CS 415: Programming Languages

Algol

Aaron Bloomfield

Fall 2005

Historical perspectiveHistorical perspective

By mid/late 50’s a lot of PLs were out thereBy mid/late 50’s a lot of PLs were out there

Interest in universal languageInterest in universal language

European and American groups got together in European and American groups got together in ZurichZurich Result was Algol 58Result was Algol 58 8 people spent 8 days working on the language8 people spent 8 days working on the language

Algol goalsAlgol goals

To be as close as possible to standard math To be as close as possible to standard math notationnotation Also very readable without much more explanationAlso very readable without much more explanation

Should be possible to use it to describe Should be possible to use it to describe algorithms in publicationsalgorithms in publications A form of it is still used todayA form of it is still used today

Should be mechanically translatable into Should be mechanically translatable into machine language programsmachine language programs

3 language versions3 language versions

Reference languageReference language Used by the committee, described in the report, and Used by the committee, described in the report, and

used in official Algol publicationsused in official Algol publications

Publication languagePublication language Allowed for differences in the character set for Allowed for differences in the character set for

different languagesdifferent languages Europeans and Americans couldn’t decide on which Europeans and Americans couldn’t decide on which

character to use for the decimal point!character to use for the decimal point!

Hardware representationsHardware representations Condensed languages for machine inputCondensed languages for machine input

More historyMore history

Was first to use BNF (Backus-Naur Form)Was first to use BNF (Backus-Naur Form) Same Backus that created FortranSame Backus that created Fortran

He also was one of the main creators of AlgolHe also was one of the main creators of AlgolAnd he created functional programmingAnd he created functional programmingAnd won the Turing award in ’77And won the Turing award in ’77

Right. Back to BNFRight. Back to BNF BNF example:BNF example:

<value> := <number> | <variable> | <expression><value> := <number> | <variable> | <expression><number> := <integer> | <float><number> := <integer> | <float><integer> := <integer><digit> | <digit><integer> := <integer><digit> | <digit><digit> := 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9<digit> := 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Language was designed by committeeLanguage was designed by committeeReport is a “paradigm of brevity and clarity”Report is a “paradigm of brevity and clarity”

Most languages today require 1000’s of pagesMost languages today require 1000’s of pages Brevity and clarity contributed to reputation as simple, elegant Brevity and clarity contributed to reputation as simple, elegant

languagelanguage

Algol language featuresAlgol language features

Block structureBlock structureAnd scope of variables within those blocksAnd scope of variables within those blocksExplicit type declaration for variablesExplicit type declaration for variablesScope rules for local variablesScope rules for local variablesDynamic lifetimes for variablesDynamic lifetimes for variablesNested if-then-else expressions and statementsNested if-then-else expressions and statementsCall by value and call by nameCall by value and call by nameRecursive subroutinesRecursive subroutinesArrays with dynamic boundsArrays with dynamic boundsUser defined data typesUser defined data types

GotosGotos

Algol did include themAlgol did include them

But with (recursive) procedures and blocks, they But with (recursive) procedures and blocks, they weren’t needed as muchweren’t needed as much

Gotos were going out of vogueGotos were going out of vogue People were realizing that it was a poor way to People were realizing that it was a poor way to

programprogram

Dijkstra’s 1968 letter to the ACM on the subjetctDijkstra’s 1968 letter to the ACM on the subjetct

Syntax and styleSyntax and style

Free formatFree format Indentation styleIndentation style

Algol defined the style of most successorsAlgol defined the style of most successors Hierarchical structureHierarchical structure Nesting of environments and control structuresNesting of environments and control structures

Identifiers could be more than 6 charactersIdentifiers could be more than 6 characters

Variables and typesVariables and types

Data types: integer, real, booleanData types: integer, real, boolean

No implicit declarationsNo implicit declarations

No double precision typesNo double precision types

No complex number typesNo complex number types

Arrays could have more than three dimensionsArrays could have more than three dimensions Could have dynamic boundsCould have dynamic bounds Can start at something other than 0/1Can start at something other than 0/1

BindingBinding

Binding of names to locations is done Binding of names to locations is done on entry on entry to a blockto a block Not at compile time, as in FortranNot at compile time, as in Fortran

Stack is central run-time data structureStack is central run-time data structure

BlocksBlocks

Can use a block of statements anywhere a Can use a block of statements anywhere a single statement is neededsingle statement is needed

beginbegin

declarations;declarations;

statements;statements;

endend

Blocks support structured Blocks support structured programmingprogramming

Algol 60Algol 60if x = 3 thenif x = 3 then

beginbegin

y := 9;y := 9;

k := 10;k := 10;

end;end;

FortranFortran IF (X .NEQ. 3) GOTO 100IF (X .NEQ. 3) GOTO 100

Y = 9Y = 9

K = 10K = 10

100 ...100 ...

Blocks allow nested scopesBlocks allow nested scopes

beginbegin

integer x;integer x;

procedure squid;procedure squid;

beginbegin

integer x;integer x;

......

end;end;

end;end;

Blocks for efficient storage Blocks for efficient storage managementmanagement

beginbegin ...... beginbegin real array x[1:1000];real array x[1:1000]; ...... end;end; ...... beginbegin real array y[1:2000];real array y[1:2000]; ...... end;end;end;end;

Control structuresControl structures

GotoGoto

If-then-elseIf-then-else

For loopFor loop

SwitchSwitch

Call by name vs. by valueCall by name vs. by value

Call by name is defaultCall by name is default

Call by name: re-evaluate the actual parameter Call by name: re-evaluate the actual parameter on every useon every use For actual parameters that are For actual parameters that are simple variablessimple variables, it’s , it’s

the same as call by referencethe same as call by reference For actual parameters that are For actual parameters that are expressionsexpressions, the , the

expression is re-evaluated on each accessexpression is re-evaluated on each access

No other language ever used call by name…No other language ever used call by name…

Call by nameCall by name

beginbegin integer n;integer n; procedure p (k: integer)procedure p (k: integer) beginbegin print (k);print (k); n := n+1;n := n+1; print (k);print (k); end;end; n := 0;n := 0; p (n+10);p (n+10);end;end;

n is set to 0

parameter is n+10 (not just 10)

prints n+10, which is 10

n is still 0; thus, n becomes 1

prints n+10, which is 11

parameter is n+10 (not just 10)

Problems with AlgolProblems with Algol

Didn’t include a I/O libraryDidn’t include a I/O library Thus, each implementation had a different means for Thus, each implementation had a different means for

I/OI/O This caused compatibility problemsThis caused compatibility problems And no standard way to write a Hello World programAnd no standard way to write a Hello World program

Algol 68Algol 68

Successor to Algol 60Successor to Algol 60

Included may new featuresIncluded may new features UnionsUnions Operator overloadingOperator overloading CastingCasting Standardized input and outputStandardized input and output Parallel processingParallel processing

Rarely implemented by the compilers, thoughRarely implemented by the compilers, though

Life of Algol 60Life of Algol 60

Didn’t achieve widespread useDidn’t achieve widespread use

Extremely important in the history of PLsExtremely important in the history of PLs

Successors: Pascal, Modula, Ada, othersSuccessors: Pascal, Modula, Ada, others

Produced important work on:Produced important work on: Lexical analysisLexical analysis ParsingParsing Compilation techniques for block-structured Compilation techniques for block-structured

languageslanguages