R101

A function with impl has no output, a type JSON can't carry (z.date(), z.bigint(), z.void(), z.custom()), or a one-way .transform() on its input (kerf build, kerf check, kerf dev).

Fix: Use a codec: z.codec(z.iso.datetime(), z.date(), { decode, encode }). z.null() says a function returns nothing.

Status in kerf@0.1.0: reported by kerf check.

More: kerf docs rust.