treeql - Tree Query Language

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# treeql(n) 1.3.1 tcllib "Tree Query Language"

## Name

treeql - Query tree objects

## Synopsis

- package require Tcl 8.2
- package require snit
- package require struct::list
- package require struct::set
- package require treeql ?1.3.1?

- treeql objectname -tree tree ?-query query? ?-nodes nodes? ?args...?
- qo query args...
- qo result
- qo discard

## Description

This package provides objects which can be used to query and transform tree objects following the API of tree objects created by the package struct::tree.

The tree query and manipulation language used here, TreeQL, is inspired by Cost (See section References for more information).

treeql, the package, is a fairly thin query facility over tree-structured data types. It implements an ordered set of nodes (really a list) which are generated and filtered through the application of TreeQL operators to each node in turn.

## API

### TreeQL CLASS API

The command treeql is a snit::type which implements the Treeql Query Language. This means that it follows the API for class commands as specified by the package snit. Its general syntax is

treeql objectname -tree tree ?-query query? ?-nodes nodes? ?args...?

  The command creates a new tree query object and returns the fully qualified name of the object command as its result. The API the returned command is following is described in the section TreeQL OBJECT API

  Each query object is associated with a single tree object. This is the object all queries will be run against.

  If the option -nodes was specified then its argument is treated as a list of nodes. This list is used to initialize the node set. It defaults to the empty list.

  If the option -query was specified then its argument will be interpreted as an object, the parent query of this query. It defaults to the object itself. All queries will be interpreted in the environment of this object.

  Any arguments coming after the options are treated as a query and run immediately, after the node set has been initialized. This uses the same syntax for the query as the method query.

  The operations of the TreeQL available for this are explained in the section about The Tree Query Language. This section also explains the term node set used above.

### TreeQL OBJECT API

As treeql has been implemented in snit all the standard methods of snit-based classes are available to the user and therefore not listed here. Please read the documentation for snit for what they are and what functionality they provide

The methods provided by the package treeql itself are listed and explained below.

qo query args...

  This method interprets its arguments as a series of TreeQL operators and interpretes them from the left to right (i.e. first to last). Note that the first operator uses the node set currently known to the object to perform its actions. In other words, the node set is not cleared, or modified in other ways, before the query is run. This allows the user to run several queries one after the other and have each use the results of the last. Any initialization has to be done by any query itself, using TreeQL operators. The result of the method is the node set after the last operator of the query has been executed.

  Note that uncaught errors will leave the node set of the object in an intermediate state, per the TreeQL operators which were executed successfully before the error occurred.

  The above means in detail that:

  1. The first argument is interpreted as the name of a query operator, the number of arguments required by that operator is then determined, and taken from the immediately following arguments.

     Because of this operators cannot have optional arguments, all arguments have to be present as defined. Failure to do this will, at least, confuse the query interpreter, but more likely cause errors.

  2. The operator is applied to the current node set, yielding a new node set, and/or manipulating the tree object the query object is connected to.

  3. The arguments used (i.e. operator name and arguments) are removed from the list of method arguments, and then the whole process is repeated from step [1], until the list of arguments is empty or an error occurred.

          # q is the query object.
          q query root children get data
          # The above query
          # - Resets the node set to the root node - root
          # - Adds the children of root to the set - children
          # - Replaces the node set with the       - get data
          #   values for the attribute 'data',
          #   for all nodes in the set which
          #   have such an attribute.
          # - And returns this information.
          # Below we can see the same query, but rewritten
          # to show the structure as it is seen by the query
          # interpreter.
          q query \
      	    root \
      	    children \
      	    get data

  The operators of the TreeQL language available for this are explained in the section about The Tree Query Language. This section also explains the term node set used above.

qo result

  This method returns a list containing the current node set.

qo discard

  This method returns the current node set (like method result), but also destroys the query object (qo). This is useful when constructing and using sub-queries (%AUTO% objects immediately destroyed after use).

## The Tree Query Language

This and the following sections specify the Tree Query Language used by the query objects of this package in detail.

First we explain the general concepts underneath the language which are required to comprehend it. This is followed by the specifications for all the available query operators. They fall into eight categories, and each category has its own section.

1. TreeQL Concepts

2. Structural generators

3. Attribute Filters

4. Attribute Mutators

5. Attribute String Accessors

6. Sub-queries

7. Node Set Operators

8. Node Set Iterators

9. Typed node support

### TreeQL Concepts

The main concept which has to be understood is that of the node set. Each query object maintains exactly one such node set, and essentially all operators use it and input argument and for their result. This structure simply contains the handles of all nodes which are currently of interest to the query object. To name it a set is a bit of a misnomer, because

1. A node (handle) can occur in the structure more than once, and

2. the order of nodes in the structure is important as well. Whenever an operator processes all nodes in the node set it will do so in the order they occur in the structure.

Regarding the possible multiple occurrence of a node, consider a node set containing two nodes A and B, both having node P as their immediate parent. Application of the TreeQL operator "parent" will then add P to the new node set twice, once per node it was parent of. I.e. the new node set will then be {P P}.

### Structural generators

All tree-structural operators locate nodes in the tree based on a structural relation ship to the nodes currently in the set and then replace the current node set with the set of nodes found Nodes which fulfill such a relationship multiple times are added to the result as often as they fulfill the relationship.

It is important to note that the found nodes are collected in a separate storage area while processing the node set, and are added to (or replacing) the current node set only after the current node set has been processed completely. In other words, the new nodes are not processed by the operator as well and do not affect the iteration.

When describing an operator the variable N will be used to refer to any node in the node set.

ancestors

  Replaces the current node set with the ancestors for all nodes N in the node set, should N have a parent. In other words, nodes without a parent do not contribute to the new node set. In other words, uses all nodes on the path from node N to root, in this order (root last), for all nodes N in the node set. This includes the root, but not the node itself.

rootpath

  Replaces the current node set with the ancestors for all nodes N in the node set, should N have a parent. In other words, nodes without a parent do not contribute to the new node set. In contrast to the operator ancestors the nodes are added in reverse order however, i.e. the root node first.

parent

  Replaces the current node set with the parent of node N, for all nodes N in the node set, should N have a parent. In other words, nodes without a parent do not contribute to the new node set.

children

  Replaces the current node set with the immediate children of node N, for all nodes N in the node set, should N have children. In other words, nodes without children do not contribute to the new node set.

left

  Replaces the current node set with the previous/left sibling for all nodes N in the node set, should N have siblings to the left. In other words, nodes without left siblings do not contribute to the new node set.

right

  Replaces the current node set with the next/right sibling for all nodes N in the node set, should N have siblings to the right. In other words, nodes without right siblings do not contribute to the new node set.

prev

  Replaces the current node set with all previous/left siblings of node N, for all nodes N in the node set, should N have siblings to the left. In other words, nodes without left siblings are ignored. The left sibling adjacent to the node is added first, and the leftmost sibling last (reverse tree order).

esib

  Replaces the current node set with all previous/left siblings of node N, for all nodes N in the node set, should N have siblings to the left. In other words, nodes without left siblings are ignored. The leftmost sibling is added first, and the left sibling adjacent to the node last (tree order).

  The method name is a shorthand for Earlier SIBling.

next

  Replaces the current node set with all next/right siblings of node N, for all nodes N in the node set, should N have siblings to the right. In other words, nodes without right siblings do not contribute to the new node set. The right sibling adjacent to the node is added first, and the rightmost sibling last (tree order).

root

  Replaces the current node set with a node set containing a single node, the root of the tree.

tree

  Replaces the current node set with a node set containing all nodes found in the tree. The nodes are added in pre-order (parent first, then children, the latter from left to right, first to last).

descendants

  Replaces the current node set with the nodes in all subtrees rooted at node N, for all nodes N in the node set, should N have children. In other words, nodes without children do not contribute to the new node set.

  This is like the operator children, but covers the children of children as well, i.e. all the proper descendants. "Rooted at N" means that N itself is not added to the new set, which is also implied by proper descendants.

subtree

  Like operator descendants, but includes the node N. In other words:

  Replaces the current node set with the nodes of the subtree of node N, for all nodes N in the node set, should N have children. In other words, nodes without children do not contribute to the new node set. I.e this is like the operator children, but covers the children of children, etc. as well. "Of N" means that N itself is added to the new set.

forward

  Replaces the current node set with the nodes in the subtrees rooted at the right siblings of node N, for all nodes N in the node set, should N have right siblings, and they children. In other words, nodes without right siblings, and them without children are ignored.

  This is equivalent to the operator sequence

      next descendants

later

  This is an alias for the operator forward.

backward

  Replaces the current node set with the nodes in the flattened previous subtrees, in reverse tree order.

  This is nearly equivalent to the operator sequence

      prev descendants

  The only difference is that this uses the nodes in reverse order.

earlier

  Replaces the current node set with the nodes in the flattened previous subtrees, in tree order.

  This is equivalent to the operator sequence

      prev subtree

### Attribute Filters

These operators filter the node set by reference to attributes of nodes and their properties. Filter means that all nodes not fulfilling the criteria are removed from the node set. In other words, the node set is replaced by the set of nodes fulfilling the filter criteria.

hasatt attr

  Reduces the node set to nodes which have an attribute named attr.

withatt attr value

  Reduces the node set to nodes which have an attribute named attr, and where the value of that attribute is equal to value (The "==" operator is string equal -nocase).

withatt! attr val

  This is the same as withatt, but all nodes in the node set have to have the attribute, and the "==" operator is string equal, i.e. no -nocase. The operator will fail with an error if they don't have the attribute.

attof attr vals

  Reduces the node set to nodes which which have an attribute named attr and where the value of that attribute is contained in the list vals of legal values. The contained-in operator used here does glob matching (using the attribute value as pattern) and ignores the case of the attribute value, but not for the elements of vals.

attmatch attr match

  Same as withatt, but string match is used as the "==" operator, and match is the pattern checked for.

  Note that match is a interpreted as a partial argument list for string match. This means that it is interpreted as a list containing the pattern, and the pattern element can be preceded by options understand by string match, like -nocase. This is especially important should the pattern contain spaces. It has to be wrapped into a list for correct interpretation by this operator

### Attribute Mutators

These operators change node attributes within the underlying tree. In other words, all these operators have side effects.

set attr val

  Sets the attribute attr to the value val, for all nodes N in the node set. The operator will fail if a node does not have an attribute named attr. The tree will be left in a partially modified state.

unset attr

  Unsets the attribute attr, for all nodes N in the node set. The operator will fail if a node does not have an attribute named attr. The tree will be left in a partially modified state.

### Attribute String Accessors

These operators retrieve the values of node attributes from the underlying tree. The collected results are stored in the node set, but are not actually nodes.

In other words, they redefine the semantics of the node set stored by the query object to contain non-node data after their completion.

The query interpreter will terminate after it has finished processing one of these operators, silently discarding any later query elements. It also means that our talk about maintenance of a node set is not quite true. It is a node set while the interpreter is processing commands, but can be left as an attribute value set at the end of query processing.

string op attr

  Applies the string operator op to the attribute named attr, for all nodes N in the node set, collects the results of that application and places them into the node set.

  The operator will fail if a node does not have an attribute named attr.

  The argument op is interpreted as partial argument list for the builtin command string. Its first word has to be any of the sub-commands understood by string. This has to be followed by all arguments required for the subcommand, except the last. that last argument is supplied by the attribute value.

get pattern

  For all nodes N in the node set it determines all their attributes with names matching the glob pattern, then the values of these attributes, at last it replaces the node set with the list of these attribute values.

attlist

  This is a convenience definition for the operator getvals *. In other words, it replaces the node set with a list of the attribute values for all attributes for all nodes N in the node set.

attrs glob

  Replaces the current node set with a list of attribute lists, one attribute list per for all nodes N in the node set.

attval attname

  Reduces the current node set with the operator hasatt, and then replaces it with a list containing the values of the attribute named attname for all nodes N in the node set.

### Sub-queries

Sub-queries yield node sets which are then used to augment, reduce or replace the current node set.

andq query

  Replaces the node set with the set-intersection of the node set generated by the sub-query query and itself.

  The execution of the sub-query uses the current node set as its own initial node set.

orq query

  Replaces the node set with the set-union of the node set generated by the sub-query query and itself. Duplicate nodes are removed.

  The execution of the sub-query uses the current node set as its own initial node set.

notq query

  Replaces the node set with the set of nodes generated by the sub-query query which are also not in the current node set. In other word the set difference of itself and the node set generated by the sub-query.

  The execution of the sub-query uses the current node set as its own initial node set.

### Node Set Operators

These operators change the node set directly, without referring to the tree.

unique

  Removes duplicate nodes from the node set, preserving order. In other words, the earliest occurrence of a node handle is preserved, every other occurrence is removed.

select

  Replaces the current node set with a node set containing only the first node from the current node set

transform query var body

  First it interprets the sub-query query, using the current node set as its initial node set. Then it iterates over the result of that query, binding the handle of each node to the variable named in var, and executing the script body. The collected results of these executions is made the new node set, replacing the current one.

  The script body is executed in the context of the caller.

map var body

  Iterates over the current node set, binding the handle of each node to the variable named in var, and executing the script body. The collected results of these executions is made the new node set, replacing the current one.

  The script body is executed in the context of the caller.

quote val

  Appends the literal value val to the current node set.

replace val

  Replaces the current node set with the literal list value val.

### Node Set Iterators

foreach query var body

  Interprets the sub-query query, then performs the equivalent of operator over on the nodes in the node set created by that query. The current node set is not changed, except through side effects from the script body.

  The script body is executed in the context of the caller.

with query body

  Interprets the query, then runs the script body on the node set generated by the query. At last it restores the current node set as it was before the execution of the query.

  The script body is executed in the context of the caller.

over var body

  Executes the script body for each node in the node set, with the variable named by var bound to the name of the current node. The script body is executed in the context of the caller.

  This is like the builtin foreach, with the node set as the source of the list to iterate over.

  The results of executing the body are ignored.

delete

  Deletes all the nodes contained in the current node set from the tree.

### Typed node support

These filters and accessors assume the existence of an attribute called @type, and are short-hand forms useful for cost-like tree query, html tree editing, and so on.

nodetype

  Returns the node type of nodes. Attribute string accessor. This is equivalent to

      get @type

oftype t

  Reduces the node set to nodes whose type is equal to t, with letter case ignored.

nottype t

  Reduces the node set to nodes whose type is not equal to t, with letter case ignored.

oftypes attrs

  Reduces set to nodes whose @type is an element in the list attrs of types. The value of @type is used as a glob pattern, and letter case is relevant.

## Examples

... TODO ...

## References

1. COST on the Tcler's Wiki.

2. TreeQL on the Tcler's Wiki. Discuss this package there.

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category treeql of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

## Keywords

Cost, DOM, TreeQL, XPath, XSLT, structured queries, tree, tree query language

## Category

Data structures

## Copyright

Copyright © 2004 Colin McCormack <coldstore@users.sourceforge.net>
Copyright © 2004 Andreas Kupries <andreas_kupries@users.sourceforge.net>