feat(third_party/bazel): Check in rules_haskell from Tweag

third_party/bazel/rules_haskell/tools/README.md

@@ -0,0 +1,3 @@
+Note: `py_library`s cannot be put into this folder, lest they produce
+a namespace collision with `@bazel_tools/tools`.
+See https://github.com/bazelbuild/bazel/issues/7051

third_party/bazel/rules_haskell/tools/coverage-reports/BUILD

@@ -0,0 +1,20 @@
+load(
+    "@io_tweag_rules_haskell//haskell:haskell.bzl",
+    "haskell_binary",
+)
+
+haskell_binary(
+    name = "coverage-report-renderer",
+    srcs = ["Main.hs"],
+    visibility = ["//visibility:public"],
+    deps = [
+        "@hackage//:MissingH",
+        "@hackage//:base",
+        "@hackage//:cmdargs",
+        "@hackage//:directory",
+        "@hackage//:filepath",
+        "@hackage//:hxt",
+        "@hackage//:hxt-xpath",
+        "@hackage//:listsafe",
+    ],
+)

third_party/bazel/rules_haskell/tools/coverage-reports/Main.hs

@@ -0,0 +1,134 @@
+{-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE NamedFieldPuns #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+import Control.Monad (forM_)
+
+import Control.Arrow.ListArrow (runLA)
+import Data.Either.Utils (maybeToEither)
+import Data.List (find)
+import Data.List.Safe (head, tail)
+import Data.List.Utils (split)
+import Data.Tree.NTree.TypeDefs (NTree(..))
+import Prelude hiding (head, tail)
+import System.Console.CmdArgs.Implicit (Data, Typeable, cmdArgs)
+import System.Directory (createDirectoryIfMissing)
+import System.Exit (exitFailure)
+import System.FilePath (FilePath, (</>), takeDirectory)
+import qualified Text.XML.HXT.Arrow.ReadDocument as XML
+import Text.XML.HXT.DOM.QualifiedName (localPart)
+import Text.XML.HXT.DOM.TypeDefs (XNode(..), XmlTree)
+import Text.XML.HXT.XPath.XPathEval (getXPath, getXPathSubTrees)
+
+data Args = Args
+  { testlog :: FilePath
+  , destdir :: FilePath
+  } deriving (Data, Typeable)
+
+data ReportFile = ReportFile
+  { content :: String
+  , filename :: FilePath
+  } deriving (Show)
+
+main :: IO ()
+main = do
+  Args {testlog, destdir} <- cmdArgs $ Args {testlog = "", destdir = ""}
+  if testlog == ""
+    then putStrLn noTestlogError >> exitFailure
+    else do
+      fileContents <- readFile testlog
+      let xmlTrees = runLA XML.xreadDoc fileContents
+      let rootTree = find isRoot xmlTrees
+      case rootTree of
+        Nothing -> do
+          putStrLn "Invalid XML format for testlog."
+          exitFailure
+        Just tree -> do
+          let reportFiles = generateReportFiles tree
+          case reportFiles of
+            Right reports ->
+              forM_ reports $ \ReportFile {content, filename} -> do
+                putStrLn $ concat ["Creating ", show $ destdir </> filename]
+                createDirectoryIfMissing
+                  True
+                  (destdir </> takeDirectory filename)
+                writeFile (destdir </> filename) content
+            Left err -> do
+              putStrLn err
+              exitFailure
+
+generateReportFiles :: XmlTree -> Either String [ReportFile]
+generateReportFiles doc =
+  let testSuites = getXPath "/testsuites/testsuite" doc
+   in concat <$> sequence (reportsForTestCase <$> testSuites)
+
+reportsForTestCase :: XmlTree -> Either String [ReportFile]
+reportsForTestCase testSuite = do
+  caseName <-
+    extractAttr =<<
+    maybeToEither
+      "Couldn't find testcase name."
+      (head (getXPathSubTrees "/testsuite/testcase/@name" testSuite))
+  let coverageOutputDirectory = takeDirectory caseName
+  testOutput <-
+    extractText =<<
+    maybeToEither
+      "Couldn't find system output."
+      (head (getXPathSubTrees "/testsuite/system-out" testSuite))
+  htmlPortion <-
+    maybeToEither
+      ("Couldn't find HTML report section in test case " ++ caseName ++ ".")
+      (head =<< tail (split testOutputSeparator testOutput))
+  let coverageReportPartXmlTrees = runLA XML.hreadDoc htmlPortion
+  traverse
+    (coveragePartToReportFile coverageOutputDirectory)
+    coverageReportPartXmlTrees
+
+coveragePartToReportFile :: FilePath -> XmlTree -> Either String ReportFile
+coveragePartToReportFile parentDirectory reportPart = do
+  filename <-
+    extractAttr =<<
+    maybeToEither
+      "Couldn't find report part name."
+      (head (getXPathSubTrees "/coverage-report-part/@name" reportPart))
+  content <- extractText reportPart
+  return $
+    ReportFile
+      { content = content
+      , filename = "coverage-reports" </> parentDirectory </> filename
+      }
+
+noTestlogError :: String
+noTestlogError =
+  unlines
+    [ "ERROR: You must specify the testlog XML file location with --testlog."
+    , "It is found inside the bazel-testlog, in the respective"
+    , "folder for the test you're interested in."
+    , "This must be after having run 'bazel coverage'."
+    ]
+
+isRoot :: XmlTree -> Bool
+isRoot tree =
+  case tree of
+    NTree (XTag name _) _ -> localPart name == "testsuites"
+    _ -> False
+
+extractAttr :: XmlTree -> Either String String
+extractAttr tree =
+  case tree of
+    NTree (XAttr _) [NTree (XText value) []] -> pure value
+    _ -> Left "Couldn't extract attribute from test XML."
+
+extractText :: XmlTree -> Either String String
+extractText tree =
+  let treeToText :: XmlTree -> String -> String
+      treeToText textTree acc =
+        case textTree of
+          (NTree (XText value) _) -> acc ++ value
+          _ -> ""
+   in case tree of
+        NTree (XTag _ _) textTree -> pure $ foldr treeToText "" textTree
+        _ -> Left "Couldn't extract text from test XML."
+
+testOutputSeparator :: String
+testOutputSeparator = "%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%"

third_party/bazel/rules_haskell/tools/os_info.bzl

@@ -0,0 +1,29 @@
+load("@bazel_tools//tools/cpp:lib_cc_configure.bzl", "get_cpu_value")
+
+_os_info_bzl_template = """
+cpu_value = "{CPU_VALUE}"
+is_darwin = cpu_value == "darwin"
+is_linux = cpu_value == "k8"
+is_windows = cpu_value == "x64_windows"
+"""
+
+def _os_info_impl(repository_ctx):
+    cpu = get_cpu_value(repository_ctx)
+    os_info_substitutions = {
+        "CPU_VALUE": cpu,
+    }
+    repository_ctx.file(
+        "os_info.bzl",
+        _os_info_bzl_template.format(**os_info_substitutions),
+        False,
+    )
+    repository_ctx.file(
+        "BUILD",
+        "",
+        False,
+    )
+
+os_info = repository_rule(
+    implementation = _os_info_impl,
+    local = True,
+)

third_party/bazel/rules_haskell/tools/runfiles/BUILD.bazel

@@ -0,0 +1,46 @@
+load(
+    "@io_tweag_rules_haskell//haskell:haskell.bzl",
+    "haskell_library",
+    "haskell_test",
+)
+
+haskell_library(
+    name = "runfiles",
+    srcs = ["src/Bazel/Runfiles.hs"],
+    src_strip_prefix = "src",
+    visibility = ["//visibility:public"],
+    deps = [
+        "@hackage//:base",
+        "@hackage//:directory",
+        "@hackage//:filepath",
+    ],
+)
+
+haskell_test(
+    name = "bin",
+    testonly = 1,
+    srcs = ["bin/Bin.hs"],
+    data = ["bin-data.txt"],
+    src_strip_prefix = "bin",
+    deps = [
+        ":runfiles",
+        "@hackage//:base",
+        "@hackage//:filepath",
+    ],
+)
+
+haskell_test(
+    name = "test",
+    srcs = ["test/Test.hs"],
+    data = [
+        "test-data.txt",
+        ":bin",
+    ],
+    src_strip_prefix = "test",
+    deps = [
+        ":runfiles",
+        "@hackage//:base",
+        "@hackage//:filepath",
+        "@hackage//:process",
+    ],
+)

third_party/bazel/rules_haskell/tools/runfiles/LICENSE

@@ -0,0 +1,201 @@
+                                 Apache License
+                           Version 2.0, January 2004
+                        http://www.apache.org/licenses/
+
+   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+   1. Definitions.
+
+      "License" shall mean the terms and conditions for use, reproduction,
+      and distribution as defined by Sections 1 through 9 of this document.
+
+      "Licensor" shall mean the copyright owner or entity authorized by
+      the copyright owner that is granting the License.
+
+      "Legal Entity" shall mean the union of the acting entity and all
+      other entities that control, are controlled by, or are under common
+      control with that entity. For the purposes of this definition,
+      "control" means (i) the power, direct or indirect, to cause the
+      direction or management of such entity, whether by contract or
+      otherwise, or (ii) ownership of fifty percent (50%) or more of the
+      outstanding shares, or (iii) beneficial ownership of such entity.
+
+      "You" (or "Your") shall mean an individual or Legal Entity
+      exercising permissions granted by this License.
+
+      "Source" form shall mean the preferred form for making modifications,
+      including but not limited to software source code, documentation
+      source, and configuration files.
+
+      "Object" form shall mean any form resulting from mechanical
+      transformation or translation of a Source form, including but
+      not limited to compiled object code, generated documentation,
+      and conversions to other media types.
+
+      "Work" shall mean the work of authorship, whether in Source or
+      Object form, made available under the License, as indicated by a
+      copyright notice that is included in or attached to the work
+      (an example is provided in the Appendix below).
+
+      "Derivative Works" shall mean any work, whether in Source or Object
+      form, that is based on (or derived from) the Work and for which the
+      editorial revisions, annotations, elaborations, or other modifications
+      represent, as a whole, an original work of authorship. For the purposes
+      of this License, Derivative Works shall not include works that remain
+      separable from, or merely link (or bind by name) to the interfaces of,
+      the Work and Derivative Works thereof.
+
+      "Contribution" shall mean any work of authorship, including
+      the original version of the Work and any modifications or additions
+      to that Work or Derivative Works thereof, that is intentionally
+      submitted to Licensor for inclusion in the Work by the copyright owner
+      or by an individual or Legal Entity authorized to submit on behalf of
+      the copyright owner. For the purposes of this definition, "submitted"
+      means any form of electronic, verbal, or written communication sent
+      to the Licensor or its representatives, including but not limited to
+      communication on electronic mailing lists, source code control systems,
+      and issue tracking systems that are managed by, or on behalf of, the
+      Licensor for the purpose of discussing and improving the Work, but
+      excluding communication that is conspicuously marked or otherwise
+      designated in writing by the copyright owner as "Not a Contribution."
+
+      "Contributor" shall mean Licensor and any individual or Legal Entity
+      on behalf of whom a Contribution has been received by Licensor and
+      subsequently incorporated within the Work.
+
+   2. Grant of Copyright License. Subject to the terms and conditions of
+      this License, each Contributor hereby grants to You a perpetual,
+      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+      copyright license to reproduce, prepare Derivative Works of,
+      publicly display, publicly perform, sublicense, and distribute the
+      Work and such Derivative Works in Source or Object form.
+
+   3. Grant of Patent License. Subject to the terms and conditions of
+      this License, each Contributor hereby grants to You a perpetual,
+      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+      (except as stated in this section) patent license to make, have made,
+      use, offer to sell, sell, import, and otherwise transfer the Work,
+      where such license applies only to those patent claims licensable
+      by such Contributor that are necessarily infringed by their
+      Contribution(s) alone or by combination of their Contribution(s)
+      with the Work to which such Contribution(s) was submitted. If You
+      institute patent litigation against any entity (including a
+      cross-claim or counterclaim in a lawsuit) alleging that the Work
+      or a Contribution incorporated within the Work constitutes direct
+      or contributory patent infringement, then any patent licenses
+      granted to You under this License for that Work shall terminate
+      as of the date such litigation is filed.
+
+   4. Redistribution. You may reproduce and distribute copies of the
+      Work or Derivative Works thereof in any medium, with or without
+      modifications, and in Source or Object form, provided that You
+      meet the following conditions:
+
+      (a) You must give any other recipients of the Work or
+          Derivative Works a copy of this License; and
+
+      (b) You must cause any modified files to carry prominent notices
+          stating that You changed the files; and
+
+      (c) You must retain, in the Source form of any Derivative Works
+          that You distribute, all copyright, patent, trademark, and
+          attribution notices from the Source form of the Work,
+          excluding those notices that do not pertain to any part of
+          the Derivative Works; and
+
+      (d) If the Work includes a "NOTICE" text file as part of its
+          distribution, then any Derivative Works that You distribute must
+          include a readable copy of the attribution notices contained
+          within such NOTICE file, excluding those notices that do not
+          pertain to any part of the Derivative Works, in at least one
+          of the following places: within a NOTICE text file distributed
+          as part of the Derivative Works; within the Source form or
+          documentation, if provided along with the Derivative Works; or,
+          within a display generated by the Derivative Works, if and
+          wherever such third-party notices normally appear. The contents
+          of the NOTICE file are for informational purposes only and
+          do not modify the License. You may add Your own attribution
+          notices within Derivative Works that You distribute, alongside
+          or as an addendum to the NOTICE text from the Work, provided
+          that such additional attribution notices cannot be construed
+          as modifying the License.
+
+      You may add Your own copyright statement to Your modifications and
+      may provide additional or different license terms and conditions
+      for use, reproduction, or distribution of Your modifications, or
+      for any such Derivative Works as a whole, provided Your use,
+      reproduction, and distribution of the Work otherwise complies with
+      the conditions stated in this License.
+
+   5. Submission of Contributions. Unless You explicitly state otherwise,
+      any Contribution intentionally submitted for inclusion in the Work
+      by You to the Licensor shall be under the terms and conditions of
+      this License, without any additional terms or conditions.
+      Notwithstanding the above, nothing herein shall supersede or modify
+      the terms of any separate license agreement you may have executed
+      with Licensor regarding such Contributions.
+
+   6. Trademarks. This License does not grant permission to use the trade
+      names, trademarks, service marks, or product names of the Licensor,
+      except as required for reasonable and customary use in describing the
+      origin of the Work and reproducing the content of the NOTICE file.
+
+   7. Disclaimer of Warranty. Unless required by applicable law or
+      agreed to in writing, Licensor provides the Work (and each
+      Contributor provides its Contributions) on an "AS IS" BASIS,
+      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+      implied, including, without limitation, any warranties or conditions
+      of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
+      PARTICULAR PURPOSE. You are solely responsible for determining the
+      appropriateness of using or redistributing the Work and assume any
+      risks associated with Your exercise of permissions under this License.
+
+   8. Limitation of Liability. In no event and under no legal theory,
+      whether in tort (including negligence), contract, or otherwise,
+      unless required by applicable law (such as deliberate and grossly
+      negligent acts) or agreed to in writing, shall any Contributor be
+      liable to You for damages, including any direct, indirect, special,
+      incidental, or consequential damages of any character arising as a
+      result of this License or out of the use or inability to use the
+      Work (including but not limited to damages for loss of goodwill,
+      work stoppage, computer failure or malfunction, or any and all
+      other commercial damages or losses), even if such Contributor
+      has been advised of the possibility of such damages.
+
+   9. Accepting Warranty or Additional Liability. While redistributing
+      the Work or Derivative Works thereof, You may choose to offer,
+      and charge a fee for, acceptance of support, warranty, indemnity,
+      or other liability obligations and/or rights consistent with this
+      License. However, in accepting such obligations, You may act only
+      on Your own behalf and on Your sole responsibility, not on behalf
+      of any other Contributor, and only if You agree to indemnify,
+      defend, and hold each Contributor harmless for any liability
+      incurred by, or claims asserted against, such Contributor by reason
+      of your accepting any such warranty or additional liability.
+
+   END OF TERMS AND CONDITIONS
+
+   APPENDIX: How to apply the Apache License to your work.
+
+      To apply the Apache License to your work, attach the following
+      boilerplate notice, with the fields enclosed by brackets "[]"
+      replaced with your own identifying information. (Don't include
+      the brackets!)  The text should be enclosed in the appropriate
+      comment syntax for the file format. We also recommend that a
+      file or class name and description of purpose be included on the
+      same "printed page" as the copyright notice for easier
+      identification within third-party archives.
+
+   Copyright [yyyy] [name of copyright owner]
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.

third_party/bazel/rules_haskell/tools/runfiles/README.md

@@ -0,0 +1,19 @@
+# Bazel Runfiles
+
+This is a small utility to enable discovery of the Bazel runfiles location. This is useful in tests for example.
+
+```haskell
+module Main (main) where
+
+import qualified Bazel.Runfiles as Runfiles
+import Control.Monad (when)
+import System.Process (callProcess)
+
+main :: IO ()
+main = do
+    r <- Runfiles.create
+    foo <- readFile (Runfiles.rlocation r "io_tweag_rules_haskell/tools/runfiles/test-data.txt")
+    when (lines foo /= ["foo"]) -- ignore trailing newline
+        $ error $ "Incorrect contents: got: " ++ show foo
+    callProcess (Runfiles.rlocation r "io_tweag_rules_haskell/tools/runfiles/bin") []
+```

third_party/bazel/rules_haskell/tools/runfiles/bazel-runfiles.cabal

@@ -0,0 +1,53 @@
+-- This file has been generated from package.yaml by hpack version 0.28.2.
+--
+-- see: https://github.com/sol/hpack
+--
+-- hash: bd1c2ce5b0ffa0157650e1c2409d31b818889736ef09aaf51de71e54d9a7d3fc
+
+name:           bazel-runfiles
+version:        0.7.0.1
+synopsis:       Locate Bazel runfiles location
+description:    Please see the README on GitHub at <https://github.com/tweag/rules_haskell/blob/master/tools/runfiles/README.md>
+category:       Build Tool
+homepage:       https://github.com/tweag/rules_haskell#readme
+bug-reports:    https://github.com/tweag/rules_haskell/issues
+author:         Tweag I/O Limited
+maintainer:     m@tweag.io
+copyright:      2018 Tweag I/O Limited
+license:        Apache-2.0
+license-file:   LICENSE
+build-type:     Simple
+cabal-version:  >= 1.10
+extra-source-files:
+    LICENSE
+    README.md
+
+source-repository head
+  type: git
+  location: https://github.com/tweag/rules_haskell
+
+library
+  exposed-modules:
+      Bazel.Runfiles
+  other-modules:
+      Paths_bazel_runfiles
+  hs-source-dirs:
+      src
+  build-depends:
+      base >=4.7 && <5
+    , directory
+    , filepath
+  default-language: Haskell2010
+
+executable bazel-runfiles-exe
+  main-is: Bin.hs
+  other-modules:
+      Paths_bazel_runfiles
+  hs-source-dirs:
+      bin
+  ghc-options: -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base
+    , bazel-runfiles
+    , filepath
+  default-language: Haskell2010

third_party/bazel/rules_haskell/tools/runfiles/bin-data.txt

@@ -0,0 +1 @@
+bar

third_party/bazel/rules_haskell/tools/runfiles/bin/Bin.hs

@@ -0,0 +1,12 @@
+module Main (main) where
+
+import qualified Bazel.Runfiles as Runfiles
+import Control.Monad (when)
+import System.FilePath ((</>))
+
+main :: IO ()
+main = do
+    r <- Runfiles.create
+    bar <- readFile (Runfiles.rlocation r "io_tweag_rules_haskell/tools/runfiles/bin-data.txt")
+    when (lines bar /= ["bar"]) -- ignore trailing newline
+        $ error $ "Incorrect contents: got: " ++ show bar

third_party/bazel/rules_haskell/tools/runfiles/package.yaml

@@ -0,0 +1,43 @@
+name:                bazel-runfiles
+version:             0.7.0.1
+github:              "tweag/rules_haskell"
+license:             Apache-2.0
+author:              "Tweag I/O Limited"
+maintainer:          "m@tweag.io"
+copyright:           "2018 Tweag I/O Limited"
+
+extra-source-files:
+- README.md
+- LICENSE
+
+synopsis:            Locate Bazel runfiles location
+category:            Build Tool
+
+# To avoid duplicated efforts in documentation and dealing with the
+# complications of embedding Haddock markup inside cabal files, it is
+# common to point users to the README.md file.
+description:         Please see the README on GitHub at <https://github.com/tweag/rules_haskell/blob/master/tools/runfiles/README.md>
+
+dependencies:
+- base >= 4.7 && < 5
+
+library:
+  source-dirs: src
+  dependencies:
+  - directory
+  - filepath
+  exposed-modules:
+  - Bazel.Runfiles
+
+executables:
+  bazel-runfiles-exe:
+    main:                Bin.hs
+    source-dirs:         bin
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - base
+    - bazel-runfiles
+    - filepath

third_party/bazel/rules_haskell/tools/runfiles/src/Bazel/Runfiles.hs

@@ -0,0 +1,62 @@
+-- | This module enables finding data dependencies ("runfiles") of Haskell
+-- binaries at runtime.
+--
+-- For more information, see: https://github.com/bazelbuild/bazel/issues/4460
+--
+-- Note: this does not currently support the RUNFILES_MANIFEST environmental
+-- variable.  However, that's only necessary on Windows, which rules_haskell
+-- doesn't support yet.
+--
+-- Additionally, this is not yet supported by the REPL.
+module Bazel.Runfiles
+    ( Runfiles
+    , create
+    , rlocation
+    , env
+    ) where
+
+import System.Directory (doesDirectoryExist)
+import System.Environment (getExecutablePath, lookupEnv)
+import System.FilePath (FilePath, (</>), (<.>))
+
+-- | A path to a directory tree containing runfiles for the given
+newtype Runfiles = Runfiles FilePath
+    deriving Show
+
+-- | Construct a path to a data dependency within the given runfiles.
+--
+-- For example: @rlocation \"myworkspace/mypackage/myfile.txt\"@
+rlocation :: Runfiles -> FilePath -> FilePath
+rlocation (Runfiles f) g = f </> g
+
+-- | Set environmental variables for locating the given runfiles directory.
+--
+-- Note that Bazel will set these automatically when it runs tests
+-- (@bazel test@).  However, it will not automatically set them
+-- during "bazel run"; thus, non-test binaries should set the
+-- environment manually for processes that they call.
+env :: Runfiles -> [(String, String)]
+env (Runfiles f) = [(runfilesDirEnv, f)]
+
+runfilesDirEnv :: String
+runfilesDirEnv = "RUNFILES_DIR"
+
+-- | Locate the runfiles directory for the current binary.
+--
+-- This behaves according to the specification in:
+-- https://docs.google.com/document/d/e/2PACX-1vSDIrFnFvEYhKsCMdGdD40wZRBX3m3aZ5HhVj4CtHPmiXKDCxioTUbYsDydjKtFDAzER5eg7OjJWs3V/pub
+--
+-- Note: it does not currently support the @RUNFILES_MANIFEST@ environmental
+-- variable.  However, that's only necessary on Windows, which rules_haskell
+-- doesn't support yet anyway.
+create :: IO Runfiles
+create = do
+    exeRunfilesPath <- fmap (<.> "runfiles") getExecutablePath
+    exeRunfilesExists <- doesDirectoryExist exeRunfilesPath
+    if exeRunfilesExists
+      then return $ Runfiles exeRunfilesPath
+      else do
+        envDir <- lookupEnv runfilesDirEnv
+        case envDir of
+            Just f -> return $ Runfiles f
+            Nothing -> error "Unable to locate runfiles directory"

third_party/bazel/rules_haskell/tools/runfiles/stack.yaml

@@ -0,0 +1,3 @@
+resolver: lts-12.16
+packages:
+- .

third_party/bazel/rules_haskell/tools/runfiles/test-data.txt

@@ -0,0 +1 @@
+foo

third_party/bazel/rules_haskell/tools/runfiles/test/Test.hs

@@ -0,0 +1,13 @@
+module Main (main) where
+
+import qualified Bazel.Runfiles as Runfiles
+import Control.Monad (when)
+import System.Process (callProcess)
+
+main :: IO ()
+main = do
+    r <- Runfiles.create
+    foo <- readFile (Runfiles.rlocation r "io_tweag_rules_haskell/tools/runfiles/test-data.txt")
+    when (lines foo /= ["foo"]) -- ignore trailing newline
+        $ error $ "Incorrect contents: got: " ++ show foo
+    callProcess (Runfiles.rlocation r "io_tweag_rules_haskell/tools/runfiles/bin") []