Letters between Backus and Dijkstra (1979)

Letters between Backus and Dijkstra (1979)
medium.com

Unfortunately, the Library of Congress does not allow scanners without prior approval, so this was the only way I could make my own copies. It did not help that all these letters were written or typed on very thin mimeograph paper.

I missed that the quote was Backus's, it makes a lot more sense now.

The quality of your photos is excellent and I doubt a scanner would make them much better. I wish there was a transcript though.

As a start, here is the first letter transcribed:

    to John Backus
    International Business Machines Corporation
    5600 Cattle Road
    SAN JOSE CA 95193
    U.S.A
    
                          Monday 29th of May, 1978
    
              “To interrupt one’s own researches in order to 
              follow those of another is a scientific pleasure
              which most experts delegate to their assistants.”
                    (Eric Temple Bell in 
                         “Development of Mathematics”)
    
    Dear John,
    
      I do not claim to be more noble or sensible than “most 
    experts”; perhaps it is only becauseI have only one 
    assistant to whom I can delegate no more than one 
    man can do. But when you open your letter with:
        “I am quite sure that you have never read 
        any paper I’ve sent you before”
    it is my pleasure to inform you that - although
    “quite sure” - you were very wrong. I very well 
    remember that you mailed me a sizeable paper 
    on reduction languages to which I owe my introduction
    to that subject. I didn’t only read it, parts of it 
    were even studied. I also remember that it left me 
    with mixed feelings.
    
      I can very well understand your excitement, although, 
    for lack of experience, I still cannot share it. I am 
    far from delighted with the state of the art of programming 
    today, and anyone suggesting an alternative has in 


    in [sic] principle my sympathy - until, of course, he loses it again, 
    like Terry Winograd did when he suggested natural language 
    programming - “natural descriptions”! - as an alternative-.
    In the long run I have more faith in any rephrasing of the 
    programming task that makes it more amendable to mathematical 
    treatment. But you must have lots of patience, for the 
    run will be very long. It isn’t just mental inertia - that 
    problem can be solved generally by education a new 
    generation of youngsters and waiting until the old ones 
    have died - . It is the development of a new set of 
    techniques needed to achieve a comparable degree of 
    proficiency.
    
    Could you get me a copy of G.A. Mago’s (not 
    yet published) “A network of microprocessors to execute 
    reduction languages”? That might whet my appetite! 
    From various angles I have looked into such networks 
    and I am not entirely pleased with what I have 
    seen. Firstly I suspect our techniques for proving the 
    correctness of such designs: each next proof seems to 
    differ so much from all the previous ones. I suspect 
    I discovered that all I could design were special 
    purpose networks, which, of course, made me suspect 
    the programming language in the Von Neumann style 
    which, already before you have chosen your problem, 
    seems to have set you on the wrong track.
    
       Semantically speaking the semicolon is, of course, 
    only a way of expressing functional composition: it 
    imposes the same order that can also be expressed 
    with brackets - innermost brackets first -. In combination 
    with the distribution you can generate many innermost 


    bracket pairs, thus expressing very honestly that it is 
    really only a partial order that matters. I like that, 
    it is honest.
    
    When you write “one can transform programs [....]
    by the use of laws [...] which are _part of the program-
    ming language_” etc. I am somewhat hesitant. I am 
    not convinced (yet?) that the traditional separation in 
    fixed program, variable data and assertions is a 
    mistake. The first and the last - program and assertions - 
    are somewhat merged in LUCID, in which correctness 
    proofs are carried out in (nearly) the same formalism 
    as the program is expressed in. On the one hand that 
    presents a tempting unification, on the other hand I 
    thought that mathematicians speerated carefully and 
    for good reasons the language they talk about and 
    the metalanguage in which they do so. To put it in 
    another way: given a functional program, I feel only 
    confident if I can do enough other significant things 
    to it besides easy[striked out three times] carrying it out. And those I don’t 
    see yet. The almost total absence of redundancy 
    is another aspect of the same worry. In the case 
    of a traditional program we know how to make it 
    redundant: by inserting assertions, combination 
    of text and assertions makes it into a logically 
    tightly knit whole, that gives me confidence. How 
    do we this with functional programs? By supplying 
    two of them and an argument that demonstrates 
    their equivalence?
    
      What about the following example? (My notation 
    because I lack the fluency in yours.) 

    
    (1)   Consider the function f defined by:
        f(0) = 0, f(1) = 1, f(2n) = f(n), f(2n+1) = f(n) + f(n+1)

    (2)   Consider the following program (“peven” = “positive and even”,
        so for “podd”)
    
    {N>=0} n, a, b := N, 1, 0;
    _do_ peven(n) -> a, n := a + b, n/2
      [] podd(n)  -> b, n := b + a, (n-1)/2
    _od_ {b=f(N)}
    
    Here (1) gives a recursive definition of f, (2) gives 
    a repetitive program. Both definitions can be translated 
    in a straightforward manner into functional programs. 
    What would be involved in the demonstration of their
    equivalence?
    
       The above seems to me little example of the appropriate 
    degree of sophistication to try your hands on. (I am 
   not going to try it myself, as I fear that my past 
   experience would misguide me: I am afraid that I
   wouldn’t rise above the level of translating (2)’s 
   traditional correctness proof - with which I am very
   familiar - in an unfamiliar notation. Good luck!)
    
       With my greetings and warmest regards, 
                       yours ever
                               _Edsger_
    
    P.S. Please note my new postal code: 5671 AL
    (capitals obligatory) _in front of_ the village name
                                                         EWD.

Some of the "verbosity" seems to arise from their apparently mutual concern to preserve their relationship by delivering criticism of ideas or behaviors without attacking the person. We could certainly use more of this style of communication today, but all it earns now is a "TL;DR" at the beginning or the end.

- Alan Kay