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

third_party/bazel/rules_haskell/haskell/platforms/BUILD.bazel

@@ -0,0 +1,39 @@
+# This file declares constraint values for each platform supported by
+# GHC. These rules follow the GHC naming convention, for example,
+# //haskell/platform:linux and //haskell/platform:x86_64. See the
+# config.guess in any GHC source distribution for possible platforms.
+#
+# These can be used in select expressions to choose platform-specifc
+# sources and dependencies.
+
+load(":list.bzl", "declare_config_settings")
+
+declare_config_settings()
+
+constraint_value(
+    name = "nixpkgs",
+    constraint_setting = "@bazel_tools//tools/cpp:cc_compiler",
+    visibility = ["//visibility:public"],
+)
+
+platform(
+    name = "linux_x86_64_nixpkgs",
+    constraint_values = [
+        # XXX using the platform names defined here results in a graph
+        # cycle for some reason.
+        "@bazel_tools//platforms:x86_64",
+        "@bazel_tools//platforms:linux",
+        ":nixpkgs",
+    ],
+    visibility = ["//visibility:public"],
+)
+
+platform(
+    name = "darwin_x86_64_nixpkgs",
+    constraint_values = [
+        "@bazel_tools//platforms:x86_64",
+        "@bazel_tools//platforms:osx",
+        ":nixpkgs",
+    ],
+    visibility = ["//visibility:public"],
+)