Humanist Discussion Group, Vol. 34, No. 27. Department of Digital Humanities, King's College London Hosted by King's Digital Lab www.dhhumanist.org Submit to: firstname.lastname@example.org Date: 2020-05-18 10:19:33+00:00 From: Willard McCarty
Subject: punctuation in the assignment statement Here follows a rather lengthy prelude to a question in the history and psychology of programming. Help, please! Anyone who has read David Crystal's Making a Point: The Pernickety Story of English Punctuation (2015) will know that many have attempted to fix the semantics of punctuation in English but none succeeded, though Lindley Murray's English Grammar (1795) established a long-lasting convention that has survived well beyond the Oxford English Dictionary. My specific interest in punctuation is in the colon as it is used in programming when combined with the equals-sign. This occurs in what are known as assignment statements, e.g. in ALGOL and other languages influenced by it, thus: X := Y + Z In FORTRAN, as you may know, the equivalent is written (disturbingly to some mathematicians) thus: X = Y + Z This looks like algebra but isn't truly algebraic because it is an imperative statement ('set the value of X equal to the sum of Y and Z') rather than a declarative one ('the value of X is equal to the sum of Y and Z'), as in algebra. Many have objected for very good reasons, mostly on the grounds that students who come to programming with a background in maths tend to become confused. Now for the historical bit. Within the first decade of digital computing it became obvious that programmers would fold under the burden of writing machine instructions. As Grace Hopper remarked, machines were getting faster but programmers weren't. One of the differences in how to design a standard high(er)-level programming language that would make programming easier and quicker centred on the assignment statement, specifically how to write it. Alan Perlis roughly sums up the story: > …Americans, following the FORTRAN approach, had the variable being > assigned to on the left. The Europeans had it on the right. The > Americans used an equals sign for the assignment, thus hiding the > fact that there is a direction to the assignment; the Europeans used > a directed arrow pointing to the right. When ALGOL 58 came out, > neither equals nor directed arrow was used, but a colon-equal and it > pointed to the left. (Wexelblat, History of Programming Languages, 1981: > 142) Hence X := Y + Z! My question is this: why the colon? What was this mark of punctuation meant to suggest, or better, what would it have suggested and what does it continue to suggest? The OED note on the colon as a mark of punctuation (rather than as a part of our anatomy) is a succinct statement of the conventional sense that would have been nigh universal in English usage in the mid 20th Century: > Its best defined use is to separate clauses which are grammatically > independent and discontinuous, but between which there is an > apposition or similar relation of sense. Thus it may introduce an > antithetic statement, an illustration, extract, etc. I suppose, then, that in the ALGOL lingo the colon functions as an appositional qualifier, a cognitive pause and a warning as to how the equals-sign is to be read. (I am getting here into the psychology of programming, into how on some level we read and write ALGOL and similar languages.) So I translate "X is to be MADE equal to the sum of Y and Z". Or, we might say, "Warning: this is NOT an algebraic statement!" Or, better, I think: "Here we must stretch our understanding of mathematics to include something new." Is there any supporting historical evidence for my supposition? Many thanks for any pointers. Yours, WM ----- P.S. Perlis' reference to "the Europeans" should have been more specific to give credit to Heinz Rutishauser, who in his foundational paper Automatische Rechenplanfertigung bei programmgesteuerten Rechenmaschinen (1951, published 1952) introduced what he called the "Ergibtzeichen", which looks to me like a right-pointing arrow superimposed on an equal-sign. Clarity! If only... For the source materials see: http://www.softwarepreservation.org/projects/ALGOL/standards/?searchterm=Proceed ings%20of -- Willard McCarty (www.mccarty.org.uk/), Professor emeritus, Department of Digital Humanities, King's College London; Editor, Interdisciplinary Science Reviews (www.tandfonline.com/loi/yisr20) and Humanist (www.dhhumanist.org) _______________________________________________ Unsubscribe at: http://dhhumanist.org/Restricted List posts to: email@example.com List info and archives at at: http://dhhumanist.org Listmember interface at: http://dhhumanist.org/Restricted/ Subscribe at: http://dhhumanist.org/membership_form.php
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