Principles of Programming Languages

1
Principles of ProgrammingLanguages

• Lecture 10
• Variables and Assignment

2
Variable ? Variable (Math) (CS)

• Math

• CS

? symbol
definite singular description
real
? referent
? symbol
indefinite singular description
ref real
? referent
real

• e.g. Prolog FP (IFP)

• e.g. Pascal Algol 68

3
Constants

• Algol 68 real pi 3.1416
• C (not quite) define PI 3.1416
• Pascal const pi 3.1416
• Ada pi constant FLOAT 3.1416

3.1416
same logical status as a literal
real
real
3.1416

• No reference available
• No ref real object exists
• pi replaced by literal
• No stack (or any other) space allocated (stored
  in instruction)
• Pure definition

4
Variables

• Algol 68 begin real x,y
• C float x,y
• Pascal var x,y real
• Ada x,y FLOAT

ref real
ref real
alloc on stack
real
real
x 2.0 y 3.0
3.0
2.0
5
Assignment (after y gets 3)

• Algol 68 x y
• C x y
• Bliss x ? .y

x
y
ref real
ref real
real
3.0
real
3.0
6
Variables Bliss v. C
Bliss
C

• Explicit dereferencing

• Context dereferencing

x .y .z a .j ?.(a .i)
x y z aj ai Synonym (aj) (ai)
rvalue
lvalue
const

Only lvalue is actually computed
7
Algol 68 aside identity declaration

• real x c NOT same as real x c
• Identity declaration real x 2.718 means
• Initialized variable real x 2.718 means
• Why? Implicit local (stack) allocation

x
2.718
x
2.718
8
identity declaration (contd)

• real x abbreviates loc real x which abbreviates
  the following identity declaration
• ref real x loc real

x
Local generator Yields a ref real
ref real (lvalue) Ascribed to identifier x
9
Initialized Variables

• Algol 68 begin real p 3.1416
• C float p 3.1416
• Pascal var p real begin p 3.1416
• Ada p FLOAT 3.1416
• What is variable about a variable?
• Not reference (location, lvalue)
• Ref cannot be changed ascribed to or
  possessed by ident
• Not identifier
• Not type (in most languages)
• The value (rvalue)

p
ref real
real
3.1416
stack
10
Algol 68 identity declaration unifies variable
constant declaration
m

• loc real m 3
• real x m
• ref real x m

m
3
m
x
3
11
Algol 68 identity declaration (contd)

• real x m ? loc real xm ? ref real x loc
  real m
• real t xm

ref real
ref real
real
real
3
3

• t const. In new environment
• xm evaluated when declaration encountered

9
12
Pointers

• Refs to variable refs
• Algol 68 ref real px
• C float px
• Pascal var px real
• Ada type PTR is access FLOAT
• px PTR (PTRaccess type to
  base type)

px
ref ref real
ref real
13
Algol 68 pointer declaration

• ref real px ?
• ref ref real px loc ref real

ref ref real
ref real
stack
14
Pointers (contd)

• Algol 68 px x
• C px x
• Bliss px ? x
• NOT Pascal, Ada picture. Closest is
• Pascal new(px) px x Ada px
  new FLOAT(x)

ref ref real
ref real
ref real
real
2.0
anonymous variable
15
Pointers in Pascal/Ada point into heap

• Defeats dangling pointers at block exit
• Pointers cannot point into the stack
• var x real var px, py real

py
x
heap
stack
2.0
16
Pascal/Ada pointers (contd)

• new(px) px x

py
x
heap
stack
2.0
2.0
anonymous variable px Pascal/Ada pointers
point only to unnamed data objects
17
Pascal/Ada pointers (contd)

• x 4.0
• new(py) py px

1.
py
x
heap
stack
4.0
18
Pascal/Ada pointers (contd)

• x 4.0
• py px

2.
py
x
heap
stack
4.0
pointer value out of legal range
19
Pascal/Ada pointers (contd)

• x 4.0
• new(py) py px

3.
py
x
heap
stack
2.0
2.0
4.0
20
Algol68 has a heap allocator

• ref ref real px heap real
• ref real hx heap real
• Abbreviated heap real hx
• Pascal lvalues of all var identifiers are in
  stack
• lvalues of all anonymous variables are in the
  heap
• Algol68 no restriction
• allocation orthogonal to declaration

hx
heap
stack
21
Refs and Aliasing

• Algol68 ref real py py px

py
stack
px
py
1102
1101
pointer aliasing
2.0
1100
1100
2.0
1100
22
Refs and Aliasing (contd)

• Algol 68 ref real y x (identity
  declaration)

x
y
ref real
ref real
no alloc!
real
2.0
name aliasing equivalencing (Fortran)
1100
23
Bliss untyped bit strings

px
x

• local x x ? 2 local px
• px ? x

px
x
ptr to x stored in px
px
x
picture after assignment
.
24
Semantics of Variable Uses C v. Bliss
x
x

• On RHS

• On LHS

/ ?
x
x

C
x
Meaning of x context-sensitive Not
referentially transparent
x
x
x
x
?
Bliss
.x
.x
Meaning of x is referentially transparent
25
Variable Uses C v. Bliss (contd)

• C not a unary operator
• (xy) meaningless
• x meaningless (cant reverse a pointer)
• 2 meaningless
• Bliss . is a unary operator
• .(xy) sensible
• ..x sensible
• .2 sensible

2 0010 .2 0010 ..2 1101
1101
1000
26
C ? Bliss.
C
x
x
Bliss

• On RHS

x
lvalue of x
x
.x
rvalue of x
x
Bliss variable occurrences are referentially
transparent Semantics of x is independent of
context