nil :time/duration converted to empty string by malli.experimental.time.transform/time-transformer

[devurandom]

It appears that malli.experimental.time.transform/time-transformer encodes nil values for schema [:maybe :time/duration] as "" empty strings:

(require '[malli.core :as ma]
         '[malli.experimental.time :as met]
         '[malli.experimental.time.transform :as mett]
         '[malli.registry :as mr])
(mr/set-default-registry!
  (mr/composite-registry
    (ma/default-schemas)
    (met/schemas)))
(ma/encode [:maybe :time/duration] nil mett/time-transformer)
;=> ""

I would expect that it preserves nil values.

This is particularly problematic, since it can then not decode the "" empty string value to an actual java.time.Duration:

(ma/decode [:maybe :time/duration] "" mett/time-transformer)
;=> ""

And thus this also fails validation; sadly with a not very helpful explanation:

(require '[malli.error :as me])
(me/humanize (ma/explain [:maybe :time/duration] ""))
;=> ["unknown error"]

Compare that to:

(ma/decode [:maybe :time/duration] nil mett/time-transformer)
;=> nil
(ma/decode [:maybe :time/duration] "P1D" mett/time-transformer)
;=> #object[java.time.Duration 0x1ab268bd "PT24H"]

This appears similar to #1204.

[opqdonut]

Yes, this is a clear bug. We'll try to have some time soon to look at it!