[transducers] Transducers crash when source is an empty string

Most transducers take a source sequence as a final argument. Since strings are iterable, so you can do this:

[...tx.filter(x => true, "abc")]
"abc"

However, if you provide an empty string, they will crash:

[...tx.filter(x => true, "")]
TypeError: tx.filter(...) is not iterable

This is obviously because the transducer functions do a truthiness test on their final argument to determine whether a source was provided:

export function map<A, B>(fn: Fn<A, B>): Transducer<A, B>;
export function map<A, B>(fn: Fn<A, B>, src: Iterable<A>): IterableIterator<B>;
export function map<A, B>(fn: Fn<A, B>, src?: Iterable<A>): any {
    return src
        ? iterator1(map(fn), src)
        : (rfn: Reducer<any, B>) => {
              const r = rfn[2];
              return compR(rfn, (acc, x: A) => r(acc, fn(x)));
          };
}

This could be corrected by replacing src with isIterable(src) (using @thi.ng/checks). This would incur the additional work of looking up Symbol.iterator, but it would seem to be more correct. I can’t think of a case where this would be a breaking change (i.e. values for src that work now but are not iterable by that test).

@postspectacular I’m willing to PR if you think this is actionable. I understand there’s a lot of branch work going on, so whatever you think best.

A workaround for the time being is to wrap the string argument in [...s].

[postspectacular]

Thank you for the in-depth analysis, @gavinpc-mindgrub! In this case I’d opt (like I guess you do too) for consistency, even if it means breaking the current behavior for top-level strings (incl. empty ones).

So I’d say we treat top-level strings as atoms (unflattenable, just like nested strings) and update the body of flattenWith to something like:

return isIterable(src)
  ? iterator(flattenWith(fn), isString(src) ? <any>[src] : src)
  : (rfn: Reducer<any, T>) => { ... }

With that change we get:

> [...tx.flatten("")]
[ '' ]
> [...tx.flatten([""])]
[ '' ]

> [...tx.flatten("abc")]
[ 'abc' ]
> [...tx.flatten(["abc"])]
[ 'abc' ]

[postspectacular]

@gavinpc-mindgrub no pressure pls! take your time!