.. index:: type

.. _types:

Types
#####

QL is a statically typed language, so each variable must have a declared **type**. A type is a set of values.
For example, the type ``int`` is the set of integers. 
Note that a value can belong to more than one of these sets, which means that it can have more 
than one type.

The kinds of types in QL are :ref:`primitive types <primitive-types>`, :ref:`classes <classes>`,
:ref:`character types <character-types>`, :ref:`class domain types <domain-types>`, 
:ref:`algebraic datatypes <algebraic-datatypes>`, and :ref:`database types <database-types>`.

.. index:: boolean, float, int, string, date
.. _primitive-types:

Primitive types
***************

These types are built in to QL and are always available in the global :ref:`namespace <namespaces>`,
independent of the database that you are querying.

   .. _boolean:

#. **boolean**: This type contains the values ``true`` and ``false``.
    
   .. _float:

#. **float**: This type contains 64-bit floating point numbers, such as ``6.28`` and ``-0.618``.
    
   .. _int:

#. **int**: This type contains 32-bit `two's complement <https://en.wikipedia.org/wiki/Two%27s_complement>`_ integers, such as ``-1`` and ``42``.
    
   .. _string:

#. **string**: This type contains finite strings of 16-bit characters.
    
   .. _date:

#. **date**: This type contains dates (and optionally times). 
    

QL has a range of built-in operations defined on primitive types. These are available by using dispatch on expressions of the appropriate type. For example, ``1.toString()`` is the string representation of the integer constant ``1``. For a full list of built-in operations available in QL, see the
section on `built-ins <https://help.semmle.com/QL/ql-spec/language.html#built-ins>`__ in the QL language specification.

.. index:: class
.. _classes:

Classes
*******

You can define your own types in QL. One way to do this is to define a **class**.

Classes provide an easy way to reuse and structure code. For example, you can:
  - Group together related values.
  - Define :ref:`member predicates <member-predicates>` on those values.
  - Define subclasses that :ref:`override member predicates <overriding-member-predicates>`.

A class in QL doesn't "create" a new object, it just represents a logical property. A value is 
in a particular class if it satisfies that logical property.

.. _defining-class:

Defining a class
================

To define a class, you write:

#. The keyword ``class``. 
#. The name of the class. This is an `identifier <https://help.semmle.com/QL/ql-spec/language.html#identifiers>`_ 
   starting with an uppercase letter.
#. The types to extend. 
#. The :ref:`body of the class <class-bodies>`, enclosed in braces.

For example::

    class OneTwoThree extends int {
      OneTwoThree() { // characteristic predicate
        this = 1 or this = 2 or this = 3
      }
 
      string getAString() { // member predicate
        result = "One, two or three: " + this.toString()
      }
    
      predicate isEven() { // member predicate
        this = 2
      }
    }

This defines a class ``OneTwoThree``, which contains the values ``1``, ``2``, and ``3``. The
:ref:`characteristic predicate <characteristic-predicates>` captures the logical property of
"being one of the integers 1, 2, or 3." 

.. index:: extends

``OneTwoThree`` extends ``int``, that is, it is a subtype of ``int``. A class in QL must always
extend at least one existing type. Those types are called the **base types** of the class. The 
values of a class are contained within the intersection of the base types (that is, they are in
the :ref:`class domain type <domain-types>`). A class inherits all member predicates from its
base types. 

A class can extend multiple types. See :ref:`multiple-inheritance` below.

To be valid, a class:
  - Must not extend itself.
  - Must not extend a :ref:`final` class. 
  - Must not extend types that are incompatible. (See :ref:`type-compatibility`.)

You can also annotate a class. See the list of :ref:`annotations <annotations-overview>`
available for classes.

.. _class-bodies:

Class bodies
============

The body of a class can contain:
  -  A :ref:`characteristic predicate <characteristic-predicates>` declaration.
  -  Any number of :ref:`member predicate <member-predicates>` declarations.
  -  Any number of :ref:`field <fields>` declarations. 

When you define a class, that class also inherits all non-:ref:`private` member predicates and
fields from its supertypes. You can :ref:`override <overriding-member-predicates>` those 
predicates and fields to give them a more specific definition.

.. _characteristic-predicates:

Characteristic predicates
-------------------------

These are :ref:`predicates <predicates>` defined inside the body of a class. They are logical
properties that use the variable ``this`` to restrict the possible values in the class.

.. _member-predicates:

Member predicates
-----------------

These are :ref:`predicates <predicates>` that only apply to members of a particular class.
You can :ref:`call <calls>` a member predicate on a value. For example, you can use the member
predicate from the :ref:`above <defining-class>` class::

    1.(OneTwoThree).getAString()

This call returns the result ``"One, two or three: 1"``.

The expression ``(OneTwoThree)`` is a :ref:`cast <casts>`. It ensures that ``1`` has type
``OneTwoThree`` instead of just ``int``. Therefore, it has access to the member predicate
``getAString()``.

Member predicates are especially useful because you can chain them together. For example, you
can use ``toUpperCase()``, a built-in function defined for ``string``::

    1.(OneTwoThree).getAString().toUpperCase()

This call returns ``"ONE, TWO OR THREE: 1"``.

.. index:: this
.. _this:

.. note::

    Characteristic predicates and member predicates often use the variable ``this``. 
    This variable always refers to a member of the class—in this case a value belonging to the 
    class ``OneTwoThree``.
    In the :ref:`characteristic predicate <characteristic-predicates>`, the variable ``this`` 
    constrains the values that are in the class.
    In a :ref:`member predicate <member-predicates>`, ``this`` acts in the same way as any 
    other argument to the predicate.

.. index:: field
.. _fields: 

Fields
------

These are variables declared in the body of a class. A class can have any number of field
declarations (that is, variable declarations) within its body. You can use these variables in 
predicate declarations inside the class. Much like the :ref:`variable <this>` ``this``, fields
must be constrained in the :ref:`characteristic predicate <characteristic-predicates>`.

For example:: 

    class SmallInt extends int {
      SmallInt() { this = [1 .. 10] }
    }
    
    class DivisibleInt extends SmallInt {
      SmallInt divisor;   // declaration of the field `divisor`
      DivisibleInt() { this % divisor = 0 }
    	
      SmallInt getADivisor() { result = divisor }
    }
    
    from DivisibleInt i
    select i, i.getADivisor()

In this example, the declaration ``SmallInt divisor`` introduces a field ``divisor``, constrains
it in the characteristic predicate, and then uses it in the declaration of the member predicate
``getADivisor``. This is similar to introducing variables in a :ref:`select clause <select-clauses>`
by declaring them in the ``from`` part. 

You can also annotate predicates and fields. See the list of :ref:`annotations <annotations-overview>`
that are available.

Kinds of classes
================

The classes in the above examples are all **concrete** classes. They are defined by 
restricting the values in a larger type. The values in a concrete class are precisely those
values in the intersection of the base types that also satisfy the 
:ref:`characteristic predicate <characteristic-predicates>` of the class.

.. _abstract-classes:

A class :ref:`annotated <abstract>` with ``abstract``, known as an **abstract** class, is also a restriction of 
the values in a larger type. However, an abstract class is defined as the union of its 
subclasses. In particular, for a value to be in an abstract class, it must satisfy the 
characteristic predicate of the class itself **and** the characteristic predicate of a subclass.

An abstract class is useful if you want to group multiple existing classes together 
under a common name. You can then define member predicates on all those classes. You can also 
extend predefined abstract classes: for example, if you import a library that contains an 
abstract class, you can add more subclasses to it.

**Example**

If you are writing a security query in QL, you may be interested in identifying 
all expressions that can be interpreted as SQL queries. 
You can use the following abstract class to describe these expressions::

    abstract class SqlExpr extends Expr {
      ... 
    }

Now define various subclasses—one for each kind of database management system. For example, you
can define a subclass ``class PostgresSqlExpr extends SqlExpr``, which contains expressions 
passed to some Postgres API that performs a database query. 
You can define similar subclasses for MySQL and other database management systems.

The abstract class ``SqlExpr`` refers to all of those different expressions. If you want to add
support for another database system later on, you can simply add a new subclass to ``SqlExpr``;
there is no need to update the queries that rely on it.

.. _overriding-member-predicates:

Overriding member predicates
============================

If a class inherits a member predicate from a supertype, you can **override** the inherited
definition. You do this by defining a member predicate with the same name and arity as the
inherited predicate, and by adding the ``override`` :ref:`annotation <override>`. 
This is useful if you want to refine the predicate to give a more specific result for the 
values in the subclass.

For example, extending the class from the :ref:`first example <defining-class>`::

    class OneTwo extends OneTwoThree {
      OneTwo() {
        this = 1 or this = 2
      }
    
      override string getAString() {
        result = "One or two: " + this.toString()
      }
    }

The member predicate ``getAString()`` overrides the original definition of ``getAString()``
from ``OneTwoThree``.

Now, consider the following query:: 

    from OneTwoThree o
    select o, o.getAString()

The query uses the "most specific" definition(s) of the predicate ``getAString()``, so the results
look like this:

+---+-------------------------+
| o | ``getAString()`` result |
+===+=========================+
| 1 | One or two: 1           |
+---+-------------------------+
| 2 | One or two: 2           |
+---+-------------------------+
| 3 | One, two or three: 3    |
+---+-------------------------+

In QL, unlike other object-oriented languages, different subtypes of the same types don't need to be 
disjoint. For example, you could define another subclass of ``OneTwoThree``, which overlaps
with ``OneTwo``::

    class TwoThree extends OneTwoThree {
      TwoThree() {
        this = 2 or this = 3
      }
    
      override string getAString() {
        result = "Two or three: " + this.toString()
      }
    }

Now the value 2 is included in both class types ``OneTwo`` and ``TwoThree``. Both of these classes 
override the original definition of ``getAString()``. There are two new "most specific" definitions, 
so running the above query gives the following results:

+---+-------------------------+
| o | ``getAString()`` result |
+===+=========================+
| 1 | One or two: 1           |
+---+-------------------------+
| 2 | One or two: 2           |
+---+-------------------------+
| 2 | Two or three: 2         |
+---+-------------------------+
| 3 | Two or three: 3         |
+---+-------------------------+

.. _multiple-inheritance:

Multiple inheritance
====================

A class can extend multiple types. In that case, it inherits from all those types.

For example, using the definitions from the above section::

    class Two extends OneTwo, TwoThree {}

Any value in the class ``Two`` must satisfy the logical property represented by ``OneTwo``, 
**and** the logical property represented by ``TwoThree``. Here the class ``Two`` contains one
value, namely 2.

It inherits member predicates from ``OneTwo`` and ``TwoThree``. It also (indirectly) inherits
from ``OneTwoThree`` and ``int``.

.. note:: If a subclass inherits multiple definitions for the same predicate name, then it
   must :ref:`override <overriding-member-predicates>` those definitions to avoid ambiguity.
   :ref:`Super expressions <super>` are often useful in this situation.

.. _character-types:
.. _domain-types:

Character types and class domain types
**************************************

You can't refer to these types directly, but each class in QL implicitly defines a character 
type and a class domain type. (These are rather more subtle concepts and don't appear very 
often in practical query writing.)

The **character type** of a QL class is the set of values satisfying the :ref:`characteristic 
predicate <characteristic-predicates>` of the class. 
It is a subset of the domain type. For concrete classes, a value belongs to 
the class if, and only if, it is in the character type. For :ref:`abstract classes 
<abstract-classes>`, a value must also belong to at least one of the subclasses, in addition to
being in the character type. 

The **domain type** of a QL class is the intersection of the character types of all its supertypes, that is, a value
belongs to the domain type if it belongs to every supertype. It occurs as the type of ``this`` 
in the characteristic predicate of a class.

.. index:: newtype
.. _algebraic-datatypes:

Algebraic datatypes
*******************

.. note:: The syntax for algebraic datatypes is considered experimental and is subject to
   change. However, they appear in the `standard QL libraries <https://github.com/Semmle/ql>`_
   so the following sections should help you understand those examples.

An algebraic datatype is another form of user-defined type, declared with the keyword ``newtype``.

Algebraic datatypes are used for creating new values that are neither primitive values nor entities from
the database. One example is to model flow nodes when analyzing data flow through a program.

An algebraic datatype consists of a number of mutually disjoint *branches*, that each define
a branch type. The algebraic datatype itself is the union of all the branch types.
A branch can have arguments and a body. A new value of the branch type is produced for each set
of values that satisfy the argument types and the body.

A benefit of this is that each branch can have a different structure. For example, if you want
to define an "option type" that either holds a value (such as a ``Call``) or is empty, you
could write this as follows::

    newtype OptionCall = SomeCall(Call c) or NoCall()

This means that for every ``Call`` in the program, a distinct ``SomeCall`` value is produced.
It also means that a unique ``NoCall`` value is produced.

Defining an algebraic datatype
==============================

To define an algebraic datatype, use the following general syntax:: 

    newtype <TypeName> = <branches>

The branch definitions have the following form::

    <BranchName>(<arguments>) { <body> }

- The type name and the branch names must be `identifiers <https://help.semmle.com/QL/ql-spec/language.html#identifiers>`_ 
  starting with an uppercase letter. Conventionally, they start with ``T``.
- The different branches of an algebraic datatype are separated by ``or``.
- The arguments to a branch, if any, are :ref:`variable declarations <variable-declarations>`
  separated by commas.
- The body of a branch is a :ref:`predicate <predicates>` body. You can omit the branch body, in which case
  it defaults to ``any()``. 
  Note that branch bodies are evaluated fully, so they must be finite. They should be kept small
  for good performance.

For example, the following algebraic datatype has three branches::

    newtype T =
      Type1(A a, B b) { body(a, b) }
      or 
      Type2(C c)
      or 
      Type3()

Standard pattern for using algebraic datatypes
==============================================

Algebraic datatypes are different from :ref:`classes <classes>`. In particular, algebraic datatypes don't have a 
``toString()`` member predicate, so you can't use them in a :ref:`select clause <select-clauses>`.

Classes are often used to extend algebraic datatypes (and to provide a ``toString()`` predicate). 
In the standard QL language libraries, this is usually done as follows:

- Define a class ``A`` that extends the algebraic datatype and optionally declares :ref:`abstract`
  predicates.
- For each branch type, define a class ``B`` that extends both ``A`` and the branch type, 
  and provide a definition for any abstract predicates from ``A``.
- Annotate the algebraic datatype with :ref:`private`, and leave the classes public.

For example, the following code snippet from the QL for C# data flow library defines classes
for dealing with tainted or untainted values. In this case, it doesn't make sense for 
``TaintType`` to extend a database type. It is part of the taint analysis, not the underlying
program, so it's helpful to extend a new type (namely ``TTaintType``):: 

    private newtype TTaintType =
      TExactValue()
      or
      TTaintedValue()
    
    /** Describes how data is tainted. */
    class TaintType extends TTaintType {
      string toString() {
        this = TExactValue() and result = "exact"
        or 
        this = TTaintedValue() and result = "tainted"
      }
    }
    
    /** A taint type where the data is untainted. */
    class Untainted extends TaintType, TExactValue {
    }
    
    /** A taint type where the data is tainted. */
    class Tainted extends TaintType, TTaintedValue {
    }
    
.. _database-types:

Database types
**************

Database types are defined in the database schema. This means that they depend on the database
that you are querying, and vary according to the data you are analyzing.

For example, if you are querying a QL database for a Java project, the database types may
include ``@ifstmt``, representing an if statement in the Java code, and ``@variable``, 
representing a variable.

.. _type-compatibility:

Type compatibility
******************

Not all types are compatible. For example, ``4 < "five"`` doesn't make sense, since you can't
compare an ``int`` to a ``string``.

To decide when types are compatible, there are a number of different "type universes" in QL.

The universes in QL are:
    - One for each primitive type (except ``int`` and ``float``, which are in the same universe
      of "numbers").
    - One for each database type.
    - One for each branch of an algebraic datatype.

For example, when defining a :ref:`class <classes>` this leads to the following restrictions:
    - A class can't extend multiple primitive types.
    - A class can't extend multiple different database types.
    - A class can't extend multiple different branches of an algebraic datatype.
