protolite

protolite is a Haskell protobuf en/decoder library that exploits GHC.Generics and DataKinds.

why protolite

• ✔ Nested message supported.
• ✔ All kinds of protobuf data supported.
• ✔ Very clean. Neither code generation nor templates. Protobuf definition inferred easily.
• ✔ Lightweight. Tiny code base, dependent only to binary, bytestring and utf8-string.

quick start

More examples in test/Spec.hs.

data ProtoSkipped = ProtoSkipped {
    skipped1 :: ProtoField Word32 1,
    skipped2 :: ProtoField (Optional Word32) 3,
    skipped3 :: ProtoField (Optional String) 10,
    skipped4 :: ProtoField OtherMessage 20
} deriving (Show, Eq, Generic)
instance ProtoBuf ProtoSkipped

The above snippet defines a protobuf message consisting of four fields. It's equivalent to the following protobuf definition.

message ProtoSkipped {
    fixed32 skipped1 = 1;
    optional fixed32 skipped2 = 3;
    optional string skipped3 = 10;
    OtherMessage skipped4 = 20;
}

Then it can be consumed as

let v = ProtoSkipped {
        skipped1 = 0x12345678,
        skipped2 = ProtoField $ Just 0x12345678,
        skipped3 = ProtoField $ Just "hello",
        skipped4 = ...
}
let bs = encode v
let v' = decode bs :: ProtoSkipped
print v
printHex bs
print v'

types from/to protobuf

prerequisites

In protobuf there're exactly three kinds of data: variant, fixed, and len-prefixed.

• variant includes integral data that has variant length depending on its value. This type is designed for space efficiency.
• fixed includes exactly what's from C-like language, int32, uint32, float, int64, uint64, double, which all have a compile-time fixed length.
• len-prefixed includes strings, bytearrays, packed arrays, and even the nested messages.

More to be found here protobuf-reference.

types

In the following is the type bimap between protobuf and protolite.
Types in the colomn protolite can replace symbol * in ProtoField (*) n to form a field signature.
The t means it is replaced by some other defined types. But exceptionally, only fixed and variant can be applied to packed.
Last thing to note, Optional is nothing more than an alias of Maybe.

kind	protobuf	protolite
general	repeated t	Repeated t
	optional t	Optional t | Maybe t
fixed	sfixed32	Int32
	fixed32	Word32
	sfixed64	Int64
	fixed64	Word64
	float	Float
	double	Double
variant	int32	Variant Int32
	int64	Variant Int64
	uint32	Variant Word32
	uint64	Variant Word64
	sint32	Variant SInt32
	sint64	Variant SInt64
	enum	Variant Int32
	bool	Variant Bool
len-prefixed	string	String
	bytes	LazyByteString
	packed t	Packed t
	message	message