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hrepl is a tool that lets you interactively develop Haskell code using Bazel. It uses Bazel to compile your code's dependencies, and then loads your modules into an interpreter.

This is not an officially supported Google product.


To use hrepl, first cd into this repository, build the //hrepl target and save the resulting binary somewhere convenient:

$ bazel build //hrepl
$ cp --no-preserve=mode bazel-bin/hrepl/hrepl ~/.local/bin
$ chmod +x ~/.local/bin/hrepl

You should also use a recent enough verson of rules_haskell in your project's WORKSPACE file. (See this repository's WORKSPACE file for an example.)

Then, to load one or more targets in the interpreter, run hrepl within your own Bazel project. You may specify the Bazel label(s) of any Haskell libraries, binaries or tests. For example, in the rules_haskell repository itself:

$ hrepl tests/binary-with-lib:lib
*Lib Lib>

Or, within a subdirectory and with multiple targets:

$ cd tests/binary-with-lib
$ hrepl :lib :binary-with-lib
*Lib Lib Main>

You may also specify individual source files, which will cause hrepl to load targets that declare those files in their srcs. (If there is more than one possibility, it will choose arbitrarily.)

$ hrepl tests/binary-with-lib/Lib.hs
*Lib Lib>

After you modify the interpreted module(s), the :reload command will pick up any changes. This approach is much faster than rebuilding with Bazel each time.

For more information about hrepl's command-line flags, run hrepl --help.

Using Build Targets

Compiled Dependencies

By default, hrepl will compile any dependencies of your target(s) with Bazel before starting the interpreter. You may load those dependencies with :module or import. For example:

$ hrepl //some:library  # depends on the "split" package
Prelude Library> import Data.List.Split
Prelude Library Data.List.Split>

However, since those modules are compiled, the interpreter will not be aware of the source files of those dependencies, and will not pick up changes to them on :reload. Instead, you will need to :quit and restart gghci. The same is true for changes to BUILD and .bzl files that affect your targets.

Note: hrepl will not let you load (compiled) modules from transitive dependencies automatically. This behavior is similar to the build rules, which only expose modules from targets listed directly in their deps. To expose a transitive dependency in the interpreter, pass --package //label/of:dep.

Interpreted Dependencies

Alternately, you can tell hrepl to interpret (not compile) certain dependencies. The --interpret-deps=PACKAGE flag specifies any dependencies that are under the given PACKAGE (either directly, or as a subpackage). For example:

$ hrepl //some/project:target --interpret-deps=//some/project

That will load not just :target into the interpreter, but also any source files from dependencies of :target that are in some/project/BUILD or any other BUILD file in a subdirectory of some/project.

You may pass the flag more than once to combine the dependencies from different subdirectories.

Warning: hrepl will combine the compiler_flags attributes of interpreted targets into a single list, and apply all of them to each source file it loads. If two targets have conflicting compiler_flags, for example enabling and disabling the same GHC extension, it may not be possible to interpret both of them at once.

Multiple build targets

You may load zero or more Bazel targets in the interpeter at once. For example, to load two targets:

$ hrepl //your:target1 //another:target2
Prelude Target1 Target2>

hrepl will also interpret (i.e., not compile) any "intermediate" targets. For example, suppose that :target1 depends on :dep and :dep depends on :target2. Then hrepl will interpret :dep as well, and :reload will pick up any changes to :dep as well as to :target1 and :target2. However, hrepl will not expose the definitions in :dep by default. If you want to use them, either specify those targets on the command-line or call import. For example:

$ hrepl //your:target1 //another:target2
Prelude Target1 Target2> import Dep
Prelude Target1 Target2 Dep>

Additional Compiled Targets

Alternately, you may tell hrepl to compile an unrelated target with the --package flag. For example:

$ hrepl //your/haskell:target --package @stackage//:split
Prelude Target>

In that case, @stackage//:split will be compiled and available for import in the interpreter:

Prelude Target> import Data.List.Split
Prelude Target Data.List.Split>

Similar to any dependencies of :target, it won't be reloaded unless you manually :quit and restart the interpreter.

You may also use this flag to expose a dependency of a target without also compiling it.

Forwarding Command-line Flags

hrepl supports forwarding flags to its subprocesses in several different ways.


You may pass compiler flags directly to hrepl. For example:

$ hrepl -XPackageImports -freverse-errors //some:target

To pass RTS options to GHC, use the --with-rtsopts flag, which takes a space-separated list of flags. For example:

$ hrepl --with-rtsopts='-t -S'  //some:target

does the equivalent of ghc +RTS -t -S -RTS.

To Bazel

You can use --bazel-args=--some-bazel-params to make hrepl pass certain flags in each call to bazel.

--bazel-args takes a space-separated list of arguments. If it's specified multiple times, the values will accumulate. For example, --bazel-args='-c opt' is equivalent to --bazel-args=-c --bazel-args=-opt. As a special shortcut, hrepl supports directly passing the Bazel -c flag to it.

For example:

$ hrepl --bazel-args='-c opt' //your/haskell:library
$ hrepl -c opt //your/haskell:library`


Interactive development for Bazel/Haskell rules



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