Algol 58: an interim design. Followed by lots of discussions in the Algol Bulletin and CACM.
Algol 60: result of an intensive 6 day design effort by a committee in Paris. A few revisions made; Algol 60 report published in CACM in Jan 1963.
The meetings were exhausting, interminable, and exhilarating. ... Progress was steady and the output, ALGOL 60, was more racehorse than camel. This language proved to be an object of stunning beauty. It was sufficiently perfect and complete so that ensuing implementations were able to append necessities, such as input-output, in the style of ALGOL 60 but their addition propagated no significant changes in the body of the original language.The ALGOL 60 report (Nauer et al., 1960) was a fitting display for the language. Nicely organized, tantalizingly incomplete, slightly ambiguous, difficult to read, consistent in format, and brief, it was a perfect canvas for a language that possessed those same properties. Like the Bible, it was meant not merely to be read, but to be interpreted.
-- Alan Perlis, "The American Side of the Development of Algol," The History of Programming Languages
One of the debates: should recursion be allowed?
dynamic array bounds; lower and upper bounds; indefinite number of dimensions
Blocks define nested scopes:
squid, the name x refers to a
different variable than in the global scope (see
Implementation of Block Structured Languages)
Algol 60 (and Modula-2, Ada, etc) use lexical scoping.
Early Lisps, APL, etc. use dynamic scoping.
Unlike Fortran, binding of variable names to locations done at block entry time (in general, it can't be done statically)
Blocks for efficient storage management
x is allocated in the first block, then deallocated,
and then the array y is allocated.
Compare with Fortran equivalence statement. The Algol solution is safe, clear, and provides no opportunity for clever abuse of the type system.
The bounds of array must be known at block entry time for example, in the declaration:
Except for procedures as parameters, Algol can be statically type checked. The report doesn't say, but a reasonable implementation will be strongly typed.
if-then-else: both statement and expression forms
switch statement -- supports a kind of computed goto; now obsolete (case statement is better)
parameter passing mechanisms: Algol 60 had call by name, call by value
(see Parameter Passing)
3 levels of representation:
hardware representations could vary from implementation to implementation
(cf comma - decimal point controversy)
Approaches to the problem of words such as INTEGER or WHILE:
A few syntactic problems:
Semicolon to separate statements.
Using the semicolon as a separator is much more error prone than using the semicolon as a statement terminator. (Ada takes the latter approach.) For example, the following Algol code is syntactically incorrect:
y:=5 actually puts a null statement
after the assignment, so that there are two statements after the "then",
resulting in an error.)
overly long names for common constructs (begin, end, comment); problem with semicolon terminating comments.
dangling else problem
Information about formal parameters too spread out; odd way to indicate return value in a function (by assigning to the name of the function)
Examples: number of characters in an identifier, number of dimensions in an array, number of arguments to a function
Algol obeys this principle much better than does Fortran
Lots of semantic ambiguities. (BNF used for a precise description of
syntax, but semantics specified in English.)
Example: are the bounds of a for loop evaluated once
before executing the loop, or at the beginning of each loop execution? A
literal reading of the Report implies that they are evaluated
each time; but this is inefficient and unclear.
As an example of the problems that can arise from such ambiguity, in the DEC System 20 implementation of SIMULA we used to have, for this loop the upper bound was evaluated once:
Some other problems: OWN variables, SWITCH, side effects in functions
labels, procedures, and strings are not first-class citizens in Algol 60. (This allows for more efficient compilation.)
no I/O statements
In the USA, Burroughs supported Algol-60, but IBM supported FORTRAN
Algol-60 is extremely important language in the history of programming languages
Many successors: Pascal, Modula-2, Ada, Euclid, Mesa, Emerald, ...
Lots of good work done on lexical analysis, parsing, compilation techniques for block-structured languages, etc, which is still used in current compilers.
Some issues of enduring interest that are prominent in Algol:
PL/I -- Swiss army knife language
kinds of extension: