Lars H, 2010-02-03: Some months ago, while working on OpenMath things, I grew tired of the XML syntax — it just felt cluttered and clumpsy with all those </>="' characters. I wanted something more lightweight (for me as author), so where to turn, if not to Tcl? First for sketching things out, and later as a format that programs would actually process, I began to use something I dubbed TDL (for Tcl Data Language, or perhaps — in analogy with Tool Command Language and Tool Protocol Language — Tool Data Language).
First, an example to illustrate the idea. A small fragment of OpenMath is
<OMA>
<OMS cd="symocat1" name="label"/>
<OMS cd="Hopf-algebra" name="mult"/>
<OMA>
<OMS cd="list1" name="list"/>
<OMV name="a"/>
</OMA>
<OMA>
<OMS cd="list1" name="list"/>
<OMV name="b"/>
<OMV name="c"/>
</OMA>
</OMA>This is rather cluttered. The same thing as TDL could be
OMA {
/OMS symocat1 label
/OMS Hopf-algebra mult
OMA {/OMS list1 list; /OMV a}
OMA {/OMS list1 list; /OMV b; /OMV c}
}or if not so specialised
OMA {
OMS cd symocat1 name label
OMS cd Hopf-algebra name mult
OMA {OMS cd list1 name list; OMV name a}
OMA {
OMS cd list1 name list
OMV name b
OMV name c
}
}giorgio_v It reminds me of xmlgen
1. A general XML element is transformed to a Tcl command with the element name as command name, and the contents as a final "body" argument. Other arguments must occur in pairs and encode the attributes in the usual key–value style. Hence
<element attr1="val1" attr2="val2"></element>
is equivalent to
element attr1 val1 attr2 val2 {}and also to
element attr1 val1 attr2 val2
since parity allows recognising the case of a missing body argument.
2. A sequence of XML elements is transformed to a sequence of commands, i.e., to a script. Hence
<element1/><element2 attr2="val2"/>
is equivalent to
element1; element2 attr2 val2
and also to
element1
element2 attr2 val23. Command names which do not fit the XML syntax for element names are used to express things other than elements and can have other syntaxes, e.g. arguments identified by position. In particular, the "/" command is used to encode character data in a sequence of elements. Thus
<OMI>3</OMI>
can be encoded as
OMI {/ 3}"/" can have any number of arguments, which are concatenated in the manner of append.
4. Elements can sometimes be given an alternative encoding, in the form of a positional command. These commands will usually have a "/" prepended to the element name. The syntax is usually that required attributes (if any) and character data contents (if any) are mandatory arguments, but no general rule defining such commands exist; each must be defined explicitly.
Example: The OM elements
<OMS cd="arith1" name="times"/>
<OMV name="x"/>
<OMI>5</OMI>could be expressed as
/OMS arith1 times
/OMV x
/OMI 5but this would be context-dependent.
Some probably think it should be a criminal offense to invent a new syntax for what is almost XML, and there are no doubt interoperability problems lurking. On the other hand, if it allows me to be more productive, then I mostly think it is a good thing.
For robust and informative handling of syntax errors, it would probably be necessary to use something like parsetcl to parse TDL, but if we're content with parsing valid TDL and throwing an error on the rest, then things are much easier. The trick is to parse TDL data by evaluating it in an empty slave interpreter.
namespace eval prettyTDL {
interp create -safe [list [namespace current]::theinterp]
theinterp hide namespace
theinterp invokehidden namespace delete ::
}The first operation on TDL code will be to prettyprint it. The central command point for prettyprinting is the prettyprint procedure, which has the call syntax
and returns the prettyprinting of the script. The supported options are:
proc prettyTDL::prettyprint {script args} {
set res ""
array set O {-indent "" -step { }}
array set O $args
theinterp eval $script
return $res
}The way the prettyprinting works is that each command appends the prettyprinted form of itself, preceded by the appropriate indentation and followed by a newline, to the local variable res in this procedure. The local array O has two entries -indent and -step which contain the current values of these parameters.
In most cases, that appending is taken care of by the unknown command in the slave interpreter, which is an alias to the following procedure in the master interpreter.
prettyTDL::theinterp alias unknown [namespace current]::prettyTDL::unknown
proc prettyTDL::unknown {name args} {
upvar 1 res res O O
switch -regexp -- $name {
{^[[:alpha:]_:][[:alnum:]_:.-]*$} {
if {[llength $args] % 2} then {
set body [lindex $args end]
set args [lreplace $args end end]
} else {
set body ""
}
append res $O(-indent) [linsert $args 0 $name]
if {[regexp {\S} $body]} then {
append res " \{\n" [
prettyprint $body {*}[array get O]\
-indent $O(-indent)$O(-step)
] $O(-indent) \}
}
append res \n
}
default {
append res $O(-indent) [linsert $args 0 $name] \n
}
}
}
proc prettyTDL::slash {args} {
upvar 1 res res O(-indent) indent
set L [list /]
for {set i 0} {$i < [llength $args]} {incr i} {
set n [string first \n [lindex $args $i]]
if {$n<0} then {
lappend L [lindex $args $i]
continue
}
if {$n>0} then {
lappend L [string range [lindex $args $i] 0 $n-1]
}
append res $indent $L { \n} \n
set L [list /]
lset args $i [string replace [lindex $args $i] 0 $n]
incr i -1
}
if {[llength $L] > 1} then {
append res $indent $L \n
}
}
prettyTDL::theinterp alias / [namespace current]::prettyTDL::slashFor example:
% prettyTDL::prettyprint {OMA {/OMS arith1 plus; /OMV a; /OMV b}}
OMA {
/OMS arith1 plus
/OMV a
/OMV b
}
To be continued…