We define the DiffTreeVertex protobuf message like so:
```protobuf
message DiffTreeVertex {
int32 diff_vertex_id = 1;
oneof diff_term {
DeletedTerm deleted = 2;
InsertedTerm inserted = 3;
ReplacedTerm replaced = 4;
MergedTerm merged = 5;
}
}
```
This is turned into two Haskell types, a toplevel `DiffTreeVertex` type
and a `DiffTreeVertexDiffTerm` type that represents the anonymous
`oneof` type. Said types looked like so:
```haskell
data DiffTreeVertexDiffTerm
= Deleted (Maybe DeletedTerm)
| Inserted (Maybe InsertedTerm)
| Replaced (Maybe ReplacedTerm)
| Merged (Maybe MergedTerm)
deriving stock (Eq, Ord, Show, Generic)
deriving anyclass (Proto3.Message, Proto3.Named, NFData)
```
This is the wrong representation, as it neglects to account for the
fact that options could be added to the `diff_term` stanza. A sum type
does not provide enough constructors to handle the case of when none
of `deleted`, `inserted`, `replaced` etc. is `Just` anything. A more
correct definition follows:
```haskell
data DiffTreeVertexDiffTerm = DiffTreeVertexDiffTerm
{ deleted :: Maybe DeletedTerm
, inserted :: Maybe InsertedTerm
, replaced :: Maybe ReplacedTerm
, merged :: Maybe MergedTerm
}
```
This patch applies the above change, using `-XPatternSynonyms` to
provide backwards-compatible API shims. Though this changes JSON
output format (through the `ToJSON` instance), it should have no
bearing on backwards compatibility in protobuf objects, since there is
no way to consume diff trees as protobufs as of this writing.
Fixes#168.
