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htsql / doc / ref / scopes.rst

Naming Scopes

Table of Contents

An identifier is a sequence of characters which contains Latin letters, underscores (_), decimal digits and those Unicode characters which are classified as alphanumeric. An identifier must not start with a digit. In HTSQL, identifiers are used to refer to classes, attributes, links, functions, constants, predefined expressions, and other objects.

Each identifier in the input query is resolved to determine the object denoted by it. How an identifier is resolved depends on the form of the identifier itself and the position of the identifier in the input query. In this section, we describe in detail how HTSQL processor resolves identifiers.

Identifiers

In HTSQL, an identifier could be used in three different forms: plain, functional and referential.

Plain Identifiers

We call an identifier plain when it does not appear in a function or a reference form. Plain identifiers are used to refer to data model objects such as classes, attributes and links, and also to global constants.

This query contains four plain identifiers: school, name, campus and null, which refer respectively to a class school, class attributes school.name and school.campus, and a constant null.

Function Calls

An identifier is said to be in a functional form (or just a function) when it is a part of a function call expression. HTSQL supports two notations for function calls: prefix (F(x,y,...)) and infix (x :F (y,...)); the choice of calling notation has no effect on name resolution.

It is convenient to treat unary and binary operators as functions with one or two arguments. In HTSQL, operators use the same mechanism for name resolution as regular functions.

This query contains:

  • count(), csv(): functions with one argument;
  • as(): a function with two arguments;
  • /: an unary operator (two occurrences);
  • ?, =: binary operators.

The query uses both prefix and infix call notation. Rewritten to use the prefix notation only, the query takes the form:

References

An identifier appears in a reference form if it is preceded by $ symbol. References are used to pass values between different parts of the query. Often, references serve as parameters of predefined expressions.

Here, reference $avg_credits denotes the average number of credits across all courses.

In this query, reference $c is a parameter of a predefined expression course_with_credits().

Scopes

A naming scope is a mapping of names to associated objects. HTSQL distinguishes two types of scopes: global and local.

Global Scope

The global scope contains built-in functions, operators and constants.

In this query, functions count() and csv(), operators ? and =, and constant null are found in the global scope.

Local Scope

In the model graph, each node together with all outgoing arrows forms a local scope.

A unit node induces a unit scope. This scope contains the names of all classes in the database model.

In this example, identifiers school and department are found in the unit scope.

A class node induces a class scope. A class scope contains names of all class attributes and links.

In this example, identifiers name, department and program are from the scope of class school.

A domain node induces a domain scope, which is generally empty because domain nodes have no outgoing arrows.

This following diagram demonstrates local scopes associated with the unit node and class node school.

Quotient Scope

The quotient class is a special type of a node in the model graph formed by the projection operator (^). The quotient class is a derived node, that is, a node which does not come from the original database model, but is constructed dynamically.

The projection operator has the form T ^ x, where T is called the base of the projection and x is called the kernel of the projection. The quotient class consists of all unique values of x as it runs over T.

Each quotient class T ^ x has a natural link back to the base node T; it relates each kernel value to all entities of the base class that produced this value. This link is called a complement link. Attributes of the quotient class are values of the kernel expression.

Quotient scope is a local scope associated with a quotient class. HTSQL processor assigns the name of the base class to the complement link. In cases when HTSQL is unable to deduce the link name, one may use a complement indicator ^.

Thus the following two queries produce identical results. The first query uses explicit attribute and link names while the second one uses a wildcard (*) and complement (^) indicators to refer to the same objects:

Resolution Rules

In an HTSQL query, each expression is associated with a collection of naming scopes, or a naming context. A naming context consists of the global scope and a stack of local scopes. When HTSQL processor resolves identifiers in an expression, it seeks for the matching name and the corresponding object in the naming context of the expression.

The naming context of the query itself consists of just one local scope: the unit scope. Some functions and operators modify the naming scope by adding a new local scope to the stack or augmenting the top local scope.

Context-Altering Operators

Some operators alter the naming context before evaluating the right operand. The following operators evaluate and add the left operand to the naming context before evaluating the right operand:

  • sieve (T ? p);
  • projection (T ^ x);
  • selection (T {x,y,...});
  • composition (T . S).

The following operators adds the unit scope to the naming context before evaluating the right operand:

  • attachment (x -> T);
  • detachment (@ T).

Scope-Augmenting Functions

Functions define() and given() allows you to add new names to the current scope.

Function define() takes one or more assignment and adds the names and associated expressions to the top local scope.

In this example, we add a calculated attribute num_prog to the scope of school.

Function define() could also be used to add functions and references to the top local scope:

Function given() takes an expression as the first parameter, a list of assignments as subsequent parameters and evaluates the expression in an augmented scope. Function given() is typically used in infix notation:

Resolving Plain Identifiers

When HTSQL processor translates a plain identifier, it uses the following rules to find the corresponding object.

  1. Search the top local scope for a matching name; done if found.
  2. Search the global scope for a matching name; done if found.
  3. Otherwise, report an error.

Note that only the top scope in the local scope stack is consulted, the other scopes are completely shadowed.

The following table summarizes naming contexts used in the query above.

Scope Content
global true, false, null
unit school, department, program, course
scope of school code, name, campus, program, department

The next query shows that attribute campus from the scope of school is not available when school is shadowed by another scope.

Resolving Function Calls

Rules for resolving identifiers in functional form mostly coincide with rules for plain identifiers. The only difference is that both the name and the number of arguments must coincide.

Compare the following three queries. The first and the third queries match functions date() with 1 and 3 arguments respectively. Note that those are different functions even though they share the same name, they are distinguished by the number of arguments.

In the example above, function date() was found in the global scope. You can use function define() to add a function to the top local scope.

Resolving References

The following rules are used for resolving references:

  1. Search for the matching name in every scope in the stack of local scopes; done if found.
  2. Otherwise, report an error.

Note that as opposed to plain identifiers, reference lookup uses all local scopes in the current naming context.

In this example, reference $avg_credits is defined in the scope of department, but used in a nested scope of course.

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