Algol

Report
CS 415: Programming
Languages
Algol
Aaron Bloomfield
Fall 2005
Historical perspective
By mid/late 50’s a lot of PLs were out there
Interest in universal language
European and American groups got together in
Zurich


Result was Algol 58
8 people spent 8 days working on the language
Algol goals
To be as close as possible to standard math
notation

Also very readable without much more explanation
Should be possible to use it to describe
algorithms in publications

A form of it is still used today
Should be mechanically translatable
machine language programs
into
3 language versions
Reference language

Used by the committee, described in the report, and
used in official Algol publications
Publication language


Allowed for differences in the character set for
different languages
Europeans and Americans couldn’t decide on which
character to use for the decimal point!
Hardware representations

Condensed languages for machine input
More history
Was first to use BNF (Backus-Naur Form)

Same Backus that created Fortran
He also was one of the main creators of Algol
And he created functional programming
And won the Turing award in ’77


Right. Back to BNF
BNF example:
<value> := <number> | <variable> | <expression>
<number> := <integer> | <float>
<integer> := <integer><digit> | <digit>
<digit> := 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
Language was designed by committee
Report is a “paradigm of brevity and clarity”


Most languages today require 1000’s of pages
Brevity and clarity contributed to reputation as simple, elegant
language
Algol language features
Block structure
And scope of variables within those blocks
Explicit type declaration for variables
Scope rules for local variables
Dynamic lifetimes for variables
Nested if-then-else expressions and statements
Call by value and call by name
Recursive subroutines
Arrays with dynamic bounds
User defined data types
Gotos
Algol did include them
But with (recursive) procedures and blocks, they
weren’t needed as much
Gotos were going out of vogue

People were realizing that it was a poor way to
program
Dijkstra’s 1968 letter to the ACM on the subjetct
Syntax and style
Free format

Indentation style
Algol defined the style of most successors


Hierarchical structure
Nesting of environments and control structures
Identifiers could be more than 6 characters
Variables and types
Data types: integer, real, boolean
No implicit declarations
No double precision types
No complex number types
Arrays could have more than three dimensions


Could have dynamic bounds
Can start at something other than 0/1
Binding
Binding of names to locations is done on entry
to a block

Not at compile time, as in Fortran
Stack is central run-time data structure
Blocks
Can use a block of statements anywhere a
single statement is needed
begin
declarations;
statements;
end
Blocks support structured
programming
Algol 60
if x = 3 then
begin
y := 9;
k := 10;
end;
Fortran
100
IF (X .NEQ. 3) GOTO 100
Y = 9
K = 10
...
Blocks allow nested scopes
begin
integer x;
procedure squid;
begin
integer x;
...
end;
end;
Blocks for efficient storage
management
begin
...
begin
real array x[1:1000];
...
end;
...
begin
real array y[1:2000];
...
end;
end;
Control structures
Goto
If-then-else
For loop
Switch
Call by name vs. by value
Call by name is default
Call by name: re-evaluate the actual parameter
on every use


For actual parameters that are simple variables, it’s
the same as call by reference
For actual parameters that are expressions, the
expression is re-evaluated on each access
No other language ever used call by name…
Call by name
parameter is n+10 (not just 10)
begin
integer n;
procedure p (k: integer)
begin
prints n+10, which is 10
print (k);
n := n+1;
n is still 0; thus, n becomes 1
prints n+10, which is 11
print (k);
end;
n := 0;
n is set to 0
p (n+10);
end;
parameter is n+10 (not just 10)
Problems with Algol
Didn’t include a I/O library



Thus, each implementation had a different means for
I/O
This caused compatibility problems
And no standard way to write a Hello World program
Algol 68
Successor to Algol 60
Included may new features





Unions
Operator overloading
Casting
Standardized input and output
Parallel processing
Rarely implemented by the compilers, though
Life of Algol 60
Didn’t achieve widespread use
Extremely important in the history of PLs
Successors: Pascal, Modula, Ada, others
Produced important work on:



Lexical analysis
Parsing
Compilation
techniques
languages
for
block-structured

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