Advanced Programming Principles CM20214/CM20221

Macro languages

Purpose: to improve readability of other code, abstraction, textual manipulation

Examples: Cpp, LATEX, M4, macros in Lisp Notable features: usually lexical (character or text) based, with some exceptions (Lisp, Rust)

1 / 103 Macro languages Feet

• LATEX: \documentclass[12pt]{article} \usepackage{latexgun,latexshoot} \begin{document} See how easy it is to shoot yourself in the foot? \\ \gun[leftfoot]{shoot} \\ \pain \end{document}

2 / 103 Often used in conjunction with another language, e.g., the C preprocessor

So not often thought about in great detail, but are used to great effect

Particularly conditional macros whose expansion depends on other factors

Macro languages

These are used widely, in a huge variety of contexts

3 / 103 So not often thought about in great detail, but are used to great effect

Particularly conditional macros whose expansion depends on other factors

Macro languages

These are used widely, in a huge variety of contexts

Often used in conjunction with another language, e.g., the C preprocessor

4 / 103 Particularly conditional macros whose expansion depends on other factors

Macro languages

These are used widely, in a huge variety of contexts

Often used in conjunction with another language, e.g., the C preprocessor

So not often thought about in great detail, but are used to great effect

5 / 103 Macro languages

These are used widely, in a huge variety of contexts

Often used in conjunction with another language, e.g., the C preprocessor

So not often thought about in great detail, but are used to great effect

Particularly conditional macros whose expansion depends on other factors

6 / 103 Then if we use NUMBER everywhere in our code NUMBER x; ... it takes only a single change to make our code use short rather than int: very useful for source code portability between architectures

Macro languages

So, in C, we can write code like

#ifdef SMALLINT #define NUMBER short #else #define NUMBER int #endif

7 / 103 Macro languages

So, in C, we can write code like

#ifdef SMALLINT #define NUMBER short #else #define NUMBER int #endif

Then if we use NUMBER everywhere in our code NUMBER x; ... it takes only a single change to make our code use short rather than int: very useful for source code portability between architectures

8 / 103 C #define _ F-->00||-F-OO--; int F=00,OO=00;main(){F_OO();printf("%1.3f\n",4.*-F/OO/OO);}F_OO() { _-_-_-_ _-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_ _-_-_-_ }

An enthusiastic use of C macros by Brian Westley

9 / 103 And the macro language is Lisp itself, not a separate language: recall that programs are data! (defmacro head (l) (list ’car l))

defmacro deﬁnes the code to be executed (in the compiler or interpreter) when expanding the macro named head

When compiling (head x), the compiler ﬁrst expands the macro head by executing (list ’car l), where l has the value x, to get (car x)

The compiler then compiles (car x)

Macro languages

In contrast to most languages, Lisp macros are not text based, but expression based

10 / 103 (defmacro head (l) (list ’car l))

defmacro deﬁnes the code to be executed (in the compiler or interpreter) when expanding the macro named head

When compiling (head x), the compiler ﬁrst expands the macro head by executing (list ’car l), where l has the value x, to get (car x)

The compiler then compiles (car x)

Macro languages

In contrast to most languages, Lisp macros are not text based, but expression based

And the macro language is Lisp itself, not a separate language: recall that programs are data!

11 / 103 defmacro deﬁnes the code to be executed (in the compiler or interpreter) when expanding the macro named head

When compiling (head x), the compiler ﬁrst expands the macro head by executing (list ’car l), where l has the value x, to get (car x)

The compiler then compiles (car x)

Macro languages

In contrast to most languages, Lisp macros are not text based, but expression based

And the macro language is Lisp itself, not a separate language: recall that programs are data! (defmacro head (l) (list ’car l))

12 / 103 When compiling (head x), the compiler ﬁrst expands the macro head by executing (list ’car l), where l has the value x, to get (car x)

The compiler then compiles (car x)

Macro languages

In contrast to most languages, Lisp macros are not text based, but expression based

And the macro language is Lisp itself, not a separate language: recall that programs are data! (defmacro head (l) (list ’car l)) defmacro deﬁnes the code to be executed (in the compiler or interpreter) when expanding the macro named head

13 / 103 The compiler then compiles (car x)

Macro languages

In contrast to most languages, Lisp macros are not text based, but expression based

When compiling (head x), the compiler ﬁrst expands the macro head by executing (list ’car l), where l has the value x, to get (car x)

14 / 103 Macro languages

In contrast to most languages, Lisp macros are not text based, but expression based

When compiling (head x), the compiler ﬁrst expands the macro head by executing (list ’car l), where l has the value x, to get (car x)

The compiler then compiles (car x)

15 / 103 (defmacro baz (l m) (if (> m 0) (list ’car l) (list ’cdr l)))

Will expand baz at compile time to a car or a cdr

Macro languages

The full power of Lisp applies to macroexpansion

16 / 103 Macro languages

The full power of Lisp applies to macroexpansion

(defmacro baz (l m) (if (> m 0) (list ’car l) (list ’cdr l)))

Will expand baz at compile time to a car or a cdr

17 / 103 These slides are written in LATEX \begin{frame} \frametitle{Macro languages} \framesubtitle{\LaTeX}

Sometimes macros are used in their own right, e.g., \LaTeX, the typesetting language is macro based

\paruncover2{These slides are written in \LaTeX} \end{frame}

Macro languages LATEX

Sometimes macros are used in their own right, e.g., LATEX, the typesetting language is macro based

18 / 103 \begin{frame} \frametitle{Macro languages} \framesubtitle{\LaTeX}

Sometimes macros are used in their own right, e.g., \LaTeX, the typesetting language is macro based

\paruncover2{These slides are written in \LaTeX} \end{frame}

Macro languages LATEX

Sometimes macros are used in their own right, e.g., LATEX, the typesetting language is macro based

These slides are written in LATEX

19 / 103 Macro languages LATEX

Sometimes macros are used in their own right, e.g., LATEX, the typesetting language is macro based

These slides are written in LATEX \begin{frame} \frametitle{Macro languages} \framesubtitle{\LaTeX}

Sometimes macros are used in their own right, e.g., \LaTeX, the typesetting language is macro based

\paruncover2{These slides are written in \LaTeX} \end{frame}

20 / 103 For example, a new chapter needs a new page, a big heading, lots of space before the next text

So you deﬁne a macro, say \chapter, that does all this

Usually, someone else has created an entire macro package so you don’t have to

Macro languages LATEX

The basic datatype is text and you deﬁne macros as convenient shorthands for things you like to do to that text

21 / 103 So you deﬁne a macro, say \chapter, that does all this

Usually, someone else has created an entire macro package so you don’t have to

Macro languages LATEX

The basic datatype is text and you deﬁne macros as convenient shorthands for things you like to do to that text

For example, a new chapter needs a new page, a big heading, lots of space before the next text

22 / 103 Usually, someone else has created an entire macro package so you don’t have to

Macro languages LATEX

The basic datatype is text and you deﬁne macros as convenient shorthands for things you like to do to that text

For example, a new chapter needs a new page, a big heading, lots of space before the next text

So you deﬁne a macro, say \chapter, that does all this

23 / 103 Macro languages LATEX

The basic datatype is text and you deﬁne macros as convenient shorthands for things you like to do to that text

For example, a new chapter needs a new page, a big heading, lots of space before the next text

So you deﬁne a macro, say \chapter, that does all this

Usually, someone else has created an entire macro package so you don’t have to

24 / 103 They are called “scripts” as they are (were originally) lists of things to be done

Often, but not exclusively, interpreted rather than compiled

Scripting Languages

Purpose: control of other elements of a system, e.g., programs, “glue” to join elements together

Examples: DOS batch, sh, Python, Sed, Perl, Ruby, JavaScript . . .

Notable features: not particularly good at classical number crunching; generally lots of string processing and process manipulation

25 / 103 Often, but not exclusively, interpreted rather than compiled

Scripting Languages

Purpose: control of other elements of a system, e.g., programs, “glue” to join elements together

Examples: DOS batch, sh, Python, Sed, Perl, Ruby, JavaScript . . .

Notable features: not particularly good at classical number crunching; generally lots of string processing and process manipulation

They are called “scripts” as they are (were originally) lists of things to be done

26 / 103 Scripting Languages

Purpose: control of other elements of a system, e.g., programs, “glue” to join elements together

Examples: DOS batch, sh, Python, Sed, Perl, Ruby, JavaScript . . .

Notable features: not particularly good at classical number crunching; generally lots of string processing and process manipulation

They are called “scripts” as they are (were originally) lists of things to be done

Often, but not exclusively, interpreted rather than compiled

27 / 103 • Sh, csh, bash: You can’t remember the syntax for anything so you spend five hours reading man pages then your foot falls asleep. You then shoot the computer and switch to Perl • Perl: You shoot yourself in the foot, but nobody can understand how you did it. Six months later, neither can you • Ruby: Your foot is ready to be shot in roughly five minutes, but you just can’t find anywhere to shoot it • JavaScript: You’ve perfected a robust, rich user experience for shooting yourself in the foot. You then find that bullets are disabled on your gun

Scripting Languages Feet • DOS batch: You aim the gun at your foot and pull the trigger, but only a weak gust of warm air hits your foot

28 / 103 • Perl: You shoot yourself in the foot, but nobody can understand how you did it. Six months later, neither can you • Ruby: Your foot is ready to be shot in roughly five minutes, but you just can’t find anywhere to shoot it • JavaScript: You’ve perfected a robust, rich user experience for shooting yourself in the foot. You then find that bullets are disabled on your gun

29 / 103 • Ruby: Your foot is ready to be shot in roughly five minutes, but you just can’t find anywhere to shoot it • JavaScript: You’ve perfected a robust, rich user experience for shooting yourself in the foot. You then find that bullets are disabled on your gun

Scripting Languages Feet • DOS batch: You aim the gun at your foot and pull the trigger, but only a weak gust of warm air hits your foot • Sh, csh, bash: You can’t remember the syntax for anything so you spend ﬁve hours reading man pages then your foot falls asleep. You then shoot the computer and switch to Perl • Perl: You shoot yourself in the foot, but nobody can understand how you did it. Six months later, neither can you

30 / 103 • JavaScript: You’ve perfected a robust, rich user experience for shooting yourself in the foot. You then ﬁnd that bullets are disabled on your gun

31 / 103 Scripting Languages Feet • DOS batch: You aim the gun at your foot and pull the trigger, but only a weak gust of warm air hits your foot • Sh, csh, bash: You can’t remember the syntax for anything so you spend five hours reading man pages then your foot falls asleep. You then shoot the computer and switch to Perl • Perl: You shoot yourself in the foot, but nobody can understand how you did it. Six months later, neither can you • Ruby: Your foot is ready to be shot in roughly five minutes, but you just can’t find anywhere to shoot it • JavaScript: You’ve perfected a robust, rich user experience for shooting yourself in the foot. You then find that bullets are disabled on your gun

32 / 103 The original scripts were the job control languages for the early mainframes

For example, IBM’s JCL was used to describe a list of programs to be loaded and run: recall batch processing

Scripting Languages

Also widely used

33 / 103 For example, IBM’s JCL was used to describe a list of programs to be loaded and run: recall batch processing

Scripting Languages

Also widely used

The original scripts were the job control languages for the early mainframes

34 / 103 Scripting Languages

Also widely used

The original scripts were the job control languages for the early mainframes

For example, IBM’s JCL was used to describe a list of programs to be loaded and run: recall batch processing

35 / 103 Scripting Languages Feet

• JCL: You send your foot down to MIS with a 4000-page document explaining how you want it to be shot. Three years later, your foot comes back deep-fried

36 / 103 Simple textual lists of things (programs) to be done

#!/bin/sh setxkbmap -option "compose:menu" -option "ctrl:nocaps" dispwin -L [ "$XAUTHORITY" ] && cp -f "$XAUTHORITY" ~/.Xauthority

Scripting Languages Sh

The Unix command line language, originally sh (the Bourne Shell), lately bash (the Bourne Again shell)

37 / 103 #!/bin/sh setxkbmap -option "compose:menu" -option "ctrl:nocaps" dispwin -L [ "$XAUTHORITY" ] && cp -f "$XAUTHORITY" ~/.Xauthority

Scripting Languages Sh

The Unix command line language, originally sh (the Bourne Shell), lately bash (the Bourne Again shell)

Simple textual lists of things (programs) to be done

38 / 103 Scripting Languages Sh

The Unix command line language, originally sh (the Bourne Shell), lately bash (the Bourne Again shell)

Simple textual lists of things (programs) to be done

#!/bin/sh setxkbmap -option "compose:menu" -option "ctrl:nocaps" dispwin -L [ "$XAUTHORITY" ] && cp -f "$XAUTHORITY" ~/.Xauthority

39 / 103 Originally reasonably simple but they have grown more complex over time

Crucially, they do not require a GUI so can be deployed automatically over large numbers of machines

Somewhat low-level, so not so good for more complex tasks, or less complex programmers

Scripting Languages Sh

Shell scripts are widely used to automate repetitive and complex tasks

40 / 103 Crucially, they do not require a GUI so can be deployed automatically over large numbers of machines

Somewhat low-level, so not so good for more complex tasks, or less complex programmers

Scripting Languages Sh

Shell scripts are widely used to automate repetitive and complex tasks

Originally reasonably simple but they have grown more complex over time

41 / 103 Somewhat low-level, so not so good for more complex tasks, or less complex programmers

Scripting Languages Sh

Shell scripts are widely used to automate repetitive and complex tasks

Originally reasonably simple but they have grown more complex over time

Crucially, they do not require a GUI so can be deployed automatically over large numbers of machines

42 / 103 Scripting Languages Sh

Shell scripts are widely used to automate repetitive and complex tasks

Originally reasonably simple but they have grown more complex over time

Crucially, they do not require a GUI so can be deployed automatically over large numbers of machines

Somewhat low-level, so not so good for more complex tasks, or less complex programmers

43 / 103 String manipulation: sed is a stream editor

Reads ﬁles one line at a time (as a stream) and edits them, one line at a time, according to given rules sed -e ’s/hello/goodbye/’ Substitutes goodbye for occurrences of hello in its input

Scripting Languages Sed

Example: Sed. A very simple scripting language, capable of just one task

44 / 103 Reads ﬁles one line at a time (as a stream) and edits them, one line at a time, according to given rules sed -e ’s/hello/goodbye/’ Substitutes goodbye for occurrences of hello in its input

Scripting Languages Sed

Example: Sed. A very simple scripting language, capable of just one task

String manipulation: sed is a stream editor

45 / 103 sed -e ’s/hello/goodbye/’ Substitutes goodbye for occurrences of hello in its input

Scripting Languages Sed

Example: Sed. A very simple scripting language, capable of just one task

String manipulation: sed is a stream editor

Reads ﬁles one line at a time (as a stream) and edits them, one line at a time, according to given rules

46 / 103 Scripting Languages Sed

Example: Sed. A very simple scripting language, capable of just one task

String manipulation: sed is a stream editor

Reads ﬁles one line at a time (as a stream) and edits them, one line at a time, according to given rules sed -e ’s/hello/goodbye/’ Substitutes goodbye for occurrences of hello in its input

47 / 103 Can edit ﬁles too large to ﬁt into memory all at once

Usually used as a component in shell scripts

Limited to line-by-line editing

More general is awk, but better is Perl

Scripting Languages Sed

Uses regular expressions for patterns

48 / 103 Usually used as a component in shell scripts

Limited to line-by-line editing

More general is awk, but better is Perl

Scripting Languages Sed

Uses regular expressions for patterns

Can edit ﬁles too large to ﬁt into memory all at once

49 / 103 Limited to line-by-line editing

More general is awk, but better is Perl

Scripting Languages Sed

Uses regular expressions for patterns

Can edit ﬁles too large to ﬁt into memory all at once

Usually used as a component in shell scripts

50 / 103 More general is awk, but better is Perl

Scripting Languages Sed

Uses regular expressions for patterns

Can edit ﬁles too large to ﬁt into memory all at once

Usually used as a component in shell scripts

Limited to line-by-line editing

51 / 103 Scripting Languages Sed

Uses regular expressions for patterns

Can edit ﬁles too large to ﬁt into memory all at once

Usually used as a component in shell scripts

Limited to line-by-line editing

More general is awk, but better is Perl

52 / 103 The basic datatype is the string

The basic operation is pattern matching

It is also procedural, has ﬁrst class functions, has objects and so on

The syntax is the usual C/Java/whatever, but with a few features

Scripting Languages Perl

Perl is/was used a lot in Web page creation and manipulation, amongst many other things

53 / 103 The basic operation is pattern matching

It is also procedural, has ﬁrst class functions, has objects and so on

The syntax is the usual C/Java/whatever, but with a few features

Scripting Languages Perl

Perl is/was used a lot in Web page creation and manipulation, amongst many other things

The basic datatype is the string

54 / 103 It is also procedural, has ﬁrst class functions, has objects and so on

The syntax is the usual C/Java/whatever, but with a few features

Scripting Languages Perl

Perl is/was used a lot in Web page creation and manipulation, amongst many other things

The basic datatype is the string

The basic operation is pattern matching

55 / 103 The syntax is the usual C/Java/whatever, but with a few features

Scripting Languages Perl

Perl is/was used a lot in Web page creation and manipulation, amongst many other things

The basic datatype is the string

The basic operation is pattern matching

It is also procedural, has ﬁrst class functions, has objects and so on

56 / 103 Scripting Languages Perl

Perl is/was used a lot in Web page creation and manipulation, amongst many other things

The basic datatype is the string

The basic operation is pattern matching

It is also procedural, has ﬁrst class functions, has objects and so on

The syntax is the usual C/Java/whatever, but with a few features

57 / 103 Scripting Languages Perl open IN, ’<’, ’infile’; open OUT, ’>outfile’; $count = 0; while () { s/world/everybody/ if (/hello/); print OUT; $count++; } close IN; close OUT; print "$count lines\n";

58 / 103 • or joined onto the filename • IN: filestream variables are syntactically different from normal variables • $count: scalar (single value) variable names are prefixed by $ • array variable names are prefixed by @ • function variable names are prefixed by & • $f is separate from @f and &f • <>: operator returns a single line of the file each time it is called • and false at end of file • it assigns to the variable $

Scripting Languages Perl • open: takes strings ’<’ to indicate input and ’>’ to indicate output; then a ﬁle name

59 / 103 • IN: filestream variables are syntactically different from normal variables • $count: scalar (single value) variable names are prefixed by $ • array variable names are prefixed by @ • function variable names are prefixed by & • $f is separate from @f and &f • <>: operator returns a single line of the file each time it is called • and false at end of file • it assigns to the variable $

Scripting Languages Perl • open: takes strings ’<’ to indicate input and ’>’ to indicate output; then a ﬁle name • or joined onto the ﬁlename

60 / 103 • $count: scalar (single value) variable names are prefixed by $ • array variable names are prefixed by @ • function variable names are prefixed by & • $f is separate from @f and &f • <>: operator returns a single line of the file each time it is called • and false at end of file • it assigns to the variable $

61 / 103 • array variable names are prefixed by @ • function variable names are prefixed by & • $f is separate from @f and &f • <>: operator returns a single line of the file each time it is called • and false at end of file • it assigns to the variable $

62 / 103 • function variable names are prefixed by & • $f is separate from @f and &f • <>: operator returns a single line of the file each time it is called • and false at end of file • it assigns to the variable $

63 / 103 • $f is separate from @f and &f • <>: operator returns a single line of the ﬁle each time it is called • and false at end of ﬁle • it assigns to the variable $

Scripting Languages Perl • open: takes strings ’<’ to indicate input and ’>’ to indicate output; then a file name • or joined onto the filename • IN: filestream variables are syntactically different from normal variables • $count: scalar (single value) variable names are prefixed by $ • array variable names are prefixed by @ • function variable names are prefixed by &

64 / 103 • <>: operator returns a single line of the ﬁle each time it is called • and false at end of ﬁle • it assigns to the variable $

65 / 103 • and false at end of ﬁle • it assigns to the variable $

66 / 103 • it assigns to the variable $

67 / 103 Scripting Languages Perl • open: takes strings ’<’ to indicate input and ’>’ to indicate output; then a file name • or joined onto the filename • IN: filestream variables are syntactically different from normal variables • $count: scalar (single value) variable names are prefixed by $ • array variable names are prefixed by @ • function variable names are prefixed by & • $f is separate from @f and &f • <>: operator returns a single line of the file each time it is called • and false at end of file • it assigns to the variable $ 68 / 103 • statement if (test); as well as the usual if (test) { statements; } (and with else) • // for pattern matching; in this case it looks to see if the default $ contains the string hello • s///: if the string contains world, replace it by the string everybody (again, using the default $ ) • $count++; lots of C-like features • untyped variables: a variable can hold numbers and strings and other types; so ++ has first to check if the value is a number or a string containing a number, which it then converts to a number • flexibility over () around function arguments

Scripting Languages Perl • $ is a default argument and can be left out of many places, e.g., print; is equivalent to print $ ;

69 / 103 • // for pattern matching; in this case it looks to see if the default $ contains the string hello • s///: if the string contains world, replace it by the string everybody (again, using the default $ ) • $count++; lots of C-like features • untyped variables: a variable can hold numbers and strings and other types; so ++ has ﬁrst to check if the value is a number or a string containing a number, which it then converts to a number • ﬂexibility over () around function arguments

70 / 103 • s///: if the string contains world, replace it by the string everybody (again, using the default $ ) • $count++; lots of C-like features • untyped variables: a variable can hold numbers and strings and other types; so ++ has ﬁrst to check if the value is a number or a string containing a number, which it then converts to a number • ﬂexibility over () around function arguments

71 / 103 • $count++; lots of C-like features • untyped variables: a variable can hold numbers and strings and other types; so ++ has ﬁrst to check if the value is a number or a string containing a number, which it then converts to a number • ﬂexibility over () around function arguments

72 / 103 • untyped variables: a variable can hold numbers and strings and other types; so ++ has ﬁrst to check if the value is a number or a string containing a number, which it then converts to a number • ﬂexibility over () around function arguments

73 / 103 • ﬂexibility over () around function arguments

Scripting Languages Perl • $ is a default argument and can be left out of many places, e.g., print; is equivalent to print $ ; • statement if (test); as well as the usual if (test) { statements; } (and with else) • // for pattern matching; in this case it looks to see if the default $ contains the string hello • s///: if the string contains world, replace it by the string everybody (again, using the default $ ) • $count++; lots of C-like features • untyped variables: a variable can hold numbers and strings and other types; so ++ has ﬁrst to check if the value is a number or a string containing a number, which it then converts to a number

74 / 103 Scripting Languages Perl • $ is a default argument and can be left out of many places, e.g., print; is equivalent to print $ ; • statement if (test); as well as the usual if (test) { statements; } (and with else) • // for pattern matching; in this case it looks to see if the default $ contains the string hello • s///: if the string contains world, replace it by the string everybody (again, using the default $ ) • $count++; lots of C-like features • untyped variables: a variable can hold numbers and strings and other types; so ++ has ﬁrst to check if the value is a number or a string containing a number, which it then converts to a number • ﬂexibility over () around function arguments

75 / 103 • single quote is unevaluated: ’hello\n’ prints as hello\n • double quote is interpolated: "hello\n" prints as hello with a newline • "The count is $count" sticks the current value of $count into the string at that point

There are several very fat books on Perl

Its ﬂexibility means it is used in a lot of places

And it is easy to write unreadable code in Perl

Scripting Languages Perl

• strings are both single and double quoted

76 / 103 • double quote is interpolated: "hello\n" prints as hello with a newline • "The count is $count" sticks the current value of $count into the string at that point

There are several very fat books on Perl

Its ﬂexibility means it is used in a lot of places

And it is easy to write unreadable code in Perl

Scripting Languages Perl

• strings are both single and double quoted • single quote is unevaluated: ’hello\n’ prints as hello\n

77 / 103 • "The count is $count" sticks the current value of $count into the string at that point

There are several very fat books on Perl

Its ﬂexibility means it is used in a lot of places

And it is easy to write unreadable code in Perl

Scripting Languages Perl

• strings are both single and double quoted • single quote is unevaluated: ’hello\n’ prints as hello\n • double quote is interpolated: "hello\n" prints as hello with a newline

78 / 103 There are several very fat books on Perl

Its ﬂexibility means it is used in a lot of places

And it is easy to write unreadable code in Perl

Scripting Languages Perl

• strings are both single and double quoted • single quote is unevaluated: ’hello\n’ prints as hello\n • double quote is interpolated: "hello\n" prints as hello with a newline • "The count is $count" sticks the current value of $count into the string at that point

79 / 103 Its ﬂexibility means it is used in a lot of places

And it is easy to write unreadable code in Perl

Scripting Languages Perl

There are several very fat books on Perl

80 / 103 And it is easy to write unreadable code in Perl

Scripting Languages Perl

There are several very fat books on Perl

Its ﬂexibility means it is used in a lot of places

81 / 103 Scripting Languages Perl

There are several very fat books on Perl

Its ﬂexibility means it is used in a lot of places

And it is easy to write unreadable code in Perl

82 / 103 Originally designed to be a scripting language to manage Web page content, it is now more likely to be thought of as a special-purpose language to enable dynamic Web content

Web applications like Google Maps are coded using JavaScript

In Firefox, the browser’s appearance, i.e., the buttons, toolbars and so on (the “chrome”), are programmed in JavaScript

This allows easy modiﬁcation and extension (using “add-ons”)

Scripting Languages JavaScript

JavaScript is an interesting case

83 / 103 Web applications like Google Maps are coded using JavaScript

In Firefox, the browser’s appearance, i.e., the buttons, toolbars and so on (the “chrome”), are programmed in JavaScript

This allows easy modiﬁcation and extension (using “add-ons”)

Scripting Languages JavaScript

JavaScript is an interesting case

Originally designed to be a scripting language to manage Web page content, it is now more likely to be thought of as a special-purpose language to enable dynamic Web content

84 / 103 In Firefox, the browser’s appearance, i.e., the buttons, toolbars and so on (the “chrome”), are programmed in JavaScript

This allows easy modiﬁcation and extension (using “add-ons”)

Scripting Languages JavaScript

JavaScript is an interesting case

Originally designed to be a scripting language to manage Web page content, it is now more likely to be thought of as a special-purpose language to enable dynamic Web content

Web applications like Google Maps are coded using JavaScript

85 / 103 This allows easy modiﬁcation and extension (using “add-ons”)

Scripting Languages JavaScript

JavaScript is an interesting case

Originally designed to be a scripting language to manage Web page content, it is now more likely to be thought of as a special-purpose language to enable dynamic Web content

Web applications like Google Maps are coded using JavaScript

In Firefox, the browser’s appearance, i.e., the buttons, toolbars and so on (the “chrome”), are programmed in JavaScript

86 / 103 Scripting Languages JavaScript

JavaScript is an interesting case

Originally designed to be a scripting language to manage Web page content, it is now more likely to be thought of as a special-purpose language to enable dynamic Web content

Web applications like Google Maps are coded using JavaScript

In Firefox, the browser’s appearance, i.e., the buttons, toolbars and so on (the “chrome”), are programmed in JavaScript

This allows easy modiﬁcation and extension (using “add-ons”)

87 / 103 The name is unfortunate: their syntax is broadly similar but their semantics are wildly different

The name was originally chosen as JavaScript was intended to be “reminiscent” of Java, but now it’s just a source of confusion

It can be argued that JavaScript is more like Scheme/Lisp than Java in the way it behaves (ﬁrst-class functions, etc.)

But you shouldn’t take this analogy too far

Scripting Languages JavaScript

Standard warning:

Java and JavaScript are different languages

88 / 103 The name was originally chosen as JavaScript was intended to be “reminiscent” of Java, but now it’s just a source of confusion

It can be argued that JavaScript is more like Scheme/Lisp than Java in the way it behaves (ﬁrst-class functions, etc.)

But you shouldn’t take this analogy too far

Scripting Languages JavaScript

Standard warning:

Java and JavaScript are different languages

The name is unfortunate: their syntax is broadly similar but their semantics are wildly different

89 / 103 It can be argued that JavaScript is more like Scheme/Lisp than Java in the way it behaves (ﬁrst-class functions, etc.)

But you shouldn’t take this analogy too far

Scripting Languages JavaScript

Standard warning:

Java and JavaScript are different languages

The name is unfortunate: their syntax is broadly similar but their semantics are wildly different

The name was originally chosen as JavaScript was intended to be “reminiscent” of Java, but now it’s just a source of confusion

90 / 103 But you shouldn’t take this analogy too far

Scripting Languages JavaScript

Standard warning:

Java and JavaScript are different languages

The name is unfortunate: their syntax is broadly similar but their semantics are wildly different

The name was originally chosen as JavaScript was intended to be “reminiscent” of Java, but now it’s just a source of confusion

It can be argued that JavaScript is more like Scheme/Lisp than Java in the way it behaves (ﬁrst-class functions, etc.)

91 / 103 Scripting Languages JavaScript

Standard warning:

Java and JavaScript are different languages

The name is unfortunate: their syntax is broadly similar but their semantics are wildly different

The name was originally chosen as JavaScript was intended to be “reminiscent” of Java, but now it’s just a source of confusion

It can be argued that JavaScript is more like Scheme/Lisp than Java in the way it behaves (ﬁrst-class functions, etc.)

But you shouldn’t take this analogy too far

92 / 103 • JavaScript: Sun/Mozilla/etc. • JScript: Microsoft • ActionScript: Adobe

Ecma: formerly European Computer Manufacturers Association, now just “Ecma International”, is a standards body

Scripting Languages JavaScript

JavaScript is more properly called ECMAScript and there are several close-but-not-perfectly-compatible versions around

93 / 103 Ecma: formerly European Computer Manufacturers Association, now just “Ecma International”, is a standards body

Scripting Languages JavaScript

JavaScript is more properly called ECMAScript and there are several close-but-not-perfectly-compatible versions around

• JavaScript: Sun/Mozilla/etc. • JScript: Microsoft • ActionScript: Adobe

94 / 103 Scripting Languages JavaScript

JavaScript is more properly called ECMAScript and there are several close-but-not-perfectly-compatible versions around

• JavaScript: Sun/Mozilla/etc. • JScript: Microsoft • ActionScript: Adobe

Ecma: formerly European Computer Manufacturers Association, now just “Ecma International”, is a standards body

95 / 103 Scripting Languages JavaScript function getdoc() { var input = document.getElementById("num"); var num = input.value;

if (parseInt(num) == num && num > 0 && num < 5000) { document.location = "http://www.rfc-editor.org/rfc/rfc" + num + ".txt"; } else { alert("Not a valid RFC number!"); input.value = ""; } return false; }

96 / 103 • variables declared by var, no type declarations • a variable can hold items of any type • syntax reminiscent of Java (and C and so on)

Scripting Languages JavaScript

• functions declared by function, no type declarations

97 / 103 • a variable can hold items of any type • syntax reminiscent of Java (and C and so on)

Scripting Languages JavaScript

• functions declared by function, no type declarations • variables declared by var, no type declarations

98 / 103 • syntax reminiscent of Java (and C and so on)

Scripting Languages JavaScript

• functions declared by function, no type declarations • variables declared by var, no type declarations • a variable can hold items of any type

99 / 103 Scripting Languages JavaScript

• functions declared by function, no type declarations • variables declared by var, no type declarations • a variable can hold items of any type • syntax reminiscent of Java (and C and so on)

100 / 103 It is OO, but in a very different way from other languages, particularly Java

We shall be talking more about JavaScript when we examine OO in detail

Scripting Languages JavaScript

JavaScript is easy to learn and use, good for prototyping

101 / 103 We shall be talking more about JavaScript when we examine OO in detail

Scripting Languages JavaScript

JavaScript is easy to learn and use, good for prototyping

It is OO, but in a very different way from other languages, particularly Java

102 / 103 Scripting Languages JavaScript

JavaScript is easy to learn and use, good for prototyping

It is OO, but in a very different way from other languages, particularly Java

We shall be talking more about JavaScript when we examine OO in detail

103 / 103