Re: pcase ` meaning [Was: Re: Replace trivial pcase occurrences in the Emacs sources]

[Clément Pit-Claudel]

On 28/10/2018 19.09, Garreau, Alexandre wrote:
> the pattern look like what the data should be. adding “`” and “,”
> destroy this feature, because the pattern no longer looks like what
> is matched.

Maybe it's a matter of perspective? Adding "`" and "," is precisely what makes 
the pattern look like the data: the invariant is that if you have a match, 
repeating the pattern produces the original data.

Here's a concrete example: (pcase '(1 2 3 4) (`(,x ,y . ,z) `(,x ,y . ,z))) is 
a no-op.
In other words, q-patterns are equations with multiple unknown variables 
(holes).  When you write a q-pattern, you're asking "what values should the 
variables in this pattern take so that evaluating the pattern would return the 
matched data?"

Clément.