On 2023-08-04, albert@cherry.(none) (albert) <albert@cherry> wrote:
I'm busy with mal
https://github.com/kanaka/mal
a dialect of lisp similar to lisp.
Most types of objects evaluate to herself. number strings.
quotation is necessary for lisp insists that the first item
of a list is a function. All exceptions must be quoted.
Suppose we adapt the rule, that we leave lists alone unless the
first item is a function or a symbol that points to a function.
PicoLisp does this, if I recall correctly. For instance, because
in (1 2 3) 1 is not a function, (1 2 3) evaluates to itself.
If you're implementing Mal, this is decide for you, I think.
The expansion of quasiquote / splice-unquote / unquote
suddenly becomes easy in this context.
Switch to an environment where only quasiquote / splice-unquote / unquote
are known. Evaluate. Then continue with the real evaluate.
(quasiquote 1 "A" ( 1 2 3) unquote(a) b ) is to expand to
(cons 1 (cons "A" (cons (1 2 3) (cons (eval a) ( cons b () ) )..)
as a first step, contrary to a tricky choice of possibilities.
The unquoting splice still needs to be handled. To achieve
that you can give the operators this semantics
(quasiquote a b c) -> (append a b c)
(unquote x) -> (list x)
(splice x) -> x
However, we need all other arguments which are not unquotes
or splices to be turned into lists somehow:
`(1 "A" (1 2 3) ,a ,@b)
(quasiquote a "A" (1 2 3) (unquote a) (splice b))
(append (list a) (list "A") (list (1 2 3)) (list a) b)
I don't see how you can easily escape from quasiquote having to analyze
the interior according to its own rules, rather than just setting up
some environment.
One possibility might be:
(quasiquote a b c) -> (funny-list a b c)
Where funny-list lists all items except for specially
annotates ones. The special annotation is produced by splice.
We have:
(unquote x) -> x
(splice x) -> (cons 'sys:splice x)
When funny-append sees an object like (sys:splice . whatever)
it will spread whatever into a list, and add the items
individually.
The only problem with this is that we are relying on this
specific funny-list run-time support function which has
to do a run-time check.
If the Lisp dialect has to handle dotted quotes and unquotes,
that is an issue also.
(quasiquote 1 "A" ( 1 2 3) unquote(a) b ) is to expand to
Then your expression reduces to
(append 1 "A" (1 2 3) a
There is a hidden quotation in cons.
I find that disenginuous.
No there isn't, except for the one you have introduced here
whereby (1 2 3), which is (1 . (2 . (3 . nil))) in CL-like
dialects, is self-evaluating. You put the hidden quotation
into (1 . <whatever>) based on the rule that 1 is not
a function.
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