Awesome
proto-lens
The proto-lens library provides an API for protocol buffers using modern Haskell language and library patterns. Specifically, it provides:
- Composable field accessors via lenses
- Simple field name resolution/overloading via type-level literals
- Type-safe reflection and encoding/decoding of messages via GADTs
This is not an official Google product.
Tutorial
You can find tutorial documentation in the proto-lens-tutorial subdir.
There is also a reference document showing the protobuf scalar type -> haskell type mappings used by the generated lenses.
Instructions
Setup
First, install the "protoc" binary somewhere in your PATH. You can get it by following these instructions.
This project requires at least protoc
version 3.12.0.
Building from HEAD
To build and test this repository from HEAD, run:
git submodule update --init --recursive
stack test
Note: building this repository requires stack-2.3.1
or newer.
Using in a Cabal or Stack package
proto-lens
is available on Hackage and Stackage. Cabal and Stack projects can use it
to auto-generate Haskell source files from the original
protocol buffer specifications (.proto
files).
Note: if using Stack, these instructions require v1.4.0
or newer.
First, edit the .cabal
file of your project to:
- Specify
build-type: Custom
, and add acustom-setup
clause that depends onproto-lens-setup
. - Add
build-tool-depends: proto-lens-protoc:proto-lens-protoc
. - List the .proto files in
extra-source-files
. Note that the field belongs at the top level of the.cabal
file, rather than once per library/executable/etc. - List the generated modules (e.g.
Proto.Foo.Bar
) inexposed-modules
orother-modules
of the rule(s) that use them (e.g. the library or executables). - Add
proto-lens-runtime
to the build-depends of those rules. - Add a
custom-setup
clause to your .cabal file.
For example, in foo-bar-proto.cabal
:
...
build-type: Custom
extra-source-files: src/foo/bar.proto
...
custom-setup
setup-depends: base, Cabal, proto-lens-setup
build-tool-depends: proto-lens-protoc:proto-lens-protoc
library
exposed-modules: Proto.Foo.Bar, Proto.Foo.Bar_Fields
autogen-modules: Proto.Foo.Bar, Proto.Foo.Bar_Fields
build-depends: proto-lens-runtime, ...
(Note: if you do not have proto-lens-{runtime/setup}
, you are probably
using a version earlier than 0.4
and should replace those packages with
proto-lens-protoc
.)
Next, write a Setup.hs
file that uses Data.ProtoLens.Setup
and specifies the
directory containing the .proto
files. For example:
import Data.ProtoLens.Setup
main = defaultMainGeneratingProtos "src"
Then, when you run cabal build
or stack build
, Cabal will generate a Haskell
file from each .proto file, and use it as part of building the
library/executable/etc.
See the proto-lens-tests
package for some more detailed examples.
Manually running the protocol compiler
Suppose you have a file foo.proto
. Then to generate bindings, run:
protoc --plugin=protoc-gen-haskell=`which proto-lens-protoc` \
--haskell_out=. foo.proto
This will generate a file Proto/Foo.hs
which contains Haskell definitions
corresponding to the messages and fields in the protocol buffer file.
Use --haskell_out
to control the location of the output file.
Use --proto_path
to specify the location of input .proto
files. For
example, suppose we have the files src/project/{foo,bar}.proto
, and
bar.proto
has the line
import "project/foo.proto";
Then running:
protoc --plugin=protoc-gen-haskell=`which proto-lens-protoc` \
--haskell_out=. \
--proto_path=src \
src/project/foo.proto src/project/bar.proto
will generate the haskell files Proto/Project/{Foo,Bar}.hs
.
Current differences from the standard
- Extensions (proto2-only) are not supported.
- Unknown proto2 enum values cause a decoding error, instead of being preserved round-trip.
Troubleshooting
Rebuilding
Due to stack issue #1891,
if you only change the .proto files then stack won't rebuild the package (that
is, it won't regenerate the Proto.*
modules).
Loading into ghci
with Stack
stack ghci
can get confused when trying to directly load a package that
generates Proto.*
modules (for example: stack ghci <proto-package>
).
To work around this issue, run instead:
stack exec ghci --package <proto-package>
And then manually import the generated modules within ghci, for example:
Prelude> import Proto.Foo.Bar
Prelude Proto.Foo.Bar>
...
Alternately, you can make those modules available at the same time as another local package, by running:
stack ghci <another-package> --package <proto-package>
Linking errors
Due to the limitations of how we can specify the dependencies of Setup
files, stack may try to link them against the terminfo
package. You
may get an error from stack build
similar to:
/usr/bin/ld: cannot find -ltinfo
On a Debian based system (like Ubuntu), the remedy is:
sudo apt-get install libncurses5-dev