This module provides a tutorial on the overall usage of the library. Note that it does not aim to provide a thorough introduction to capnproto itself; see https://capnproto.org for general information.

Each of the example programs described here can also be found in the examples/ subdirectory in the source repository.

This module provides an overview of the capnp library.

In order to generate code from schema files, you will first need to make sure the capnp and capnpc-haskell binaries are in your $PATH. The former ships with the capnproto reference implementation; see https://capnproto.org/install.html. The latter is included with this library; to install it you can run the command:

cabal install capnp --installdir=$DIR

which will compile the package and create the capnpc-haskell executable at $DIR/capnpc-haskell.

This package is in them midst of transitioning many existing APIs over to a new design. As such, in this tuotrial we refer to the new api and the old API.

The old API will eventually be removed, but not before there is at least one release where both APIs are present and the new API has reached feature pairty. Right now, the primary missing functionality is in implementing RPC servers (clients work fine, better even).

This tutorial only covers the new API, but the tutorial for the old APIs is still available (and still correct) in the documentation for version 0.10 of this package: https://hackage.haskell.org/package/capnp-0.10.0.1

For more information about the reasons behind the new API, see: http://zenhack.net/TODO. TODO: link to blog post.

The serialization API is roughly divided into two parts: a low level API and a high level API. The high level API eschews some of the benefits of the wire format in favor of a more convenient interface.

The high level API exposes capnproto values as regular algebraic data types.

On the plus side:

• This makes it easier to work with capnproto values using idiomatic Haskell code
• Because we have to parse the data up-front we can *validate* the data up front, so (unlike the low level API), you will not have to deal with errors while traversing the message.

Both of these factors make the high level API generally more pleasant to work with and less error-prone than the low level API.

The downside is that you can't take advantage of some of the novel properties of the wire format. In particular:

• It is slower, as there is a marshalling step involved, and it uses more memory.
• You can't mmap a file and read in only part of it.
• You can't modify a message in-place.

As a running example, we'll use the following schema (borrowed from the C++ implementation's documentation):

# addressbook.capnp
@0xcd6db6afb4a0cf5c;

struct Person {
  id @0 :UInt32;
  name @1 :Text;
  email @2 :Text;
  phones @3 :List(PhoneNumber);

  struct PhoneNumber {
    number @0 :Text;
    type @1 :Type;

    enum Type {
      mobile @0;
      home @1;
      work @2;
    }
  }

  employment :union {
    unemployed @4 :Void;
    employer @5 :Text;
    school @6 :Text;
    selfEmployed @7 :Void;
    # We assume that a person is only one of these.
  }
}

struct AddressBook {
  people @0 :List(Person);
}

Once the capnp and capnpc-haskell executables are installed and in your $PATH (see the Setup section above), you can generate code for this schema by running:

capnp compile -ohaskell addressbook.capnp

This will create the following files relative to the current directory:

• Capnp/Gen/Addressbook.hs
• Capnp/Gen/Addressbook/Pure.hs
• Capnp/Gen/Addressbook/New.hs
• Capnp/Gen/ById/Xcd6db6afb4a0cf5c/Pure.hs
• Capnp/Gen/ById/Xcd6db6afb4a0cf5c/New.hs
• Capnp/Gen/ById/Xcd6db6afb4a0cf5c.hs

The modules under ById are an implementation detail. Capnp/Gen/Addressbook.New.hs is generated code for use with the new API. Other files are for use with the old API, and not covered here.

The generated moule will export declarations like the following (cleaned up and abbreviated for readability):

import qualified Capnp.Repr as R
import qualified Capnp.Classes.New as C
import qualified Capnp.Repr.Parsed as RP
import GHC.Generics (Generic)

data Person

type instance (R.ReprFor Person) = R.Ptr (Just R.Struct)

instance (C.TypedStruct Person) where { ... }
instance (C.Allocate Person) where { ... }

data instance C.Parsed Person
    = Person
        { id :: Word32
        , name :: RP.Parsed Basics.Text
        , email :: RP.Parsed Basics.Text
        , phones :: RP.Parsed (R.List Person'PhoneNumber)
        , employment :: RP.Parsed Person'employment
        }
    deriving(Generic, Show, EQ)

instance HasField "id" Slot Person Std_.Word32 where { ... }
instance HasField "name" Slot Person Basics.Text where { ... }
instance HasField "email" Slot Person Basics.Text where { ... }
instance HasField "phones" Slot Person (R.List Person'PhoneNumber) where { ... }
instance HasField "employment" Group Person Person'employment where { ... }

data Person'employment

type instance R.ReprFor Person'employment = R.Ptr (Std_.Just R.Struct)
instance C.TypedStruct Person'employment where { ... }
instance C.Allocate Person'employment where { ... }

data instance C.Parsed Person'employment
    = Person'employment'
        { union' :: C.Parsed (GH.Which Person'employment)
        }
    deriving(Generic, Show, Eq)

instance (GH.HasUnion Person'employment) where
    unionField = ...
    data RawWhich mut_ Person'employment
        = RW_Person'employment'unemployed (R.Raw mut_ ())
        | RW_Person'employment'employer (R.Raw mut_ Basics.Text)
        | RW_Person'employment'school (R.Raw mut_ Basics.Text)
        | RW_Person'employment'selfEmployed (R.Raw mut_ ())
        | RW_Person'employment'unknown' Word16
    data Which Person'employment

instance GH.HasVariant "unemployed" GH.Slot Person'employment () where { ... }
instance GH.HasVariant "employer" GH.Slot Person'employment Basics.Text where { ... }
instance GH.HasVariant "school" GH.Slot Person'employment Basics.Text where { ... }
instance GH.HasVariant "selfEmployed" GH.Slot Person'employment () where { ... }

data instance C.Parsed (Which Person'employment)
    = Person'employment'unemployed
    | Person'employment'employer (RP.Parsed Basics.Text)
    | Person'employment'school (RP.Parsed Basics.Text)
    | Person'employment'selfEmployed
    | Person'employment'unknown' Std_.Word16
    deriving(Generic, Show, Eq)

instance C.Parse (GH.Which Person'employment) (C.Parsed (GH.Which Person'employment)) where
    ...

data Person'PhoneNumber

type instance R.ReprFor Person'PhoneNumber = R.Ptr (Std_.Just R.Struct)

...

data Person'PhoneNumber'Type
    = Person'PhoneNumber'Type'mobile
    | Person'PhoneNumber'Type'home
    | Person'PhoneNumber'Type'work
    | Person'PhoneNumber'Type'unknown' Std_.Word16
    deriving(Generic, Eq, Show)

type instance R.ReprFor Person'PhoneNumber'Type = R.Data R.Sz16

instance Enum Person'PhoneNumber'Type where { ... }

...

Note that we use the single quote character as a namespace separator for namespaces within a single capnproto schema.

So, we see that capnpc-haskell generates:

• For each struct type or group:
• An uninhabited type corresponding to that type
• An instance of the ReprFor type family, marking the type as having a struct as its representation.
• An instance of HasField for each field in the struct.
• An instance of the Parsed data family, which is an idiomatic Haskell ADT corresponding to the structure of the capnproto type.
• If the struct has an anonymous union, some instances related to this, including a data family instance for Parsed (Which a), which is an ADT representation of the union. Note that there is an unknown' variant, which is used for variants found on the wire that are not known to the schema (usually because the value was constructed using a newer version of the schema).
• For each enum:
• An ADT corresponding to that enum. There is no uninhabited type, and no Parsed data family instance; the type itself serves as both. As with unions, there is an unknown' variant for unrecognized variants.
• An instance of ReprFor, recording the wire representation of the enum (always 16-bit).

Some additional things are generated for interfaces, but we cover those in the RPC section below.

The module Capnp.New exposes the most frequently used functionality from the capnp package. We can write an address book message to standard output using the high-level API like so:

{-# LANGUAGE OverloadedStrings     #-}
-- Note that DuplicateRecordFields is usually needed, as the generated
-- code relys on it to resolve collisions in capnproto struct field
-- names:
{-# LANGUAGE DuplicateRecordFields #-}
import Capnp.Gen.Addressbook.New

-- Note that Capnp.New re-exports `def`, as a convienence
import Capnp.New (putParsed, def)

import qualified Data.Vector as V

main = putParsed AddressBook
    { people = V.fromList
        [ Person
            { id = 123
            , name = "Alice"
            , email = "alice@example.com"
            , phones = V.fromList
                [ def
                    { number = "555-1212"
                    , type_ =  Person'PhoneNumber'Type'mobile
                    }
                ]
            , employment = Person'employment $ Person'employment'school "MIT"
            }
        , Person
            { id = 456
            , name = "Bob"
            , email = "bob@example.com"
            , phones = V.fromList
                [ def
                    { number = "555-4567"
                    , type_ = Person'PhoneNumber'Type'home
                    }
                , def
                    { number = "555-7654"
                    , type_ = Person'PhoneNumber'Type'work
                    }
                ]
            , employment = Person'employment $ Person'employment'selfEmployed
            }
        ]
    }

putValue is equivalent to hPutValue stdout; hPutValue may be used to write to an arbitrary handle.

We can use getParsed (or alternately hGetParsed) to read in a message:

-- ...

{-# LANGUAGE TypeApplications #-}
import Capnp.New (getParsed, defaultLimit)

-- ...

main = do
    value <- getParsed @AddressBook defaultLimit
    print value

Note the use of TypeApplications; there are a number of interfaces in the library which dispatch on return types, and depending on how they are used you may have to give GHC a hint for type inference to succeed.

The type of getParsed is:

@getParsed :: (R.IsStruct a, Parse a pa) => WordCount -> IO pa

...and so it may be used to read in any struct type.

defaultLimit is a default value for the traversal limit, which acts to prevent denial of service vulnerabilities; See the documentation in Capnp.TraversalLimit for more information. getValue uses this argument both to catch values that would cause excessive resource usage, and to simply limit the overall size of the incoming message. The default is approximately 64 MiB.

If an error occurs, an exception will be thrown of type Error from the Capnp.Errors module.

The low level API exposes a much more imperative interface than the high-level API. Instead of algebraic data types, There is an opaque wrapper type Raw:

newtype Raw (mut :: Mutability) a = ...

which accepts as type parameters the mutability of the underlying message, and a phantom type indicating the capnproto type. This second type parameter will be instantiated with the (for structs, uninhabited) type generated by the schema compiler plugin. The accessors in Capnp.New.Accessors (re-exported by Capnp.New) are used to read and write the fields. This API is much closer in spirit to that of the C++ reference implementation.

Because the low level interfaces do not parse and validate the message up front, accesses to the message can result in errors. Furthermore, the traversal limit needs to be tracked to avoid denial of service attacks.

Because of this, access to the message must occur inside of a monad which is an instance of MonadThrow from the exceptions package, and MonadLimit, which is defined in Capnp.TraversalLimit. We define a monad transformer LimitT for the latter.

We'll use the same schema as above for our example. The snippet below prints the names of each person in the address book:

{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE TypeApplications #-}

import           Capnp.Gen.Addressbook.New
import qualified Capnp.New                 as C
import           Control.Monad             (forM_)
import           Control.Monad.Trans       (lift)
import           Data.Function             ((&))
import qualified Data.Text                 as T

main :: IO ()
main = do
    addressbook <- C.getRaw @AddressBook C.defaultLimit
    C.evalLimitT C.defaultLimit $ do
        people <- C.readField #people addressbook
        forM_ [0..C.length people - 1] $ \i -> do
            people
                & C.index i
                >>= C.parseField #name
                >>= lift . putStrLn . T.unpack

Writing messages using the low-level API has a similarly imperative feel. The below constructs the same message as in our high-level example above:

{-# LANGUAGE DataKinds        #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE TypeApplications #-}

import Data.Function ((&))

import Capnp.Gen.Addressbook.New

import qualified Capnp.New as C
import qualified Data.Text as T

main :: IO ()
main =
    let Right msg = C.createPure C.defaultLimit buildMsg
    in C.putMsg msg

buildMsg :: C.PureBuilder s (C.Message ('C.Mut s))
buildMsg = do
    -- newMessage allocates a new, initially empty, mutable message. It
    -- takes an optional size hint:
    msg <- C.newMessage Nothing

    -- newRoot allocates a new struct as the root object of the message.
    -- The unit argument is a hint to the allocator to determine the size
    -- of the object; for types whose size is not fixed (e.g. untyped structs,
    -- lists), this may be something more meaningful.
    addressbook <- C.newRoot @AddressBook () msg

    -- newField can be used to allocate the value of a field, for pointer
    -- types like lists. The number is the allocation hint, as used by newRoot.
    -- We can use the OverloadedLabels extension to pass in fields by name.
    people <- C.newField #people 2 addressbook

    -- Index gets an object at a specified location in a list. Cap'N Proto
    -- lists are flat arrays, and in the case of structs the structs are
    -- unboxed, so there is no need to allocate each element:
    alice <- C.index 0 people

    -- encodeField takes the parsed form of a value and marshals it into
    -- the specified field. For basic types like integers & booleans, this
    -- is almost always what you want. For larger values, you may want to
    -- use newField as above, or separately create the value and use setField,
    -- as shown below.
    C.encodeField #id 123 alice
    C.encodeField #name (T.pack "Alice") alice
    C.encodeField #email (T.pack "alice@example.com") alice

    -- We would probably use newField here, but to demonstrate, we can allocate
    -- the value separately with new, and then set it with setField.
    phones <- C.new @(C.List Person'PhoneNumber) 1 msg
    C.setField #phones phones alice

    mobilePhone <- C.index 0 phones
    -- It is sometimes more ergonomic to use (&) from Data.Function. You might
    -- ask why not just make the container the first argument, but it works
    -- out better this way for the read examples.
    mobilePhone & C.encodeField #number (T.pack "555-1212")
    mobilePhone & C.encodeField #type_ Person'PhoneNumber'Type'mobile

    -- Since named unions act like unnamed unions inside a group, we first have
    -- to get the group field:
    employment <- C.readField #employment alice

    -- Then, we can use encodeVariant to set both the tag of the union and the
    -- value:
    employment & C.encodeVariant #school (T.pack "MIT")

    bob <- C.index 1 people
    bob & C.encodeField #id 456
    bob & C.encodeField #name (T.pack "Bob")
    bob & C.encodeField #email (T.pack "bob@example.com")

    phones <- bob & C.newField #phones 2
    homePhone <- phones & C.index 0
    homePhone & C.encodeField #number (T.pack "555-4567")
    homePhone & C.encodeField #type_ Person'PhoneNumber'Type'home
    workPhone <- phones & C.index 1
    workPhone & C.encodeField #number (T.pack "555-7654")
    workPhone & C.encodeField #type_ Person'PhoneNumber'Type'work
    employment <- bob & C.readField #employment
    employment & C.encodeVariant #selfEmployed () -- Note the (), since selfEmploy is Void.

    pure msg

This package supports level 1 Cap'n Proto RPC. The tuotrial will demonstrate the most basic features of the RPC system with example: an echo server & client. For a larger example which demos more of the protocol's capabilities, see the calculator example in the source repository's examples/ directory.

Note: for now, we only show the client here, as the new API does not yet support implementing rpc servers -- for that you can use the old API, see old docs for more info.

Note that capnproto does not have a notion of "clients" and "servers" in the traditional networking sense; the two sides of a connection are symmetric. In capnproto terminology, a "client" is a handle for calling methods, and a "server" is an object that handles methods -- but there may be many of either or both of these on each side of a connection.

Given the schema:

@0xd0a87f36fa0182f5;

interface Echo {
  echo @0 (query :Text) -> (reply :Text);
}

In the low level module, the code generator generates an unihabited type Echo, with its ReprFor instance indicating that it is a capability.

There is a Client type exported by Capnp.New, which is parametrized over a phantom type indicating the type of the remote capability. So a Client Echo allows you to call methods on an Echo interface.

Actually invoking methods uses the functions in Capnp.Repr.Methods, re-exported by Capnp.New. callP, callB, and callR provide different ways of supplying arguments to a call, but all are intended to be used with the OverloadedLabels extension for specifying the method name.

Pipelining onto a field can be done with the pipe function. waitPipeline blocks until the result is available.

Here is an an echo (networking) client using this interface:

{-# LANGUAGE OverloadedLabels  #-}
{-# LANGUAGE OverloadedStrings #-}

import Data.Function      ((&))
import Data.Functor       ((<&>))
import Network.Simple.TCP (connect)

import qualified Capnp.New as C
import           Capnp.Rpc
    (ConnConfig(..), fromClient, handleConn, socketTransport)

import Capnp.Gen.Echo.New

main :: IO ()
main = connect "localhost" "4000" $ \(sock, _addr) ->
    handleConn (socketTransport sock C.defaultLimit) C.def
        { debugMode = True
        , withBootstrap = Just $ \_sup client ->
            let echoClient :: C.Client Echo
                echoClient = fromClient client
            in
            echoClient
                & C.callP #echo C.def { query = "Hello, World!" }
                <&> C.pipe #reply
                >>= C.waitPipeline
                >>= C.evalLimitT C.defaultLimit . C.parse
                >>= print
        }