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From: Andrey Orst <andreyorst@gmail.com>
Date: Thu, 18 Feb 2021 22:26:52 +0300
Message-ID: <CAAKhXoZx+p+_fgvgnYHz3vacK+qvu9Ra2m3vq+nxP7nvDm3LSg@mail.gmail.com>
Subject: nil-safe nested table access operator
To: ~technomancy/fennel@lists.sr.ht
Content-Type: text/plain; charset="UTF-8"

Hi I wanted to suggest new operator for Fennel for nil-safe nested table access.

Currently Fennel has dot operator to access values stored under keys
in tables. This operator accepts several keys to look up value in
nested set of tables:

    >> (local t {:a {:b {:c 42}}})
    >> (. t :a :b :c)
    42

However if any of keys before :c were not present in respective
tables, we will get error of trying to index nil:

    >> (. t :a :nope :c)
    ;; error

I would like to propose ?. operator which will short-circuit if any of
keys are not present.

    >> (?. t [:a :nope :c])
    nil

This operator accepts keys in sequential table, because optional
default value can be specified as third argument:

    >> (?. t [:a :nope: :c] 27)
    27

This is handy when you're dealing with tables that can be altered over
the liftetime of the application. Below are two sample
implementations:

    (fn ?. [tbl keys not-found]
      (match keys
        [k] (match (. tbl k)
              v (if (. keys 2)
                    (?. v [(table.unpack keys 2)] not-found)
                    v)
              not-found)
        tbl))

    (fn ?. [tbl keys not-found]
      (var res tbl)
      (var t tbl)
      (each [_ k (ipairs keys)]
        (match (. t k)
          v (set [res t] [v v])
          (do (set res not-found)
              (lua :break))))
      res)

The first one is recursive and a bit more verbose after code
generation, also constructs new table on each step which may be not
optimal. The second one iterates over table of keys directly, but
modifies local variables by setting, and uses nasty (lua :break). I'm
not sure which one is better, but the second one is prematurely
optimized. Maybe a less complex implementation can be made, but I
couldn't think of one yet. And maybe this should be a macro instead?

As for the name, I think ?. is good name, as it has similar semantics
embedded in it as for -?> or -?>>. Some fonts with ligatures have
support for this character combination as well. Other variants include
get-in, access-in, maybe others have some ideas too?

-- 
Best regards,
Andrey Listopadov