Why study Programming Languages?

1
Why study Programming Languages?

• Increase your capacity to express ideas (both
  programming and in general)
• Increase your ability to select a PL for a
  specific use
• Increase your ability to learn new languages
• Provide you with a better understanding of
  implementation issues in programming
• Increase your ability to design a new PL
• Provide you with a better understanding of
  computational theory

2
Language Evaluation Criteria

• We will evaluate languages based on 5 criteria
• Readability
• Writability
• Reliability
• Cost
• Tradeoffs

3
Readability

• Simplicity of instructions
• Simpler is better (consider how many ways you can
  incr in C/Java)
• Orthogonality of instructions
• Do we treat operations differently for different
  types?
• for instance, do we add 2 floats differently from
  2 ints
• Operator and method/function overloading keeps
  instructions orthogonal
• too much orthogonality can cause problems though,
  in Smalltalk, everything is an object, even a
  number
• Control statements
• Early languages had insufficient control
  structures leading to extensive use of GO TO
  statements
• most modern languages have adequate control
  statements so that GO TO statements are not
  needed and in many cases, not part of the
  language
• what about statements like break and continue?
• Data Types and Structures
• Enumerable types, boolean type, add to readability

4
More on Readability

• Another issue that impacts readability is of
  syntactic expression
• identifier rules
• early languages restricted length of names
• modern languages may differentiate between upper
  and lower case letters
• special words
• those words that make up the language are known
  as special (or reserved) words, the choice of
  words can make a language more readable
• explicit end statements (endif, endfor, enddo)?
• form and meaning
• are words reserved or based on context?
• in C, the usage of static is based on context
• in FORTRAN, INTEGER and REAL can be types and
  names
• INTEGER REAL // REAL is an INTEGER
• REAL INTEGER // INTEGER is a REAL

5
Writability

• Simplicity and Orthogonality
• simplicity has the same meaning as with
  readability, but the opposite effect
• the more complex an instruction, the better for
  the programmer
• Cs for loop is very flexible good for writing,
  not so good for reading
• we still want orthogonal languages
• if you have to learn different ways to accomplish
  some task in different contexts, it becomes more
  difficult to learn the language
• Support for Abstraction (two types)
• data abstraction (for data structures)
• procedural abstraction (for modularity)
• Expressitivity
• does the language offer convenient ways to
  express ideas
• are there sufficient control structures?
• is the definition for a data type difficult or
  not available? etc.

6
Reliability

• How reliable can a program be written in the
  given language?
• Type Checking
• does the language perform compile-time type
  checking?
• how thoroughly are variables checked?
• a strongly typed language will find all type
  mismatches at compile time
• but few languages are strongly typed
• subprogram parameter and global variable type
  checking is difficult
• Exception Handling
• pioneered in PL/I, not available in most
  languages until recently (C, Java)
• Aliasing
• two or more variables reference the same named
  item
• most commonly associated with pointers and
  parameters
• changing the item through one of the aliased
  names may be an unnoticed or undesired side
  effect
• most languages do nothing to prevent aliases
• Readability and Writability
• the more readable the language, the more reliable
  it will be
• the less writable the language, the greater the
  chance of hard-to-find errors

7
Cost and Other Factors

• Cost is primarily a function of time
• Training/Learning Curve
• readability positively influences this,
  writability requires more training
• Time to write programs (writability positively
  influences this)
• Compilation Speed/Efficiency (affects programmer
  time)
• Execution Speed/Efficiency
• execution speed is desired because this what the
  user sees
• most languages will sacrifice compilation speed
  for execution speed
• Maintenance
• a language that is more readable and reliable is
  easier to maintain
• Other factors include portability, generality,
  well-definedness and standardization and legacy
  issues
• Language selection can often be a tradeoff
  between
• readability and flexibility/writability (more
  flexible makes it less readable)
• cost and expressiveness
• type checking/reliability and abstraction/flexibil
  ity