# `GeoGenius.Providers.Fields`
[🔗](https://github.com/agoodway/geo_genius/blob/main/lib/geo_genius/providers/fields.ex#L1)

Coerces a raw payload value -- read out of a GeoJSON feature's properties
or a CSV row's cells -- into either a present string or `nil`.

Every provider pulls a code, a name, an alias, or an external code's value
out of a payload the same way, and needs the same answer for "is this cell
actually present": a value is present when it is a non-blank string or a
number, absent otherwise. Before this module, `GeoGenius.Providers.GeoJSON`
passed an empty string through and re-checked `value in [nil, ""]` at every
call site, while `GeoGenius.Providers.CSV` folded `""` into `nil` up front
and checked `is_nil/1`. Same concern, two contracts -- a third-party
provider author copying either exemplar got a different answer depending
on which one they read.

# `presence`

```elixir
@spec presence(term()) :: String.t() | nil
```

Returns a trimmed string for a present value, `nil` otherwise.

A binary is trimmed and turned to `nil` if the result is empty; an integer
or float is stringified; anything else -- a list, a map, a boolean -- is
`nil` rather than raising, since a payload value's shape is the source's
to guarantee, not this function's.

Trimming matters most for a code: `area_key` is
`<authority>:<area_type>:<code>`, a stable, public identifier, so a code
cell padded with spaces -- common in a fixed-width FIPS or ZIP export --
must not carry that padding into it. Without trimming, a later re-import
of the same source with the padding stripped would produce a different
`code` and therefore a different `area_key`, creating a duplicate area
instead of updating the existing one, and orphaning any `relation` rows
keyed on the old code. A value that is entirely whitespace is treated the
same as an absent one. What a provider does with an absent code is its own
call -- `GeoGenius.Providers.CSV` skips the row, since a manifest may name
a column its source fills only sometimes, while
`GeoGenius.Providers.SimpleMaps.Rows` fails it, since its source fills
every one of those columns in every row -- and a blank name or alias is
simply not added.

---

*Consult [api-reference.md](api-reference.md) for complete listing*