Context Dependent Syntax, Lisp setf, Python a[i]=x

By Xah Lee. Date: . Last updated: .

Common Lisp setf is Worst Computer Language Design

the Common Lisp setf is the most fk'd design in computer languages possible.

languages with a[i]=x, are also fk'd. It's the confounding of meaning. It is exception, irregularity.

here's elisp manual explaining setf.

elisp manual generalized variable 2017 04 07
(info "(elisp) Generalized Variables")

Let's talk about a[i] = x, and see what's wrong.

the gist here, is that it uses the same notation to indicate a “place”. For example, a[3] is the 3rd element in array. So,

a[3]=4 means make the 3rd place to have value of 4.

The problem here, is that there's different meaning of a[3] in the 2 cases. Normally, a[3] evaluates to a value. But in a[3]=4, the a[3] does not evaluate. Rather, it's part of the syntax that tells the compiler of a “storage location”.

So, you see, the a[3] in print a[3] and a[3]=4 have diff meaning, and is context dependent.

And of course Forth also explicitly has fetch and store ops so you can say 3 a @ . and x @ 3 a ! for printing and assignment

The Bliss language (DEC Systems lang) had a unary dot operator to return a value so you'd say a[3]=.x and print .a[3]

—from Bahstin Beer Bum @mmaug, 2016-10-05

Context Dependent Syntax

  1. JavaScript Syntax Soup: 「p in o」 vs 「for (p in o) {…}」
  2. Python's Context Dependent Syntax Soup: 「… in …」 And 「… not in …」
  3. Context Dependent Syntax, Lisp setf, Python a[i]=x

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