Flat Codec
flat@1.0 is a simple codec device to flatten/unflatten object paths since HTTP headers and body cannot handle nested object structures. It's internally used by httpsig@1.0 to resolve object paths.
Codec devices have to and from methods to encode and decode, but they are not exposed to external URLs. We can create custom methods for our custom device to expose them.
-export([ flat_to/3, flat_from/3 ]).
flat_to(Msg1, Msg2, Opts) ->
Body = maps:get(<<"body">>, Msg1),
OBJ = dev_codec_json:from(Body),
FLAT = dev_codec_flat:to(OBJ),
JSON = dev_codec_json:to(FLAT),
{ok, JSON}.
flat_from(Msg1, Msg2, Opts) ->
Body = maps:get(<<"body">>, Msg1),
OBJ = dev_codec_json:from(Body),
FLAT = dev_codec_flat:from(OBJ),
JSON = dev_codec_json:to(FLAT),
{ok, JSON}.One thing to note is that HTTP header keys cannot contain /, so if you ever need to send keys with / you need to push them into multipart body. We will handle this in the next chapter with Httpsig Codec. Flat codec only handles map structures. List structures are handled by Structured Codec. Also Flat Codec is an intermediary step used by Httpsig Codec, so values also have to be strings with dev_codec_flat:to.
const cases = [
{ a: { b: "v" } },
{ a: "v", b: { c: "v2", d: "v3" } },
{ a: { b: { c: { d: "v" } } } },
]
for (const v of cases) {
const { body } = await hb.post({
path: "/~mydev@1.0/flat_to",
body: JSON.stringify(v),
})
console.log(JSON.parse(body))
}{ a: { b: "v" } }->{ "a/b": "v" }{ a: "v", b: { c: "v2", d: "v3" } }->{ a: "v", "b/c": "v2", "b/d": "v3" }{ a: { b: { c: { d: "v" } } } }->{ "a/b/c/d": "v" }
const cases = [
{ "a/b": "v" },
{ a: "v", "b/c": "v2", "b/d": "v3" },
{ "a/b/c/d": "v" },
]
for (const v of cases) {
const { body } = await hb.post({
path: "/~mydev@1.0/flat_from",
body: JSON.stringify(v),
})
console.log(JSON.parse(body))
}Running Tests
You can find the working test file for this chapter here:
Run tests:
yarn test test/codec-flat.test.js