a program that implicitly embeds a schema and whose purpose is conversion. This surely works, but I think of it as a one-off sort of solution, whereas this page is dedicated to describing a general approach, so again I think an explicit schema would be more appropriate.
... I hate to say it, but there is a fourth way to go. PLEASE do not use this. [tcl::unsupported::representation] tells you what C-based type the script most recently wanted any given value to be. Again, don't do this, since it will result in a very brittle and unpredictable program, requiring the end user to do anti-Tcl contortions to get the desired results, and even then with surprises. The only reason I mention this at all (aside from taking the opportunity to warn against it) is because a real-world Tcl extension does it: tcom. And it is sometimes very shocking [L1 ].
AMG: What is the point of applying an extra level of list encoding to each command's argument? It's already a single word on account of being one argument. I don't see how this helps in disambiguating its type, if that's even a goal (see above).
AMG: The format you present is script-oriented, meaning that you use newlines as field separators. Do you also allow semicolons? How about substitutions? Comments? Double quotes instead of braces? Backslashes? Blank lines? Is whitespace significant, e.g. indenting? What about non-data commands such as looping and conditionals, such as those I demonstrate in Config file using slave interp?
Except for those showing type tagging, all the examples you present could instead be viewed as dicts, since newlines work as list/dict element separators just as well as any other form of whitespace. What benefit does your approach have over nested dicts?