Loop design

This is a rough WIP draft to stimulate the discussion. ☺

Syntax:

(loop ?INTO BIND ITER ?KEY VAL)

where

• INTO is either {}, [], existing TABLE, or (SYM ?VAL) for accumulation
• BIND is either SYM or (SYM SYM…) to be bound by ITER
• ITER is either an iterator function or [MIN MAX ?STEP] for numeric iteration

P.S. Too bad loop cannot be named for. ☹

• Forms

  Old: for, each, collect, icollect, fcollect, accumulate, faccumulate

  New: loop

• Design

  Design goals in no particular order:

  • Use one form instead of an incomplete zoo of forms with strange names.
  • Use parentheses for multiple bindings, to match the rest of Fennel.
  • Make it immediately clear whether the loop collects, accumulates, or performs side effects.
  • Enable all input/output combinations.

    See the motivating example.

• Limitations

  No guard (or until) yet. To pull its weight, loop would have to elegantly replace while as well; otherwise, one can simply type if or when in the loop.

• Motivating example

  Create a set of all integers in the given interval.

  This is currently impossible with fcollect because it produces a sequence.

  (loop {} i [1 10] i true)
  

• Other examples
  • Create a copy of a sequence with every element doubled

    Before:

    (icollect [_ elm (ipairs lst)] (* elm 2))
    

    After:

    (loop [] (_ elm) (ipairs lst) (* elm 2))
    

  • Sum a sequence

    Before:

    (accumulate [sum 0 _ elm (ipairs tbl)] (+ sum elm))
    

    After:

    (loop (sum 0) (_ elm) (ipairs tbl) (+ sum elm))
    

    Note. (sum 0) would read better as [sum 0], like in let, but that would be ambiguous.

  • Side effecting loop

    Before:

    (each [idx (pairs tbl)] (proc idx))
    

    After:

    (loop idx (pairs tbl) (proc idx))