Add documentation for abstract types

Author: MilesCranmer

docs/src/api.md

@@ -7,6 +7,18 @@ Quantity
 Dimensions
 ```
 
+There are also abstract types available. There are no required
+functions to build an interface, most relevant functions are
+defined on the abstract functions (including constructors).
+
+```@docs
+AbstractDimensions
+AbstractQuantity
+```
+
+Note also that the `Quantity` object can take a custom `AbstractDimensions`
+as input, so there is often no need to subtype `AbstractQuantity` separately.
+
 ## Utilities
 
 The two main general utilities for working

src/types.jl

@@ -3,9 +3,38 @@ import Tricks: static_fieldnames, static_fieldtypes
 const DEFAULT_DIM_BASE_TYPE = FixedRational{Int32,2^4 * 3^2 * 5^2 * 7}
 const DEFAULT_VALUE_TYPE = Float64
 
-abstract type AbstractQuantity{T,D} end
+"""
+    AbstractDimensions{R}
+
+An abstract type for dimension types. `R` is the type of the exponents of the dimensions,
+and by default is set to `DynamicQuantities.DEFAULT_DIM_BASE_TYPE`.
+AbstractDimensions are used to store the dimensions of `AbstractQuantity` objects.
+Together these enable many operators in Base to manipulate dimensions.
+This type has generic constructors for creating dimension objects, so user-defined
+dimension types can be created by simply subtyping `AbstractDimensions`, without
+the need to define many other functions.
+
+The key function that one could wish to overload is
+`DynamicQuantities.dimension_name(::AbstractDimensions, k::Symbol)` for mapping from a field name
+to a base unit (e.g., `length` by default maps to `m`). You may also need to overload
+`DynamicQuantities.constructor_of(::Type{T})` in case of non-standard construction.
+"""
 abstract type AbstractDimensions{R} end
 
+"""
+    AbstractQuantity{T,D}
+
+An abstract type for quantities. `T` is the type of the value of the quantity,
+and `D` is the type of the dimensions of the quantity. By default, `D` is set to
+`DynamicQuantities.DEFAULT_DIM_TYPE`. `T` is inferred from the value in a calculation,
+but in other cases is defaulted to `DynamicQuantities.DEFAULT_VALUE_TYPE`.
+It is assumed that the value is stored in the `:value` field, and the dimensions
+object is stored in the `:dimensions` field. These fields can be accessed with
+`ustrip` and `dimension`, respectively. Many operators in `Base` are defined on
+`AbstractQuantity` objects, including `+, -, *, /, ^, sqrt, cbrt, abs`.
+"""
+abstract type AbstractQuantity{T,D} end
+
 """
     Dimensions{R} <: AbstractDimensions{R}