add an Elm toolchain which will download Elm

.buckconfig

@@ -1,6 +1,7 @@
 [repositories]
 root = .
 prelude = prelude
+prelude-nri = prelude-nri
 toolchains = toolchains
 none = none
 

prelude-nri/elm/toolchain.bzl

@@ -0,0 +1,42 @@
+ElmToolchainInfo = provider(fields=[
+    "elm",
+])
+
+def _system_elm_toolchain_impl(_ctx) -> [[DefaultInfo.type, ElmToolchainInfo.type]]:
+    """
+    An Elm toolchain that assumes the current environment has all the binaries
+    it needs in PATH.
+    """
+    return [
+        DefaultInfo(),
+        ElmToolchainInfo(
+            elm = RunInfo(args = ["elm"]),
+        ),
+    ]
+
+
+system_elm_toolchain = rule(
+    impl = _system_elm_toolchain_impl,
+    attrs = {},
+    is_toolchain_rule = True,
+)
+
+def _elm_toolchain_impl(ctx: "context") -> [[DefaultInfo.type, ElmToolchainInfo.type]]:
+    """
+    An Elm toolchain which you can source binaries from wherever it makes sense
+    to you.
+    """
+    return [
+        DefaultInfo(),
+        ElmToolchainInfo(
+            elm = ctx.attrs.elm[RunInfo],
+        ),
+    ]
+
+elm_toolchain = rule(
+    impl = _elm_toolchain_impl,
+    attrs = {
+        "elm": attrs.dep(providers = [RunInfo]),
+    },
+    is_toolchain_rule = True,
+)

toolchains/BUCK

@@ -1,6 +1,36 @@
 load("@prelude//toolchains:genrule.bzl", "system_genrule_toolchain")
+load("@prelude-nri//elm:toolchain.bzl", "elm_toolchain")
 
 system_genrule_toolchain(
     name = "genrule",
     visibility = ["PUBLIC"],
 )
+
+elm_toolchain(
+    name = "elm",
+    visibility = ["PUBLIC"],
+    elm = ":elm_compiler_binary",
+)
+
+ELM_COMPILER_URL = select({
+    "config//os:linux": "https://github.com/elm/compiler/releases/download/0.19.1/binary-for-linux-64-bit.gz",
+    "config//os:macos": "https://github.com/elm/compiler/releases/download/0.19.1/binary-for-mac-64-bit.gz",
+})
+
+ELM_COMPILER_SHA256 = select({
+    "config//os:linux": "e44af52bb27f725a973478e589d990a6428e115fe1bb14f03833134d6c0f155c",
+    "config//os:macos": "05289f0e3d4f30033487c05e689964c3bb17c0c48012510dbef1df43868545d1",
+})
+
+http_file(
+    name = "elm_compiler_archive",
+    urls = [ELM_COMPILER_URL],
+    sha256 = ELM_COMPILER_SHA256,
+)
+
+genrule(
+    name = "elm_compiler_binary",
+    cmd = "gzip --decompress --stdout --keep $(location :elm_compiler_archive) > $OUT && chmod +x $OUT",
+    out = "elm",
+    executable = True,
+)