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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: When Is A High/Low-Level Language Not A High/Low-Level Language?
Date: Sun, 18 Aug 2024 00:20:49 +0100
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On 17/08/2024 23:11, Lawrence D'Oliveiro wrote:
> On Sat, 17 Aug 2024 11:19:30 +0100, Bart wrote:
> 
>> ... what does this have to do with C, or anything at all?
> 
> C is supposed to be the epitome of the low-level language that can do bit-
> fiddling and unsafe type conversions and the like. This is an example of
> an unsafe type conversion (offering a typesafe interface to the caller, of
> course) done dynamically, in a language which is generally considered to
> be “higher-level” than C.
> 
> In sum: types as first-class objects + low-level bit-fiddling = a
> combination unavailable in traditional “low-level” languages like C.
> 
>> Apart from being an apallingly bit of code.
> 
> How would you it less “apallingly”?
> 
> (This sentence no verb. Also speling.)

It's an adverb. Although there should have been two P's.

> 
>> However I can't see the switch-expression; there is a Dict constructor,
>> where all elements are evaluated, not just the one selected. That is not
>> how 'switch' works.
> 
> How does a switch-expression work, then? Can you give us an example?

Take this Python code that has a similar dict constructor:

   def prnt(x): print(x); return len(x)

   i=3
   a={1:prnt("One"), 2:prnt("Two"), 3:prnt("Three")}[i]

   print(a)

It selects the third element keyed with '3', but the output is:

   One
   Two
   Three
   5

So 'prnt' has been called 3 times instance of just once. (Also using a 
non-existent key gives an error.)

The equivalent using 'switch' in one of my languages (or anything 
expression-based that used any form of multi-way select) is this:

     fun prnt(x) = (println x; x.len)

     i := 3
     a :=
       switch i
       when 1 then prnt("One")
       when 2 then prnt("Two")
       when 3 then prnt("Three")
       else 0
       end

     println a

Output is:

     Three
     5

Only one branch has been evaluated. Plus there is a default value (it's 
required). Also, since the index values here are in sequence, I can use 
N-way select:

    a := (i | prnt("One"), prnt("Two"), prnt("Three") | 0)

Same result. You can't use a list here plus normal indexing, as again 
all elements would be evaluated.