# Shared vocabulary types

Some types are needed by nearly every package and app, and each would
otherwise invent its own. Geospatial libraries disagree on whether a
point is `[lat, lng]` or `[lng, lat]`, and payment libraries on whether
ten pounds is `10.00`, `"10.00"` or `1000`. When the language owns the
type, every package and every app uses the same one, and that whole class
of bug goes away. So the standard library defines a type wherever two
packages would otherwise each define their own, even when the heavy
operations on it live in a package or on another page.

- **Each one is defined once.** It has one construction that validates
  it, one shape on the wire and one shape in storage. Nothing else may
  give it a second.
- **Ambiguity is designed out.** Fields are named, never positional:
  `Point { latitude: ...  longitude: ... }`, never a pair. An axis order
  appears only in a conversion whose name says it: `geo.from_lng_lat(list)`,
  `geo.from_lat_lng(list)`, `geo.from_geojson`.
- **The types are global names.** Every standard type is a global name,
  as `Email` and `Instant` are, so you write `PostalAddress`, never
  something like `geo.Point`. Only the functions live in namespaces
  (`geo.`, `money.`, `list.`).
- **Your app names its own types for their purpose.** Don't declare your
  own `Address`. Name a type for what it is for and what it holds:
  `DeliveryStop { address: PostalAddress }`, `BillingContact`.
- **A standard type's name can't be reused.** Declaring a type with a
  standard type's name is the error `type.standard_clash`
  ([A release that adds a standard type](standard-type-clashes.md)),
  as reusing any built-in name is an error
  ([Values and types](values-and-types.md)). So the shape check below
  only has to catch equivalents under other names.
- **Packages and apps use them.** When a package's public types, or your
  app's own code, declare their own equivalent of a standard type, the
  compiler reports the error `type.standard_exists`, pointing at the
  standard type ([Packages use the standard types](packages-use-the-standard-types.md)).
  You can decline it with a reason. It is an error rather than a warning
  because Jadpo has no warnings
  ([Declining a judgement call](declining-a-judgement-call.md)).