1. Conference on the History of Programming Languages III
Modula and Oberon
Niklaus Wirth
Conference on the History of Programming Languages III
San Diego, 9 June 2007
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2. Overview Background The languages’ main characteristics
Their environments
Oberon
Trends and goals
Outlook
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3. Background A sequence of langauges with a common style and purpose:
Algol 60 – Algol W – Pascal – Modula-2 – Oberon
Reducing the difficulties and the sources of mistakes in programming by structure and strict typing of all data
Rigorously defined, convenient syntax
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4. ALGOL 60 Defined in 1960 by international committee
Target: Numerical algorithms
First language with strictly defined syntax
Abstraction from computers and their particular instruction sets
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5. ALGOL-W
Result of discussions in IFIP WG 2.1 on a successor of Algol 60.
Proposal by Wirth, perfected with C.A.R. Hoare in 1965
Implemented in for IBM 360
Adds pointer types and record handling
Complex multi-pass compiler
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6. Pascal “My idea of the successor of Algol 60”
Strong influence of Structured Programming
Designed in 1968, implemented
Published in 1970
Success in teaching due to clear structure and ready availability at low cost
Pascal-S, Pascal-P
UCSD-Pascal, Borland-Pascal
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7. Modula-2 Pascal for System Programming
Designed in 1977, implemented 77-80
System for workstation Lilith (1980)
Published in 1979
Mixture of Pascal and Mesa (Xerox)
Modules and Information Hiding
Separate compilation -> libraries
Low-level facilities for device access
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8. Environments Algol: Small scale computers in academia
Pascal: Era of batch processing and remote program submission, closed shops
Modula: Beginning of era of work stations,
essentially still used in batch mode
Oberon: Era of high-power work stations,
interactive use, graphics, windows
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9. Environments Algol 60 1960 Fortran 1957 Algol-W 1965 PL/1 1965
Pascal 1970 C (?) 1970
Modula Ada 1984
Oberon 1988 Java 1995
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10. Environments Algol 60 1960 Gier, Electrologica Algol-W 1965 IBM 360
Pascal 1970 CDC 6000, IBM 360,
PDP-10, ICL, Univac
Modula PDP-11, Lilith, Apple-2
Oberon 1988 NS-32000, M-68000, Int-8086, IBM Power, Sparc, MIPS
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11. What is essential for programming?
Languages are too complex, fraught with dispensable features and facilities.
Research in programming methodology: assertions, invariants, predicate transformers suggest that a language must rest on a sound, mathematical basis
Language to be defined in abstract terms, without reference to a mechanism (computer)
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12. Oberon Simplified, reduced Modula-2 (Almost) same syntax, same style
Fewer standard data types
Fewer statement kinds
Unified definitions and implementations
Separation of the notions of unit of program and unit of action: Module vs. procedure
(Almost) no low-level facilities (loopholes)
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13. Type extension (of record types)
Only two concepts added in Oberon:
Type extension (of record types)
with type test and type guard T0 T1 x x
a T1 is also a T0 y y z
2. Type inclusion
LONGREAL  REAL  LONGINT  INTEGER  SHORTINT
Procedure-typed record fields + Type extension are the tools for object-oriented programming
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14. Trends and Goals
The goal of a language must be to assist the programmer to design correct, reliable, provable, efficient software
To this aim, it defines a formalism, an abstraction to replace any actual computer
This suggests simplicity and regularity of concept
The language must be defined without reference to an actual mechanism
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15. Have we reached this goal?
Algol 60: reasonably for numeric programs
Pascal: better; bad features were
the variant record as loophole, rigid static typing
Modula-2: poorly
variant record, type transfer functions, loopholes
Oberon: best of all, very close, water-tight type checks
Only very few SYSTEM operators (Put, Get)
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16. Outlook
New languages tend to go in the wrong direction: They become increasingly complex
Teaching programming is dominated by teaching about the language and its features and libraries: It has become a nightmere!
- Correctness is reached by trial and error
Effectiveness irrelevant, computers are fast
Programming: The School of Poor Design
Can we learn from history?
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17. Education
The essence of engineering education is to teach the art of good design.
Success in teaching is strongly determined by the tools we use.
Our tool is the Programming Langauge.
It must present the proper, clearly defined abstractions with which we operate.
Yet it must remain on solid grounds to allow for economic, realistic solutions
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