Scripting Languages LECTURE 1 INTRODUCTION

1. Scripting LanguagesLECTURE 1INTRODUCTION • Georges Khazen

2. by John K. Ousterhout

3. Introduction • Scripting languages are designed for different tasks than are system programming languages like C, C++ or Java • System programming languages were designed for building data structures and algorithms from scratch. • Scripting languages were designed for gluing • They use existing powerful components and connect them together.

4. Introduction • - Scripting languages and system programming languages are complimentary. • - Most major computer platforms since the 1960s include both kinds of languages • - Several recent trends, such as faster machines, graphical user interfaces (GUI) and the growth of the Internet, have greatly expanded the applicability of scripting languages.

5. System programming Languages • They were introduced as an alternative to assembly languages (low level programming, register allocation and procedure-calling sequence) • By the late 1950s, higer-level languages such as List, Fortran and Algol began to appear. • No more need for machine instructions • Compilers are needed to translate each statement in the source program into a sequence of binary instructions • Over the time, other system programming languages evolved including PL/1, Pascal, C, C++ and Java • Quicker than assembly languages for developing applications but less efficient

6. System programming LanguagesHigher Level • System programming languages differ from assembly languages in two ways: they are higher level and they are strongly typed. • • Register allocation is handled by the compiler • • Procedure calling sequences are generated automatically; • • Programmers can use simple keywords such as while and if for control structures; the compiler generates all the detailed instructions to implement the control structures. • On average, each line of code in a system programming language translates to about five machine instructions.

7. Typing • Typing refers to the degree to which the meaning of information is specified in advance of its use. • In a strongly typed language, the programmer declares how each piece of information will be used, and the language prevents the information from being used in any other way. • In a weakly typed language, there are no a priori restrictions on how information can be used; the meaning of information is determined solely by the way it is used, not by any initial promises.

8. Typing • Today’s system programming languages are strongly typed. For example: • • Each variable in a system programming language must be declared with a particular type such as integer or pointer to string, and it must be used in ways that are appropriate for the type. • • Data and code are segregated; it is difficult if not impossible to create new code on the fly. • • Variables can be collected into structures or objects with well-defined substructure and procedures or methods to manipulate them. • An object of one type cannot be used where an object • of a different type is expected.

9. TYPING • Typing has several advantages • It makes large programs more manageable by clarifying how things are used and differentiating among things that must be treated differently. • Compilers use type information to detect kinds of errors, such as an attempt to use a floating-point value as a pointer. • Typing improves performance by allowing compilers to generate specialized code. For example, if a compiler knows that a variable always holds an integer value, then it can generate integer instructions to manipulate the variable; • if the compiler does not know the type of a variable, then it must generate additional instructions to check the variable’s type at runtime.

10. Degree of Typing • - System programming are strongly typed to help manage complexity • - Scripting languages are typed less to simplify connections among components and provide rapid application development.

11. Scripting Languages • Generalizations about Scripting Languages • Interpreted (some compiled internally) • Rapid Development — no compilation required • Portability — cross platform (since not compiling — no machine code) • Slower than compiled languages • Frequently no “main” • Good for controlling and connecting other applications • Flexibility • Easy to extend • Vast libraries and extensions available

12. Introduction • Popular Applications of Scripting Languages • System Administration • System Automation • Text Manipulation • Web Development • Client-side • Server-side • Graphical User Interfaces • System Prototyping • Hacking & Cracking • Merging • Interfacing Incompatible Programs • Connecting Programs

13. Other Observations • Used for gluing together existing components • Higher level than other languages • Abstract operations are built-in — regular expressions, string operations are part of language level constructs • Tcl 1-liner example in paper same work as 25 lines using Microsoft Foundation Classes • More work per line of code • Weaker typing means variables can hold whatever, need not explicitly declare them of a given type • Interpreted — run inside another application (the interpreter) • Trade-off between slower execution for faster development

14. Different Tools for Different Tasks • Both types of languages have their strengths and weaknesses. Each type tends to lend itself better for some tasks. • You should consider choosing a scripting programming language when: • The application's main task is to connect preexisting components • The application will manipulate a variety of different things • The application requires a GUI • The application will do a lot of string manipulation • The functions of the application evolve rapidly over time • The application needs to be easily extensible • You should consider choosing a system programming language when: • The application implements complex algorithms or data structures • The speed of the application is absolutely critical • The functions of the application are well defined and change slowly

15. Object Oriented Programming • Object Oriented Programming (OOP) is commonly held as a significant step in the evolution of programming languages. • OOP features such as strong typing and inheritance are often claimed to produce dramatic reductions in development time. • Strong typing encourages narrowly defined interfaces, which do not lend themselves for reuse • Inheritance poses a number of challenges — ties implementation of inherited classes together, adds complexity, and then there is multiple inheritance • OOP does provide some good features: • Data Encapsulation — combine data and code in a manner that hides implementation • Interface Inheritance — same API, multiple implementations (i.e. Java Interface) • These same OOP benefits have been applied to scripting languages: • Perl (historically procedural) adopted OOP along side its standard procedural style • Python is a semi-object oriented scripting language • Ruby treats everything as an object (including literals and so called “primitives”)

16. Scripting Languages vs. Systems Languages • Scripting LanguagesSystems Languages • Higher Level Lower Level • Gluing Components Building Components • Loosely Typed Strongly Typed • Interpreted Compiled • Runs Slower Runs Faster • Faster Development Slower Development • Dynamic Code Static Code • Smaller Code Size Larger Code Size

17. Examples (FiBONACCI) • C C++ • Java

18. Examples (FiBONACCI) • Perl Python Ruby

19. Text EDitors • Problems with Normal Text Editors • - They include formatting and layout information in addition to the text • - This might corrupt your files by filling them with un-manipulating text data and or scripts. • Some Recommended Text Editors • - Emacs, vi, vm for Linux • - Emacs, TextWrangler BBEdit, TextMate for MAC • - Emacs, TextWrangler, Notepad++ for Windows

20. Scripting Languages Covered • Shell Scripting • Python • Perl • Ruby • PHP and Javascript (not in depth)