Re: [External] : Re: Better documentation for non-binding clauses of if-let and friends

[Joost Kremers <joostkremers@fastmail.fm>]

On Tue, Nov 12 2024, Drew Adams wrote:
> This is your if-let*, with space separation of
> bindings from conditions:
>
>> (if-let* ((           (parsebib--char "@"))
>>           (           (parsebib--keyword '("string")))
>>           (open       (parsebib--char "{("))
>              ^^^^1
>>           (definition (parsebib--assignment))
>              ^^^^^^^^^^1
>>           (           (parsebib--char
>                           (alist-get
>                             open '((?\{ . "}") (?\( . ")"))))))
>                             ^^^^2
>>     definition
>       ^^^^^^^^^^2
>>   (signal 'parsebib-error
>>           (list (format "Malformed @String definition at position %d,%d"
>>                         (line-number-at-pos) (current-column)))))
>
> Implied and-ing is all the if-let* gets you, at
> least in this example.  At the cost of a loss of
> obvious distinction between binding and use
> occurrences.

I don't see the loss. The `if-let*` version has two occurrences of `open`
and `definition`: the first is the binding, the second is the use. Without
`if-let*`, I need to repeat both once more to test if they're non-nil.
Those occurrences and the `and`s are there to make the code do what I want,
but they don't help express the idea behind the code, so to my mind they're
boilerplate.

> I guess your parser grammar rule (not shown) just
> has the (parsebib-*...) conditions as literal
> match patterns, and it has `open' and `definition'
> as nonterminals.

`post` is also a literal, I'm just capturing it so I can make sure I get
the corresponding closing delimiter. Otherwise you could close a curly
brace { with a parenthesis ) or vice versa. (Yes, this is cheating, I
know...) `definition` is a non-terminal (and should probably be called
`assignment`).

> In any case, you said you didn't know what an
> "equivalent without `if-let*` would look like".
> So I mentioned that `macroexpand' has the answer.

True, it didn't occur to me to look at it.

> And the answer isn't very complex.  In this case,
> at least, it just makes the and-ing of binding
> dependencies explicit/clear.

Which I feel is unnecessary and even a hindrance.

-- 
Joost Kremers
Life has its moments