Dict.Nonempty

Similarly to Nonempty String and Nonempty List provides a type that is guaranteed to have at least one entry.

It consist of a key-value pair and a regular Dict

type NonemptyDict comparable v
    = NonemptyDict ( ( comparable, v ), Dict comparable v ) -- the constructor is opaque

hence it only allows comparable keys (same as Dict) and it exposes function head : NonemptyDict comparable v -> ( comparable, v ) that is guaranteed to succeed (no Maybe) and returns the key-value pair with the lowest key (Note on performance: Dict is always internally ordered to make the operations on it fast - that's why it allows only comparable key - so this head functions is inexpensive as well as in only adds one more comparison of keys)

Common usage

Has very similar functions to the regular Dict with a few exceptions

Dict.Nonempty.remove

returns Maybe

remove : comparable -> NonemptyDict comparable v -> Maybe (NonemptyDict comparable v)

Dict.Nonempty.fromList

has one more parameter (comparable,v) so that non-emptiness is guaranteed

fromList : ( comparable, v ) -> List ( comparable, v ) -> NonemptyDict comparable v

Why would you use this?

Not sure about that but I am using it for tracking errors when constructing a record out of user input (form) where key is field index. So it is something like a nonempty Array (Maybe error) but you don't have to deal with inserting the Nothings as Arrays can't have holes i.e.

Array.fromList ["a","b"]
  |> Array.set "d"            -- there is gap between last index (1 - where "b" is) and index we are trying to set
  |> Array.toList ["a","b"]   -- hence the the "d" is lost

The construction fails only when there is an error so there is a guarantee that there will be at least one entry and it should be possible to ask - give me only the first error that occurred (hence the above-mentioned function head)