TDDC74 Programmering:
Abstraktion och modellering
VT 2015
Johannes Schmidt
Institutionen för datavetenskap
Linköpings universitet
Lecture 7
State and environment model
Part 2
Definition: a binding

A binding refers to association of a value
to a name, for example,
x:10, the value of x is 10
f:<procedure>, the value of f is a procedure
Definition: Frame
A frame is simply a set of bindings
 The set could be empty
 Graphical representation:

x: 10
y: 20
Definition: Environment
An environment is a finite ordered set (or
sequence) of frames
 Graphical representation:

E1
x: 42
y: #f
a: 33
y: 20
x: 10
y: 20
E4
x: 30
z: 20
a: 10
b: 1729
E5
E2
E3
E6
w: 127
y: 20
Definition: The Global Environment
The so called global environment (GE) is
distinguished from other environments
and contains the global state.
 GE contains only one frame; this frame is
the final frame of all other environemts as
well
 Graphical representation:

GE
Lots of Environments
GE
pi: 3.1415
E1
x: 42
y: #f
a: 33
y: 20
x: 10
y: 20
E4
x: 30
z: 20
a: 10
b: 1729
E5
E2
E3
E6
w: 127
y: 20
Definition: Procedure
A procedure is an object that contains
two parts:
- one part contains the parameters and
the body of the procedure and
- another part contains the environment
in which the procedure was created.
 Graphical representation:

Env
Para:
Body:
Definition:
The Current Environment
The current environment (CE) indicates the
environment in which we are currently
computing.
 Graphical representation:

CE
The Current Environment
GE
pi: 3.1415
E1
x: 42
y: #f
a: 33
y: 20
x: 10
y: 20
E4
x: 30
z: 20
a: 10
b: 1729
E5
E2
E3
CE
E6
w: 127
y: 20
The Creation of
Environment Diagrams
Initially, we have
GE
CE
define, lambda, set! and procedure
application change the diagram
define

(define <name> <value>)
creates a new binding in the current (or
first) frame of the current environment
The Creation of
Environment Diagrams: define
(define y 69)
GE
pi: 3.1415
a: 33
y: 20
x: 10
y: 20
E4
x: 30
y:69
z: 20
a: 10
b: 1729
E5
E2
E3
CE
E6
w: 127
y: 20
lambda

(lambda (par1 par2 ...)
body)
creates a procedure object in the current
environment
The Creation of
Environment Diagrams: lambda
GE
pi: 3.1415
CE
Para:
Body:
a: 33
y: 20
x: 10
y: 20
E4
x: 30
z: 20
a: 10
b: 1729
E5
E2
E3
E6
w: 127
y: 20
set!

(set! <name> <new-value>)
changes the old value of <name> in the
current environment to <new-value>. The
most recent binding in the current
environment is affected.
The Manipulation of
Environment Diagrams: set!
(set! y 69)
GE
pi: 3.1415
Para:
Body:
a: 33
y: 20 69
x: 10
y: 20
E4
x: 30
z: 20
a: 10
b: 1729
E5
E2
E3
CE
E6
w: 127
y: 20
The Manipulation of
Environment Diagrams: set!
(set! y 69)
GE
pi: 3.1415
Para:
Body:
a: 33
y: 20
x: 10
y: 20
E4
x: 30
z: 20
a: 10
b: 1729
E5
w: 127
y: 20 69
CE
E2
E3
E6
Procedure Application

When applying a procedure object to
arguments,
- create a new environment by creating a
new frame in which we bind the
parameters to the given arguments
- connect this frame to the environment of
the procedure and let CE point to this new
environment
- evaluate the body of the procedure in CE
Evaluation of let
Translate let to its lambda-equivalent
and evaluate as usual
 let*, letrec and others are treated
similarly

Programming Environments
The concept of programming environments
should not be confused with the notion of
environment we use here.
A Programming Environment usually referes to a
user-friendly interactive system that enables a
programmer to write, edit, execute, analyse, test,
debug and simulate programs
DrRacket is an example of a programming
environment
Programming Paradigms
Programmeringsparadigmer
Functional
 Imperative
 Object Oriented
 Equation-based

We have concentrated on functional
programming so far. We will now see
more and more examples of Imperative
and object-oriented style.