Overview:

1
Overview

• Lecture 6
• Dolores Zage

2
What is a program?

• Operations that are to be applied to certain data
  in a certain sequence (definition holds for all
  languages)
• Basic differences among languages exist in
• types of data allowed
• types of operations available
• mechanisms for controlling the sequence of the
  operations applied to the data

3
Focus

• Chapter 4 concentrates data, types (highlighting
  imperative languages)
• Chapter 5 procedures (extensions to the usual
  built in types defined by the programmer)
  encapsulation (a higher level abstraction or
  representation) actually defining the syntax and
  operations on this construct!

4
Type Checking

• Computer hardware data storage representations
  usually include no type information
• 1110000101010.111
• Can you tell if this is an integer, real,
  sequence of chars, an instruction?
• Common error in assembly languages is to invoke
  an operation such as integer addition on
  arguments of the wrong type

5
Type Checking

• These errors are difficult to locate because the
  operation works since the result (usually
  meaningless) is just a bit string.
• A major advantage of using a higher level
  language is that the language implementation can
  provide type checking.
• Type checking means that each operation executed
  by a program receives the proper number of
  arguments of the proper data type

6
Dynamic type checking

• At run time
• done immediately before operation
• usually implemented by storing a type tag in each
  data object
• each operation must attach the appropriate type
  tags to the results
• done in languages that do not require
  declarations
• dynamic types advantage is flexibility in design
  - programmer is freed from most concerns about
  data types

7
Disadvantages of Dynamic Type Checking

• Errors lurk in the program until they are
  executed (in large programs can be several hours
  of execution)
• not all program paths are executed
• type information can be substantial
• implemented in software and each operation must
  be checked before -gt speed of execution is likely
  to be greatly reduced

8
Static Type Checking

• Eliminate or minimize dynamic checking-gt perform
  type checking at compile time.
• Collect info in symbol table
• In most languages, static type checking is not
  possible for some constructs
• dynamic type checking
• leaving the operations unchecked

9
Typing

• Strong - all type errors can be detected
  statically (strongly typed)
• Text book definition - so only one language can
  fit in this category - ML
• Notice that when a language has either arrays,
  enumeration, subranges -gt it cannot be strong
• How do you know that A1number will be in the
  range of your array?
• or that Monday Sunday will still be part of the
  enumerated set?

10
Typing

• So the question is what will you give up to get a
  strongly typed language !(or to get a safe
  run-time program)
• Another question you may have How does a
  strongly typed language do any operations? Every
  overflow condition is defined in the language.
• There is much confusion on strong typing. You
  either catch ALL errors or you are weak. PERIOD.
  But I guess there are scales of weakness..
• So there is much confusion out in the PUBLIC.
  And there is a difference between strong type
  and structured type!

11
Typing

• what-is definition
• by what-is.com
• what is a Merry Strongly-Typed Christmas
• Garbage Collection for Strongly-Typed Languages
• typing and binding,
• page by Carl Erickson
• PERL
• for slow connections, I simply downloaded the
  page and made a local copy

12
Other PL sites

• Hotprog.com

13
Weak - Safe
All Programs
Weak
Safe
There are weak programs that are safe (type
errors are caught at run time)