Add get_tree and set_tree methods to irmin-graphql server

[zshipko]

Unfortunately there isn't a good way to model the tree input arguments as recursive objects (as I had hoped):

{
    "a": {"b": {"c": "123"}}, 
    "foo": "bar"
}

Instead, we have to use a flat array of objects:

[
    {"key": "a/b/c", "value": "123"}, 
    {"key": "foo", "value": "bar"}
]

Despite this one minor flaw, I think this is a pretty nice way of supporting read/write transactions in irmin-graphql!

[samoht]

Unfortunately there isn't a good way to model the tree input arguments as recursive objects

Can you detail why it's not possible? I am fine to merge that PR but I would like to understand the design constraints :-)

[zshipko]

There is just no way to construct a recursive (or mutually recursive) input argument. For example, my initial try:

type tree = {
  key: string;
  tree: tree option;
  value: string option;
  metadata: string option;
 }

let rec tree () = Schema.Arg.(
  obj "Tree"
    ~fields:[
       arg "key" ~typ:(non_null string);
       arg "tree" ~typ:(tree ());
       arg "value" ~typ:string;
       arg "metadata" ~typ:string;
    ]
    ~coerce:(fun key tree value metadata -> {key; tree; value; metadata})
)

creates a stack overflow.. Also, making tree lazy would raise Lazy.Undefined in this case.

GraphQL really only works with recursion of a known depth.¹ which means the return value from get_tree would pose a problem as well.

This final implementation seems to be more in-line with the way GraphQL is meant to be used, so even if there are workarounds to the infinite loop issue, I think it makes sense to stick with the array-based solution regardless.

¹ graphql/graphql-spec#91 (comment)

[samoht]

Hum ok I see. Maybe we can allow to pass "arbitrary" JSON trees and consider them as Json_tree contents? That seems a bit annoying to serialise a nice tree into a collection of key/values pairs. But I guess it's fine as a first step.

[zshipko]

Yeah, it would definitely be possible to pass trees as JSON encoded strings — I will spend some time today to see if that’s a better approach.