David Evans CS655: Programming Languages University of Virginia Computer Science Lecture 5: Languages Bugs and/or Features Start working with your language group to answer questions 1-6 now. Applicants must also have extensive knowledge of Unix, although they should have sufficiently good programming taste to not consider this an achievement. Hal Abelson, MIT job advertisement

1 Feb 2000University of Virginia CS 6552 Menu Background on Algol68 Project Update Fill in the rest of the language properties table Present results (starting promptly at 1:15)

1 Feb 2000University of Virginia CS 6553 Algol 68 Committee Unruly, political, infighting, unresolved conflicts 1965 Meeting: two proposals –Wirth and Hoare: extend Algol 60 with records –van Wijngaarden: design new language, minimal concepts combined in orthogonal way, describe using W-grammar, incomplete and incomprehensible Kept postponing meetings because drafts weren’t ready Accepted report (18 of 28 original members) First implementation: 1970 Revised Report: 1975 (first readable presentation)

1 Feb 2000University of Virginia CS 6554 Othogonality in Design m n Combination Techniques Building Blocks m*n things Some of those m*n things will be hard to understand or implement, so you end up with either exceptions or complexity.

1 Feb 2000University of Virginia CS 6555 Orthogonality in Algol68 References ref x was a type, so ref ref x was a type. refs for parameters, so refs for local varaibles. Procedures procs for parameters, so proc variables, parameters, results and in-line constructors Compare to Pascal: “References are dangerous and complicated, so only allow them where they are absolutely necessary.” –Functions can only return simple types, passed procedures cannot have ref parameters, etc.

1 Feb 2000University of Virginia CS 6556 W-Grammars Invented by Van Wijngaarden Can define any computable language! (Contrast BNF: only context-free languages) Simplified example: reference to MODE assignation :: reference to MODE destination, :=, MODE source. MODE :: real ; integral ; reference to MODE; …

1 Feb 2000University of Virginia CS 6557 Minority Report Signed by Edsger Dijkstra, C. A. R. Hoare and 5 others Some complaints about complexity of language (reference concept taken too far) Many complaints about description of language (complexity of W-Grammars) Controversy over publication (suppressed by IFIP) Copy on back of your manifest

1 Feb 2000University of Virginia CS 6558 The reason you have only 4 pages of reading this week and no writing assignments due until Feb 16 is: A.Your PS1’s were so great I thought everyone deserved a vacation for the rest of the term. B.There aren’t any interesting papers about types. C.The office staff has revolted from having so much copying to do for this class, and refuses to make any more copies for me. D.I wanted to make sure everyone has plenty of time to prepare a good Valentine’s Day gift. E.So you can get a good start on your projects and get into the habit of working on them consistently.

1 Feb 2000University of Virginia CS 6559 By end of week, all groups should have: Picked a problem for your project. Found and read the most relevant research papers to your project (divide among team members) Figured out how you will divide the proposal writing work: be your own harshest critics –Each proposal fragment should have one or more writers, and one or more different reviewers Bounced your project idea off me or John

1 Feb 2000University of Virginia CS Back to work.... Be ready to present at 1:15!