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The ALGOL Family
CSE341
Programming Languages
Overview Historical perspective ALGOL Goals BNF (Backus-Naur Form) ALGOL 58 ALGOL 60 ALGOL 68 Success and Failure of ALGOL Conclusion
History
ALGOrithmic Language Developed in Europe by an international group,
consisting of 7 different countries, in the late 1950’s Very similar to FORTRAN Peter Naur and J.W. Backus worked on the project.
Was the debut of the BNF syntax. Designed specifically for programming scientific
computations
History (Continued)
Never became as commercially popular as FORTRAN or COBOL Was not compatible with IBM
Is considered the most important programming language in terms of influence on later language development
Many similar languages can (and are) referred to as “ALGOL-like” JAVA, C, C++, Pascal, Ada, FORTRAN, etc.
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 into machine language programs
BNF (Backus-Naur Form)
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
Three 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
ALGOL 58The beginning June 1957 – ACM requested a committee to look
into a universal programming language European and American groups got together in
Zurich. No current language covers everything Creating more non-ideal languages doesn’t help the
situation Each passing month more people start using other
languages How can the logical structure of existing languages be
adjusted
ALGOL 58New Features Types
Integer Real Boolean
Formal vs Actual parameters Declaration for statements Switch Compound statements Begin end delimiters Three level language description Call by name
ALGOL 58Function vs Procedure Nature of the body
Expression Compound statement
Parameter evaluation mechanism Value of any actual parameter Text of any actual parameter
Identifier collision Non-parameter identifiers are global to the definition Communication only through parameters
Place of use An operand in an expression A statement or an operand in an expression
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
begin integer n; procedure p (k: integer) begin print (k); n := n+1; print (k); end; n := 0; p (n+10);end;
parameter is n+10 (not just 10)
prints n+10, which is 10
parameter is n+10 (not just 10)
n is still 0; thus, n becomes 1
prints n+10, which is 11
n is set to 0
Example Computing Illustrate ( Compute The Mean ):
// the main program (this is a comment) begin
integer N; Read Int(N);
begin real array Data[1:N]; real sum, avg; integer i; sum:=0; for i:=1 step 1 until N do
begin real val; Read Real(val); Data[i]:=if val<0 then -val else val
end;
for i:=1 step 1 until N do sum:=sum + Data[i];
avg:=sum/N; Print Real(avg)
end end
Example(Continued) I/O Illustrate: because ALGOL had no IO facilities. The following
code could run on an ALGOL implementation for a Burroughs A-Series mainframe.
BEGIN
FILE F (KIND=REMOTE);
EBCDIC ARRAY E [0:11];
REPLACE E BY "HELLO WORLD!";
WHILE TRUE DO
BEGIN
WRITE (F, *, E);
END;
END.
Problems with Algol58
Didn’t include a I/O libraryThus, each implementation had a different
means for I/OThis caused compatibility problemsAnd no standard way to write a Hello World
program
ALGOL 60 Basic Language of 1960
Simple imperative language + functions Successful syntax, BNF -- used by many successors
statement oriented begin … end blocks (like C { … } ) if … then … else
Recursive functions and stack storage allocation Type discipline was improved by later languages Very influential but not widely used in US
Tony Hoare: “Here is a language so far ahead of its time that it was not only an improvement on its predecessors but also on nearly all of its successors.”
ALGOL 60Proposed Changes The empty statement was adopted Modifications to operation hierarchy
Unary operators having higher precedence than binary operators was adopted Implied multiplication was rejected Relation sequences were rejected
The else clause was adopted Strings, lists, trees, matrices, and complex numbers were rejected as types Multiple assignment statements were adopted
Created a new problem, A[i] := i := e Recursion became possible in obvious ways Constants were rejected
ALGOL 60What’s New Block Call by value/name Typed procedures Declaration scope Dynamic arrays Side effects Global and local variables Step, until, while, if then else Activation records Recursive decent parsers No I/O
Algol 60 Sample
real procedure average(A,n);real array A; integer n;begin
real sum; sum := 0;for i = 1 step 1 until n do
sum := sum + A[i];
average := sum/nend;
no array bounds
no ; here
set procedure return value by assignment
Algol Oddity
Question: Is x := x equivalent to doing nothing?
Interesting answer in Algol
integer procedure p;
begin
….
p := p
….
end; Assignment here is actually a recursive call
Problems with Algol60
Holes in type discipline Parameter types can be arrays, but
No array bounds Parameter types can be procedures, but
No argument or return types for procedure parameters Problems with parameter passing mechanisms
Pass-by-name “Copy rule” duplicates code, interacting badly with side effects
Pass-by-value expensive for arrays Some awkward control issues
goto out of block requires memory management
ALGOL 68 Considered difficult to understand
Idiosyncratic terminology Types were called “modes” Arrays were called “multiple values”
Used vW grammars instead of BNF Context-sensitive grammar invented by van Wijngaarden
Elaborate type system Complicated type conversions
Fixed some problems of Algol 60 Eliminated pass-by-name
Not widely adopted
ALGOL 68What’s New Many new types, called modes
Primatives Bits Bytes String Sema (a semaphore) Complex File Pipe Channel format
Additional Flex (Flexible array) Heap (space on the heap) Loc (local space on the stack) Ref (pointer) Long (bigger ints/reals) Short (smaller ints/reals)
Declaration of custom types/modes
Algol 68 Modes
Primitive modes Compound Modes
int --arrays Real --structures Char --procedures bool --sets string --pointers Compl(complex) bits bytes Sema (semaphore) Format (I/O) file
Rich, structured, and orthogonal type system is a major contribution of Algol 68.
Other Features of Algol 68
Storage management Local storage on stack Heap storage and garbage collection
Parameter passing Pass-by-value Use pointer types to obtain pass-by-reference
Assignable procedure variables
Structure
ALGOL68 is block structured w/ static scope rules ALGOL68's model of computation:
static stack: block/procedure AR's; local data objects heap: “heap” -- dynamic-- data objects
ALGOL68 is an expression-oriented language
Basic Syntax
Addition : “ + ” Subtraction : “ - ” Multiplication : “ * ” Division : “ / ” Exponentiation : “ ** ” Assignment : “ := ” Boolean Expressions
= , > , < , <= , >= , /=
Recursion
Algol 68 Supports recursion
Example:
real procedure factorial (n);
begin
if n = 1 then
factorial := 1;
else
factorial := n* factorial(n-1);
end;
Arrays
Three types of arrays: real, integer, Boolean Each array must contain all the same types All arrays are of type real unless specified Can have multidimensional arrays Declarations:
array name1[1:100]; (1D array of type real) real array name2(-3:6,20:40); (2D array of type real) integer array name3,name4(1:46);(2 1D arrays of type integer) Boolean array name5(-10:n); (1D array of type Boolean)
(Allocated Dynamically)
Block Structure
First language to implement a block structure Similar in form to pascal
begin
…..
end; Each block can have its own variables, visible only to
that block (local variables). After the block is exited the values of all the local variables are lost.
Block Structure example
Example:
begin
own integer i; integer j,k;
i := j + k;
end; The integer i will have the value of j+k stored the next time
the block is entered By using the “own” statement the variable will retain its
value for the next time the block is entered
Parameter Passing
Two types of parameter passing: by Value, by Name
Pass by Value works the same as in most other languages
Pass by Name is similar to pass by reference, but it adds flexibility
All parameters are pass by name unless otherwise specified Example: can make a call “sum(i,2,5,x+6)” to the procedure sum
procedure sum(i,j,k,l); value i,j,k;begin
i := i + j + k + lend;
(will execute as i := i + 2 + 5 + (x+6))
ALGOLSuccesses and Failures Programming computers – Partial Success
Core language is strong, no I/O is a serious shortcoming Publication of algorithms – Very Successful Stimulus to compiler design – Success with a seed of
corruption It only stimulated compiler design because it was difficult
Stimulated formal language research – Success Not the goal of the ALGOL effort
Description of ALGOL 60 – Failure Difficult to understand for the uninitiated reader Needs an informal explanation
Conclusion
General purpose algorithmic language with a clean consistent and unambiguous syntax.
Comprehensive fully-checked type-system covering structures, unions, pointers, arrays and procedures.
Procedures may be nested inside procedures and can deliver values of any type without you having to worry about where the storage is coming from.
User-defined operators including user-defined operator symbols.
Powerful control structures can deliver values of any type.
Conclusion (Continued)
Dynamic sized arrays know their current bounds. Array and structure displays can be used in any context. Parallel programming with semaphores. Complex arithmetic. Declarations can be interleaved with statements. Clear distinction between value semantics and reference
semantics. No distinction between compile-time constants and run-
time values.
References
Lindsey, C.H., “A History of ALGOL 68.” History of Programming Languages, ACM Press. 1993. 97-132
Naur, Peter, “Successes and failures of the ALGOL effort”, ALGOL Bulletin #28, Computer History Museum. 1986. 58-62
Thomson, C.M., “Algol 68 as a Living Language”, ALGOL Bulletin #47, Computer History Museum. 1981. 21-24
Perlis, Alan, “The American side of the development of Algol”, The first ACM SIGPLAN conference on History of programming languages, ACM Press. 1978. 3-14
Naur, Peter. “The European side of the last phase of the development of ALGOL 60”, The first ACM SIGPLAN conference on History of programming languages, ACM Press. 1978. 15-44