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

third_party/bazel/rules_haskell/examples/rts/BUILD.bazel

@@ -0,0 +1,29 @@
+load(
+    "@io_tweag_rules_haskell//haskell:haskell.bzl",
+    "cc_haskell_import",
+    "haskell_library",
+    "haskell_toolchain_library",
+)
+
+haskell_toolchain_library(name = "base")
+
+haskell_library(
+    name = "add-one-hs",
+    srcs = ["One.hs"],
+    deps = [":base"],
+)
+
+cc_haskell_import(
+    name = "add-one-so",
+    dep = ":add-one-hs",
+)
+
+cc_test(
+    name = "add-one",
+    srcs = [
+        "main.c",
+        ":add-one-so",
+    ],
+    visibility = ["//visibility:public"],
+    deps = ["@ghc//:threaded-rts"],
+)

third_party/bazel/rules_haskell/examples/rts/One.hs

@@ -0,0 +1,6 @@
+module One () where
+
+add_one_hs :: Int -> Int
+add_one_hs x = x + 1
+
+foreign export ccall add_one_hs :: Int -> Int

third_party/bazel/rules_haskell/examples/rts/main.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include "HsFFI.h"
+
+extern HsInt add_one_hs(HsInt a0);
+
+int main(int argc, char *argv[]) {
+  hs_init(&argc, &argv);
+  printf("Adding one to 5 through Haskell is %ld\n", add_one_hs(5));
+  hs_exit();
+  return 0;
+}