Names, Bindings, Types and Scope

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Names, Bindings, Types and Scope

• Juan Carlos Guzmán
• CS 3123 Programming Languages Concepts
• Southern Polytechnic State University

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What is a Variable

• A memory location?
• An object?
• Sebesta gives them six characteristics name,
  address, value, type, lifetime, scope
• I do not agree, but . these are the features
  that we will first study in this course.

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Constants

• These do have the following properties
• value
• type
• lifetime
• They are usually called values

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What is a Name

• An identifier -- its a way to refer to an object
• To which object a name refers?
• Address
• Aliases
• When is the name valid?

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Is a variable a constant?

• No, a variable is a name
• A constant is a computational object
• They are related, but they are not the same!
• You can name virtually any object, but there are
  restrictions.

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Binding Time

• Binding an association between an attribute and
  its entity
• Binding Time when does it happen?
• and, When can it happen?

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When it can happen!

• Static
• Language design time
• Language implementation time
• Program Writing time
• Compile Time
• Link Time
• Load Time

• Dynamic
• Run Time
• At the start of execution (program)
• When the function is called
• When the expression is evaluated
• When the data is accessed

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Trade offs

• The earlier the binding time, the more
  information the compiler has about the attribute
• The later the binding time, the more flexible the
  entity may be
• Ex. arrays

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What do we bind?

• What is the entity?
• What is the attribute?

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Types

• static
• type checking (implicit and explicit types)
• type inference
• strong typing
• dynamic
• type is bound at assignment
• more flexible
• more prone to errors

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Lifetime

• How long will last the object that I just
  created?

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Storage

• for how long do I need space?
• static the space is available in the whole
  program
• stack-allocation allocate on function call,
  deallocate on function return
• heap-allocation
• malloc, new
• implicit
• Garbage Collection

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Scope

• The region of the program in which a binding is
  active is its scope
• Name scope
• lexical scope (static)
• dynamic scope

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Name Scope

• Information Hiding
• procedures
• modules
• objects
• overloading
• separate compilation

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Static vs. Dynamic Scope

• Under lexical scope, sub1 will always reference
  the x defined in big
• Under dynamic scope, the x it references depends
  on the dynamic state of execution

• procedure big
• var x integer
• procedure sub1
• begin sub1
• ... x ...
• end sub1
• procedure sub2
• var x integer
• begin sub2
• ...
• sub1
• ...
• end sub2

begin big ... sub1 sub2
... end big