Add a library for looking up runfiles.

Relevant Bazel issue: bazelbuild/bazel#4460.

I followed the approach documented here:
https://docs.google.com/document/d/e/2PACX-1vSDIrFnFvEYhKsCMdGdD40wZRBX3m3aZ5HhVj4CtHPmiXKDCxioTUbYsDydjKtFDAzER5eg7OjJWs3V/pub
Other than not bothing yet with `RUNFILES_MANIFEST`, which is only needed on
WINDOWS.

tools/runfiles/BUILD

@@ -0,0 +1,42 @@
+load(
+  "@io_tweag_rules_haskell//haskell:haskell.bzl",
+  "haskell_binary",
+  "haskell_library",
+  "haskell_test",
+)
+
+haskell_library(
+    name = "runfiles",
+    srcs = ["Bazel/Runfiles.hs"],
+    prebuilt_dependencies = [
+        "base",
+        "directory",
+        "filepath",
+    ],
+    visibility = ["//visibility:public"],
+)
+
+haskell_binary(
+    name = "bin",
+    srcs = ["Bin.hs"],
+    main_file = "Bin.hs",
+    deps = [":runfiles"],
+    data = ["bin-data.txt"],
+    prebuilt_dependencies = [
+        "base",
+        "filepath",
+    ],
+    testonly = 1,
+)
+haskell_test(
+    name = "test",
+    srcs = ["Test.hs"],
+    main_file = "Test.hs",
+    deps = [":runfiles"],
+    data = ["test-data.txt", ":bin"],
+    prebuilt_dependencies = [
+        "base",
+        "filepath",
+        "process",
+    ],
+)

tools/runfiles/Bazel/Runfiles.hs

@@ -0,0 +1,63 @@
+-- | 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
+    , getRunfiles
+    , rlocation
+    , runfilesEnv
+    ) 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@).
+runfilesEnv :: Runfiles -> [(String, String)]
+runfilesEnv (Runfiles f) = [(runfilesDirEnv, f)]
+
+runfilesDirEnv :: String
+runfilesDirEnv = "RUNFILES_DIR"
+
+-- | Locate the runfiles directory for the current binary.
+--
+-- This behaves according ot 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.
+getRunfiles :: IO Runfiles
+getRunfiles = 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"
+
+
+

tools/runfiles/Bin.hs

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

tools/runfiles/Test.hs

@@ -0,0 +1,15 @@
+module Main (main) where
+
+import Bazel.Runfiles
+import Control.Monad (when)
+import System.FilePath ((</>))
+import System.Process (callProcess)
+
+
+main :: IO ()
+main = do
+    r <- getRunfiles
+    foo <- readFile (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 (rlocation r "io_tweag_rules_haskell/tools/runfiles/bin") []

tools/runfiles/bin-data.txt

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

tools/runfiles/test-data.txt

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