A Critique of the Cap'n Proto Schema Language.

25 Jun 2019

Recently on the Cap’n Proto mailing list, someone asked for feedback on an initial design for an Elm implementation. I chimed in with some advice and some lessons learned from writing the Haskell implementation and working with the Go implementation. One thing I mentioned was this:

I suggest optimizing for “well-written” schema, and making peace with the fact that certain constructs may generate unpleasant APIs.

David Renshaw asked for details on what bits were difficult to map. I talked a bit about difficulties with nested namespaces, and mentioned that I had a longer critique in mind. This blog post is that critique.

It’s mostly a critique of the schema language itself – not the wire format, not the RPC protocol. There are things worth saying about those, but to keep things at least somewhat focused, I’m just going to stick to the schema language proper.

Even so, this is one of my longer posts; get comfortable.

Design Principles

Some of the critiques below have some motivating design principles, which I’ll call out up front:

Name Spaces

In the Cap’n Proto schema language, each struct or interface definition introduces its own namespace. So you can define:

struct Foo {
    struct Bar {

struct Baz {
    struct Bar {

Which in C++, will generate types Foo, Foo::Bar, Baz, and Baz::Bar. Rust and OCaml both do something similar.

In languages without any kind of intra-module namespaces, code generators usually come up with some kind of naming convention as a substitute. Both the C and Go implementations use underscores to separate name space segments, e.g. Foo_Bar. The Haskell implementation uses the single quote, like Foo'Bar.

The length of identifiers like this gets out of hand quickly. Here’s a particularly nasty example, from the output of the Go code generator on Sandstorm’s web-session.capnp:


An alternative idea is to generate multiple modules, but this can introduce cyclic dependencies, and the implementation is much more complex at best. In some cases it is actually impossible.

Furthermore, as Kenton pointed out, having things properly namespaced in the target language doesn’t actually make this much nicer. He provided this example line of code from Cloudflare Workers:

case PipelineDef::Stage::Worker::Global::Value::JSON:

Nested namespaces violate the “optimize for the call site” principle; they are sometimes convenient at the definition site, but lead to APIs that are annoying to work with in actual code. I generally recommend just not using nested namespaces in schema code, and consider them to be a misfeature.

Implicitly Named Entities

Relatedly, there are a few constructs which the schema author doesn’t have to give a name to, but which (for most backends) generate named entities:

All of these things feel quite natural to work with at the schema level, but then the code generator is left having to come up with a name for you, and normally it doesn’t have a heck of a lot to work with.

Anonymous Unions

Cap’n Proto includes support for tagged unions/variants. However, unlike the variants in ML-family languages (e.g. Haskell, OCaml, Elm), Cap’n Proto unions are not first class types. Instead, they are fields of structs, which can also be “anonymous.”

To someone coming from an ML-like language, The fact that Cap’n Proto’s unions aren’t first class types is a bit odd. The official docs make a good justification for this, and I think it is a-priori a reasonable design. Nonetheless, the fact that the feature maps poorly to existing languages is a real downside.

I have a suggestion on how to make this better, which I think avoids losing the good points of the current design. Split the use of unions into two cases:

  1. Structs which are entirely one big union.
  2. Structs which contain fields outside the union.

The first case is very common, and would be worth making more ergonomic. I would suggest that rather than writing:

struct Foo {
    union {

You should just be able to write:

union Foo {

In the second case I would disallow anonymous unions – if there are other fields, you have to give them a name.

You could still add common fields to a top-level union; just change it to a struct, move all of the existing variants into a (named) union field, and add your fields per usual.

In languages where it is most natural, this would allow unions to cleanly map to variants, and structs to records. The code generator would also not have to invent field names for anonymous unions, and you don’t lose any of the advantages of the current approach.

The Haskell implementation currently looks for structs which are one big anonymous union so that it can drop the outer wrapper. By itself this isn’t too complicated, but it was yet another thing, and they add up.


As the official documentation describes, groups are primarily useful in conjunction with unions. They allow more than one field as part of the same union variant, and they can be a little more self documenting, since the members of that variant can be given names.

But stand-alone groups are far less compelling, and present challenges for code generators. They’re another entity that (usually) needs to have a name generated for it, and this is again going to end up being longer than ideal.

I think coupling groups to unions would make this a better. There was actually a surprising amount of complexity in avoiding name collisions between the full generality of groups and anonymous unions.

Parameters And Results

The huge Go identifier mentioned earlier comes from this (comparatively reasonable) schema definition:

interface WebSession @0xa50711a14d35a8ce extends(Grain.UiSession) {
    # ...
    interface WebSocketStream {
        sendBytes @0 (message :Data);
        # ...

At the definition site, we just name the method and its arguments (and results, if any). But at the protocol level, arguments and results are their own structs, and the schema compiler generates a struct type for each parameter list, and each result list. The name websession.WebSession_WebSocketStream_sendBytes_Results_Future refers to something that doesn’t even carry any data – sendBytes has no results!

It’s possible to instead define the parameter & result types explicitly, e.g:

interface WebSocketStream {
    sendBytes @0 SendBytesReq -> SendBytesResp;

struct SendBytesReq {
    message :Data;

struct SendBytesResp {

It’s tempting to argue that in this case, you don’t gain much – the name I’ve chosen for the struct is no more informative that the auto-generated one, and the schema is much more verbose. It’s not like I picked an especially lousy example, either; most of the grpc examples I’ve seen (where this style is required) end up with similarly boring names.

But forcing the schema author to do this also forces them to deal with the namespacing problem themselves, so the code generator can drop not one but two levels of nesting. In the Haskell implementation this makes the type names significantly shorter; in the original you have:

data WebSocketStream'sendBytes'params = WebSocketStream'sendBytes'params
    { message :: Data

whereas with the separately declared struct you get:

data SendBytesReq = SendBytesReq
    { message :: Data

The resulting identifier is less than half the length of the original.

Identifier Conventions

Cap’n Proto currently uses case to distinguish between types and constants/annotations, but some programming languages (including both Haskell and Go) use case to distinguish other things. This can cause trouble when the code generator has to modify a name to conform with the target language.

For example, in persistent.capnp there is both a type called Persistent and an annotation called persistent. Go uses capitalization to distinguish between public and private identifiers, so if left to its own devices, the Go backend capitalizes persistent, which causes a name collision. The Haskell implementation tries to be smarter about this, but it results in significant implementation complexity that could have been avoided by a little thought during design.

We could solve this problem by having all identifiers use the same case conventions (standardizing on smallCamelCase), and have them live in the same namespace, so that anything that might cause a name collision in the target language will also cause a collision in the schema language.

Default Field Values

Finally something other than names!

Cap’n Proto allows the schema author to define default values for fields, if they are not otherwise set. So if you write:

struct Foo {
    bar @0 :Int32;
    baz @1 :Text = "hello";

If a message does not have the baz field set, it will be treated as being the string "hello".

I strongly argue for disallowing these for pointer types (strings, lists, structs…). They are already rarely used. Between the schema that ship with Cap’n Proto and the ones in Sandstorm – over nine thousand lines of schema source – the feature is used exactly twice, both to set the default value of a parameter to the empty string.

For the Haskell implementation, this feature actually introduced a significant amount of complexity – enough that, given how infrequently the feature is actually used – I just decided not to support it. The Haskell implementation just prints a warning to the console and ignores custom defaults for pointer fields.

The usual way to implement this is to modify the message on first access, which for immutable messages doesn’t work. I considered just returning a constant in the immutable case, but its storage would be in a different message than the pointer the user followed to it, which could lead to surprises if the user tries to fish out the underlying segment from the new value. There’s probably a decent design to be found here, but after not coming up with a great answer quickly, I eventually decided it was a waste of time.

Default values for non-pointer fields are much less of an issue – they are implemented by storing fields xor’ed with their default values, so the immutability issue doesn’t come up. For Haskell it’s totally fine.

But they do cause a problem for Go when using the pogs package, since by default Go zeros the memory for structs, so if you have a custom default value in a Cap’n Proto struct declaration, The default value of the Go struct will disagree. It’s not an issue when you’re working with messages in the wire format, since you’re not using Go structs directly, but when performance permits Go structs are nicer to work with.

Supposedly this was also the motivation for dropping custom default values from Protobufs in version 3.

Kenton mentioned on the mailing list that he uses custom defaults for non-pointer fields “all the time,” particularly booleans. My own sense was that these were more common as well, but I decided to do some measurements, again on the core & Sandstorm schema. Here’s what I found:

This leaves only 5 uses of the feature that actually do anything. The five cases are:

The first two could be solved by changing release to retain and flipping the semantics. Kenton had expressed concern about double-negatives making things hard to read, and my original intent in doing these measurements was to collect a list of such fields, pull out a thesaurus, and try to get a sense of how hard it actually was to avoid that. I was surprised to find that I didn’t get a sample size big enough to tell.

I had previously been on the fence as to whether this feature was worth the awkwardness it introduced for Go, but given how little it seems to be used in reality, I’m more solidly of the opinion that it should have been left out. Given Kenton’s experience report I have to assume it’s used more inside Cloudflare, but I no longer believe the feature carries its weight.


I would make a few changes to the way inheritance of interfaces works in the schema language:

First, signal an error if an interface declares method with the same name as a method in one of its parents. Right now, handling this correctly requires more name spacing logic in the code generator, and I know at least the Go implementation just punts on this – which is probably the right thing to do, given that I’ve never seen a schema that actually does this.

I would also drop multiple inheritance. This would get rid of the diamond problem, which is a source of API design challenges. One that came up in the context of the Haskell implementation:

Haskell has good support for message-passing concurrency, and it would be fairly natural to map the receive end of objects to a channel that you pull calls out of, so for an interface like this:

interface Foo {
    bar @0 BarReq -> BarResp;
    baz @1 BazReq -> BazResp;

you would get code like:

data Foo'call
    = Foo'Bar BarReq (BarResp -> IO ())
    | Foo'Baz BazReq (BazResp -> IO ())

And then a server would handle these by doing something like:

handleFoo calls = forever $ do
    call <- recv calls
    case call of
        Foo'Bar req reply -> do
            reply returnValue
        Foo'Baz req reply -> do

This is nice for a lot of reasons; it means the library doesn’t have to care as much abount managing the lifetimes of servers/threads, the API is small and transparent, it’s easy to have multiple threads service the same object, or fork off a thread to handle something and reply later, etc. It affords a lot of flexibility and is fairly simple to do.

Adding single inheritance to this situation isn’t too hard; just add a super variant to the call type. Say we have:

interface Quux extends (Foo) {
    echo @0 EchoReq -> EchoResp;

Then this becomes:

data Quux'call
    = Quux'Echo EchoReq (EchoResp -> IO ())
    | Quux'super Foo'call

Multiple inheritance gets awkward though; you could add variants Quux'super'Foo and Quux'super'Bar, but you can now potentially have more than one path to the same object. It’s probably possible to find a reasonable solution here, but I don’t think multiple inheritance buys you enough for it to be worth imposing the burden on implementers (and users of the resulting API).

I ended up mapping interfaces to type classes instead, but if I were to do it over again I’d do what I describe above; type classes solve the diamond problem, but the implementation needs to care about a bunch of things it otherwise wouldn’t, and the inversion of control makes writing servers harder.


There are a lot of good things to say about Cap’n Proto. The two big ones for me:

…but all of the incidental difficulties add up to a lot of unnecessary implementation work, and clumsy APIs. The experience of implementing Cap’n Proto has really cemented my belief in being ruthless in leaving things out.