Investigate where `lst` argument should go

[codereport]

I recently got this issue request:

• adjacent-map arguments order #9

which was to fix the argument order on adjacent-map to be consistent with Racket's builtin map. However, when playing around with Racket I noticed many other functions like take and drop have lst as the first argument. Opening this issue to determine what the best practice should be. At the end of the day, Racket has the threading module with thread-first, thread-last and the _ placeholder, but still want to know the motivation for differences in builtin Racket functions.

(require threading)

(define l '(1 2 3))

(~>> l
    (reverse _)
    (take _ 2)
    (drop _ 1))

[codereport]

@filiplajszczak Let me know if you have thoughts.

[filiplajszczak]

I think that difference between map family of procedures and take family is that first kind is higher order procedure that accepts other procedures in the first place and the second kind accepts position.

Compare

• (map proc lst ...+)
• (andmap proc lst ...+)
• (ormap proc lst ...+)
• (for-each proc lst ...+)
• (foldl proc init lst ...+)
• (foldr proc init lst ...+)
• (findf proc lst)
• (filter-map proc lst ...+)
• (append-map proc lst ...+)

with

• (list-ref lst pos)
• (list-tail lst pos)
• (take lst pos)
• (drop lst pos)
• (split-at lst pos)
• (take-right lst pos)
• (drop-right lst pos)
• (split-at-right lst pos)

[bhrgunatha]

I think it's just due to being a loooong running project.

I believe Racket started, with Scheme as its base, to research and teach programming languages and started . It inherited Scheme's procedures - including the argument order. That's why for example map, filter, foldl, foldr et.c have the procedures at heir first argument - they were inherited from Scheme.

As time progressed, more and more useful utility procedures for lists were developed, like take, drop, split-at, group-by, ... Since they all have different behaviour, but they all act on lists, it makes a lot of sense to have the list as their first argument. It also makes sense to extend that, as a standard, to other data structures and their procedures, so their data structure are also the first argument too.

You can't simply change the order of arguments for those older, well-established, ancestral procedures. It would just break so much existing code (including internal to Racket!), it would be madness.