Humanist Discussion Group, Vol. 34, No. 53. Department of Digital Humanities, King's College London Hosted by King's Digital Lab www.dhhumanist.org Submit to: firstname.lastname@example.org Date: 2020-05-23 09:06:12+00:00 From: Norman Gray
Subject: Re: [Humanist] 34.45: punctuation in the assignment statement [The following seems to have gone astray, but this shepherd found it. Apologies on behalf of naughty software and my own sloppy habits. --WM] Willard, hello. Just to throw a little more petrol into the fire... In a Lisp one might write > (define x 1) > (let ((x (+ x 2))) x) ...evaluates to 3 In the first line, 'x' is defined to be a synonym for '1'. In the second line, a variable 'x' is given a value. What value? The value of the expression (+ x 2) (which is lisp-speak for x+2), as evaluated in the context outside the 'let', namely 1+2, or 3. Then the 'let' expression has, as its overall value, the value of x, as evaluated in the context inside the let, namely 3. The fact that the two variables happen to have the same name is irrelevant (but it does confuse the student most instructively). Now you can take the view that there's technically no _assignment_ here, merely a sequence of rewritings. The 'let' indicates that, within the 'let', when x is seen it should be taken to have the value 3, produced by (+ x 2). Some variants of this language (there are a silly number of such variants) don't have any mechanism for changing the value of a variable -- that is, _re_assignment is inexpressible. In case you're thinking this is some weird new-fangled language, Lisp is only just younger than Fortran (one year, I suspect you know this, Willard), and marginally precedes Algol, so was presumably known to the Algol designers. In case you doubt Lisp's mathematical chops, it emerged as a concrete version of Church's lambda calculus for describing the theory of computation, and the story goes that when John McCarthy's student Steve Russell reported that he'd managed to make an actual computer evaluate Lisp, he was reproached by McCarthy for having missed the point. I mention this, not to be contrary or whimsical, but to observe that there's more than one way of expressing computation; that notations always have a meaning only in context, so that they are arbitrary and the free choice of the author; and that the point of notation is to label a particular operation or expression or even thought, rather than the notation having any intrinsic meaning that can be argued over. And I think that leads us back to recognising the arbitrariness of ':=' as an Algol notation. The language designers decided it would be useful to lexically distinguish assignment from an equality test, they presumably cast around in their memories for a suitable symbol available using the keyboards in front of them, which would have some level of intuitive affordance to the language's users, ':=' fitted the bill, and thus made they it so. The only meanings available to be interrogated by the close-reader may be about those intuitions, or perhaps what that specific keyboard actually looked like. At  there is a page of keyboard layouts for 'typewriters' used with early machines: the second one has ':' physically closer to the user's fingers than '<' or '>'. Lisp's mathematical background may also touch on your admitted question, Willard, in the sense that Lisp concretises an abstract mathematical question, and so seems to perch comfortably on the boundary between computation (in the sense of adding up) and the theory of computation. I also observe in passing that the Wikipedia page about the lambda calculus  uses ':=' with an assignment-like meaning which would be read aloud as something like 'taken to be' or 'replaced by'. I don't know if that notation is one that Church or McCarthy, or the Algol authors, would have used. Best wishes, Norman  https://en.wikipedia.org/wiki/Lambda_calculus  http://www.quadibloc.com/comp/kyb01.htm -- Norman Gray : https://nxg.me.uk SUPA School of Physics and Astronomy, University of Glasgow, UK _______________________________________________ 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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