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. 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 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 • C’s 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. 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 Reliability

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/flexibility