INSIDE PERL MOHAMMAD T. IRFAN 11/6/13 Origin Scripting language - - PDF document

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CSCI-2325: INSIDE PERL

MOHAMMAD T. IRFAN 11/6/13

Origin

• Scripting language based on sh and awk
• Larry Wall: “People often argue over the difference between a scripting

language and a programming language. If you ask me, I’ll tell you that a script is what you hand the actors, and a program is what you hand the audience.”

• It is very much an imperative programming language
• Always compiled before execution

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Variables

• Implicitly declared
• 3 namespaces for variables (with special start symbols for name)
• Scalar: $
• Array: @
• Hash: %
• Larry Wall’s explanation: It’s similar to natural languages
• Implicit variables
• Example: @_

Scalar Variables

• May contain both numbers and strings
• Depending on the context numbers are converted to strings and vice

versa

• Allows changing the value of a scalar variable to any type– number of

string

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Arrays

• Arrays are dynamic
• Can have “empty” cells, which are denoted as “undefined”
• Array indexing cannot be checked

Scoping

• Dynamic scoping: References to names are resolved dynamically
• Allows my keyword to avoid unexpected effects
• local keyword can be used for dynamic scoping

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Example: Dynamic Scoping

$x = 5; $y = 10; myFunction(); # call subroutine– no argument print "x = $x, y = $y\n"; sub myFunction { local($x, $y); #make it my ($x,$y) to swap global var $x = 100, $y = 200; swap(); # no argument print "After swap: x = $x, y = $y\n"; } sub swap { $t = $x; #Is this the x in myFunciton or the global x? $x = $y; $y = $t; }

Type System

• Review [Caution: there are different definitions of these terms]
• A language is statically typed, if the types of all variables are fixed when

they are declared at compile time

• A language is dynamically typed if the type of a variable can vary at run

time depending on the value assigned

• A language is strongly typed if its type system can detect all type errors

either at compile time or at run time

• “Weakly typed” is not well-defined – may not be related to strongly typed
• Examples
• C is statically typed, but not strongly typed (hint: pointer, array index)
• Java is both statically and strongly typed
• Perl is both dynamically and strongly typed

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Semantics

• Expression, assignment – similar to C
• Copy semantics
• Iterative statements
• A variety of for, while, and do-while statements
• Larry Wall’s philosophy: Often, there are many ways of expressing the same

idea using natural languages

• Conditional statements
• if-elsif-else statements, other types of if and unless statements
• Subroutine
• Parameters are passed implicitly: array @_
• Can return multiple items using an array
• Always returns the last computed value, even if there is no return statement
• Object-oriented features
• package
• Exception handling

Applications

• System administration
• Text processing
• Computational biology and AI

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BUILDING A LEXICAL ANALYZER FOR CLITE

Input File

void main() { int i; double jVar2; jVar2 = 10 + i + 50.99; }

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Desired Output

Type void main main ( ( ) ) { { Type int Identifier i ; ; Type double Identifier jVar2 ; ; Identifier jVar2 = = IntegerLiteral 10 + + Identifier i + + DoubleLiteral 50.99 ; ; } }

How to run perl with arguments?

• perl lexer.pl source.clite
• You may enter it in Padre: Run  Run Command

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lexer.pl: A first attempt

Open input and output files Read each line from the file Process that line

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• Subroutine to print a token-lexeme pair to the output file
• Used by the match() subroutine

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A MORE ELEGANT WAY

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