Understanding Scope and Namespaces in Modern Programming Languages, Slides of Advanced Computer Programming

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Scope

Chapter Ten Modern Programming Languages, 2nd ed. (^) Docsity.com 1

Reusing Names

 Scope is trivial if you have a unique name

for everything:

 But in modern languages, we often use the

same name over and over:

 How can this work?

Chapter Ten Modern Programming Languages, 2nd ed. 2

fun square n = n * n; fun double n = n + n;

fun square a = a * a; fun double b = b + b;

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Definitions

 When there are different variables with the

same name, there are different possible

bindings for that name

 Not just variables: type names, constant

names, function names, etc.

 A definition is anything that establishes a

possible binding for a name

Chapter Ten Modern Programming Languages, 2nd ed. (^) Docsity.com 4

Examples

Chapter Ten Modern Programming Languages, 2nd ed. 5

fun square n = n * n; fun square square = square * square;

const Low = 1; High = 10; type Ints = array [Low..High] of Integer; var X: Ints;

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Examples

 Each occurrence must be bound using one

of the definitions

 Which one?

 There are many different ways to solve this

scoping problem

Chapter Ten Modern Programming Languages, 2nd ed. 7

- fun square square = square * square; val square = fn : int -> int - square 3; val it = 9 : int

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Outline

 Definitions and scope

 Scoping with blocks

 Scoping with labeled namespaces

 Scoping with primitive namespaces

 Dynamic scoping

 Separate compilation

Chapter Ten Modern Programming Languages, 2nd ed. (^) Docsity.com 8

Different ML Blocks

 The let is just a block: no other purpose

 A fun definition includes a block:

 Multiple alternatives have multiple blocks:

 Each rule in a match is a block:

Chapter Ten Modern Programming Languages, 2nd ed. 10

fun cube x = xxx;**

fun f (a::b::_) = a+b | f [a] = a | f [] = 0;

case x of (a,0) => a | (_,b) => b

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Java Blocks

 In Java and other C-like languages, you can

combine statements into one compound

statement using { and }

 A compound statement also serves as a

block:

Chapter Ten Modern Programming Languages, 2nd ed. 11

while (i < 0) { int c = iii; p += c; q += c; i -= step; }** Docsity.com

Classic Block Scope Rule

 The scope of a definition is the block

containing that definition, from the point of

definition to the end of the block, minus the

scopes of any redefinitions of the same

name in interior blocks

 That is ML’s rule; most statically scoped,

block-structured languages use this or some

minor variation

Chapter Ten Modern Programming Languages, 2nd ed. (^) Docsity.com 13

Example

Chapter Ten Modern Programming Languages, 2nd ed. 14

let val n = 1 in let val n = 2 in n end end

Scope of this definition is A-B

Scope of this definition is B

A

B

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Labeled Namespaces

 A labeled namespace is any language

construct that contains definitions and a

region of the program where those

definitions apply, and also has a name that

can be used to access those definitions from

outside the construct

 ML has one called a structure…

Chapter Ten Modern Programming Languages, 2nd ed. (^) Docsity.com 16

ML Structures

 A little like a block: a can be used

anywhere from definition to the end

 But the definitions are also available

outside, using the structure name: Fred.a

and Fred.f

Chapter Ten Modern Programming Languages, 2nd ed. 17

structure Fred = struct val a = 1; fun f x = x + a; end;

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Example

 The variables min and max would be

visible within the rest of the class

 Also accessible from outside, as

Month.min and Month.max

 Classes serve a different purpose too

Chapter Ten Modern Programming Languages, 2nd ed. 19

public class Month { public static int min = 1; public static int max = 12; … }

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Namespace Advantages

 Two conflicting goals:

• Use memorable, simple names like max
• For globally accessible things, use uncommon

names like maxSupplierBid , names that

will not conflict with other parts of the program

 With namespaces, you can accomplish both:

• Within the namespace, you can use max
• From outside, SupplierBid.max

Chapter Ten Modern Programming Languages, 2nd ed. (^) Docsity.com 20