Add documentation for StructuralContext to README

Author: TotalVerb

README.md

@@ -106,3 +106,167 @@ JSON.lower(p::Point2D) = [p.x, p.y]
 
 Define a custom serialization rule for a particular data type. Must return a
 value that can be directly serialized; see help for more details.
+
+### Customizing JSON
+
+Users may find the default behaviour of JSON inappropriate for their use case. In
+such cases, JSON provides two mechanisms for users to customize serialization. The
+first method, `JSON.Writer.StructuralContext`, is used to customize the cosmetic
+properties of the serialized JSON. (For example, the default pretty printing vs.
+compact printing is supported by provided two different `StructuralContext`s.)
+Examples of applications for which `StructuralContext` is appropriate include:
+particular formatting demands for JSON (maybe not in compliance with the JSON
+standard) or JSON-like formats with different syntax.
+
+The second method, `JSON.Serializations.Serialization`, is used to control the
+translation of Julia objects into JSON serialization instructions. In most cases,
+writing a method for `JSON.lower` (as mentioned above) is sufficient to define
+JSON serializations for user-defined objects. However, this is not appropriate for
+overriding or deleting predefined serializations (since that would globally affect
+users of the `JSON` module and is an instance of dangerous
+[type piracy](https://docs.julialang.org/en/v1/manual/style-guide/index.html#Avoid-type-piracy-1)).
+For these use-cases, users should define a custom instance of `Serialization`.
+An example of an application for this use case includes: a commonly requested
+extension to JSON which serializes float NaN and infinite values as `NaN` or `Inf`,
+in contravention of the JSON standard.
+
+Both methods are controlled by the `JSON.show_json` function, which has the following
+signature:
+
+```
+JSON.show_json(io::StructuralContext, serialization::Serialization, object)
+```
+
+which is expected to write to `io` in a way appropriate based on the rules of
+`Serialization`, but here `io` is usually (but not required to be) handled in a
+higher-level manner than a raw `IO` object would ordinarily be.
+
+#### StructuralContext
+
+To define a new `StructuralContext`, the following boilerplate is recommended:
+
+```julia
+import JSON.Writer.StructuralContext
+[mutable] struct MyContext <: StructuralContext
+    io::IO
+    [ ... additional state / settings for context goes here ... ]
+end
+```
+
+If your structural context is going to be very similar to the existing JSON
+contexts, it is also possible to instead subtype the abstract subtype
+`JSONContext` of `StructuralContext`. If this is the case, an `io::IO` field (as
+above) is preferred, although the default implementation will only use this
+for `write`, so replacing that method is enough to avoid this requirement.
+The default implementations will also use an internal 
+
+The following methods should be defined for your context, regardless of whether it
+subtypes `JSONContext` or `StructuralContext` directly. If some of these methods
+are omitted, then `CommonSerialization` cannot be generally used with this context.
+
+```
+# called when the next object in a vector or next pair of a dict is to be written
+# (requiring a newline and indent for some contexts)
+# can do nothing if the context need not support indenting
+JSON.Writer.indent(io::MyContext)
+
+# called for vectors/dicts to separate items, usually writes ","
+# unless this is the first element in a JSON array
+# (default implementation for JSONContext exists, but requires a mutable bool
+#  `first` field, and this is an implementation detail not to be relied on; 
+#  to define own or delegate explicitly)
+JSON.Writer.delimit(io::MyContext)
+
+# called for dicts to separate key and value, usually writes ": "
+JSON.Writer.separate(io::MyContext)
+
+# called to indicate start and end of a vector
+JSON.Writer.begin_array(io::MyContext)
+JSON.Writer.end_array(io::MyContext)
+
+# called to indicate start and end of a dict
+JSON.Writer.begin_object(io::MyContext)
+JSON.Writer.end_object(io::MyContext)
+```
+
+For the following methods, `JSONContext` provides a default implementation,
+but it can be specialized. For `StructuralContext`s which are not
+`JSONContext`s, the `JSONContext` defaults are not appropriate and so are
+not available.
+
+```julia
+# directly write a specific byte (if supported)
+# default implementation writes to underlying `.io` field
+# note that this enables JSONContext to act as any `io::IO`,
+# i.e. one can use `print`, `show`, etc.
+Base.write(io::MyContext, byte::UInt8)
+
+# write "null"
+# default implementation writes to underlying `.io` field
+JSON.Writer.show_null(io::MyContext)
+
+# write an object or string in a manner safe for JSON string
+# default implementation calls `print` but escapes each byte as appropriate
+# and adds double quotes around the content of `elt`
+JSON.Writer.show_string(io::MyContext, elt)
+
+# write a new element of JSON array
+# default implementation calls delimit, then indent, then show_json
+JSON.Writer.show_element(io::MyContext, elt)
+
+# write a key for a JSON object
+# default implementation calls delimit, then indent, then show_string,
+# then seperate
+JSON.Writer.show_key(io::MyContext, elt)
+
+# write a key-value pair for a JSON object
+# default implementation calls show key, then show_json
+JSON.Writer.show_pair(io::MyContext, s::Serialization, key, value)
+```
+
+What follows is an example of a `JSONContext` subtype which is very similar
+to the default context, but which uses `None` instead of `null` for JSON nulls,
+which is then generally compatible with Python object literal notation (PYON). It
+wraps a default `JSONContext` to delegate all the required methods to. Since
+the wrapped context already has a `.io`, this object does not need to include
+an `.io` field, and so the `write` method must also be delegated, since the default
+is not appropriate. The only other specialization needed is `show_null`.
+
+```julia
+import JSON.Writer
+import JSON.Writer.JSONContext
+mutable struct PYONContext <: JSONContext
+    underlying::JSONContext
+end
+
+for delegate in [:indent,
+                 :delimit,
+                 :separate,
+                 :begin_array,
+                 :end_array,
+                 :begin_object,
+                 :end_object]
+    @eval JSON.Writer.$delegate(io::PYONContext) = JSON.Writer.$delegate(io.underlying)
+end
+Base.write(io::PYONContext, byte::UInt8) = write(io.underlying, byte)
+
+JSON.Writer.show_null(io::PYONContext) = print(io, "None")
+pyonprint(io::IO, obj) = let io = PYONContext(JSON.Writer.PrettyContext(io, 4))
+    JSON.print(io, obj)
+    return
+end
+```
+
+The usage of this `pyonprint` function is as any other `print` function, e.g.
+
+```julia
+julia> pyonprint(stdout, [1, 2, nothing])
+[
+    1,
+    2,
+    None
+]
+
+julia> sprint(pyonprint, missing)
+"None"
+```