The Lowly Assert: Roundtrips

Ryan James Spencer

Data "roundtrips" when it goes from one value, to another, and back to the same value without any data loss, gain, or corruption. If you write code, you have probably roundtripped JSON, YAML, TOML, or some other serialization format in your time. You have also probably written versions of functions that do a similar 'cycle' of some data. Any time you care about data being the same after it's gone through the ringer, you want to write a roundtrip test.

Pretend we have a system where data comes in as JSON. We slurp up that JSON into a type using serde (rust's idiomatic, type-driven serialization + deserialization library). That data might later go onto being a type unrelated to JSON, so we might write some From instances. This will be our adaptive layer so we can keep the shape of the JSON and our core types distinct. I mention this approach briefly in my post "Safely Shape Code with Curtains". The From instance would normally be trivial, but we don't want the JSON layer and the core types to look the same, do we? That would make the point of the JSON types moot:

struct JsonType {
  names: Option<Vec<String>>,
  ids: Vec<i64>,

struct CoreType {
  names: Vec<String>,
  ids: Vec<i64>,

impl From<JsonType> for CoreType {
  fn from(x: JsonType) -> Self {
    Self {
      names: x.names.unwrap_or(vec![]),
      ids: x.ids,

impl From<CoreType> for JsonType {
  fn from(x: CoreType) -> Self {
    Self {
      names: Some(x.unwrap())
      ids: x.ids,

We could test each direction in isolation, but that would mask the actual mistake here. Can you spot it? The roundtrip test in a property based testing context would find the failure quite quickly. I'll do it by hand here to demonstrate the mistake:

let beg = JsonType {
  names: None,
  ids: vec![1,2,3],
let roundtrip_fwd: CoreType = expected.into();
let end: JsonType = roundtrip_fwd.into();
assert_eq!(beg, end);

When the data comes back to the JSON layer, unless we tell serde that empty vectors are always Nones for this field, we've now lost information. Clients might care a lot that their POST of some JSON for creating an entity in this make-believe system is non-symmetric. Developers might be going between the core and the JSON types regularly, and they may even be using the JSON types to write to disk, too, which would mean what was passed up from the client is now not the same as what is stored.

We can extrapolate this sort of information loss or corruption to other conversions. If you author an automatic code formatter, say prettier, gofmt, mix fmt, rustfmt, and so on, you'd want to make sure that any time you save a file and the formatter runs that your code is still the same code, semantically, as it was before saving the file. Although things might possibly look the same by eye, it could be another program entirely when run.

Food for thought

A quick refresher on functions.

  • Functions can be seen as mappings from one type of value to another
  • All possible values that can go into our mapping are known as the domain of a function
  • The set of all possible values our mapping can produce is called the codomain
  • The set of all values the mapping realistically produces is called the range or image

Ok, onto the concepts with fancy names:

  • An injective mapping is when a mapping goes from values in the domain to unique values in the codomain.

  • A surjective mapping is when a mapping goes from values in the domain to every value in the codomain, even if some mappings overlap.

  • A bijective mapping is simultaneously injective and surjective which means every value in the domain maps to every value in the codomain exactly once.

Why does this matter?

Bijective mappings give you an inverse function for free. If you are a value in the codomain and you know the mapping is bijective, then you can be sure that there must be one, and only one, value where you came from in the domain. One could prove bijections using classical means but we don't need to for production usage. Instead, it suffices to simply show the action going forwards and backwards.