> In Clojure I use threading macros often, but one thing that always
> bothers me is that the argument position is implicit (for most of the
> family of macros).  It is ok while working on it, but once I leave the
> coding for a while and then look at the code, it's extremely confusing.

+1

> I ended up always marking the inserted argument position with commas,
> which are treated as whitespaces in Clojure.
>
> srfi-197 allows to use <> as the insertion points, and I can always use
> it even the argument position is the last position:
>
>    (chain (a b) (c d <>) (e f <>))

This is a nice notation.

I've always found the <> marker a bit confusing since it stands for "not
equal" in Pascal and many other languages. But it's inherited from SRFI
26 `cut` and `cute` which I guess set a strong precedent. Starting from
scratch, I would prefer underscore which is a placeholder marker in many
languages:

(chain (a b) (c d _) (e f _))

> And I wonder if we gain anything by allowing <> in the last position to
> be omitted.
>
> Sure it may shorten the code a bit.  However, the spec is effectively
> the same as  "show the argument position with <> except when it's the
> last position, in which case you can omit it."    It looks somewhat an
> arbitrary and irregular rule.

+1

> Code is read far more times than written.  Saving a few keystrokes
> trading off readability doesn't seem a good deal.
>
> Is everyone comfortable with implicit argument?  Certainly, when I see
> (cons "x"), I can see there's an implicit argument, since the fact that
> cons takes two arguments is engraved in my brain (the same circuit fires
> when I'm reading Haskell code).
> But it isn't always necessary the case, e.g. (list 'a 'b)

Good point.