set up elm format diffs for the library and component-catalog

BUCK

@@ -1,5 +1,5 @@
 # A list of available rules and their signatures can be found here: https://buck2.build/docs/api/rules/
-load("@prelude-nri//:elm.bzl", "elm_docs")
+load("@prelude-nri//:elm.bzl", "elm_docs", "elm_format_diffs")
 load("@prelude-nri//:node.bzl", "node_modules", "npm_bin", "npm_script_test")
 
 elm_docs(
@@ -44,3 +44,8 @@ npm_script_test(
         "script/puppeteer-tests.js": "script/puppeteer-tests.js",
     },
 )
+
+elm_format_diffs(
+    name = "elm_format_diffs",
+    srcs = glob(["src/**/*.elm"]),
+)

component-catalog/BUCK

@@ -1,4 +1,4 @@
-load("@prelude-nri//:elm.bzl", "elm_app")
+load("@prelude-nri//:elm.bzl", "elm_app", "elm_format_diffs")
 
 elm_app(
     name = "app",
@@ -26,3 +26,8 @@ filegroup(
         "//:puppeteer",
     ],
 )
+
+elm_format_diffs(
+    name = "elm_format_diffs",
+    srcs = glob(["src/**/*.elm"]),
+)

prelude-nri/elm.bzl

@@ -111,3 +111,44 @@ elm_app = rule(
         ),
     },
 )
+
+def _elm_format_diffs_impl(ctx: "context"):
+    suggested_changes = []
+
+    for src in ctx.attrs.srcs:
+        suggestion = ctx.actions.declare_output("__suggested_changes__", src.short_path)
+        ctx.actions.run(
+            [
+                "bash",
+                "-xo", "pipefail",
+                "-c",
+                """
+                $1 --stdin < $2 | diff -u $2 - > $3
+                if test "$?" -eq 1; then
+                  # we're fine with files being different; we'll catch that in
+                  # the BXL script.
+                  exit 0
+                fi
+                """,
+                "--",
+                ctx.attrs._elm_toolchain.get(ElmToolchainInfo).elm_format,
+                src,
+                suggestion.as_output(),
+            ],
+            category = "elm_format_diffs",
+            identifier = src.short_path,
+        )
+        suggested_changes.append(suggestion)
+
+    return [DefaultInfo(default_outputs = suggested_changes)]
+
+elm_format_diffs = rule(
+    impl = _elm_format_diffs_impl,
+    attrs = {
+        "srcs": attrs.list(attrs.source()),
+        "_elm_toolchain": attrs.toolchain_dep(
+            default="toolchains//:elm",
+            providers = [ElmToolchainInfo],
+        ),
+    }
+)

prelude-nri/elm/BUCK

@@ -26,3 +26,33 @@ genrule(
     executable = True,
     visibility = ["PUBLIC"],
 )
+
+ELM_FORMAT_URL = select({
+    "config//os:linux": "https://github.com/avh4/elm-format/releases/download/0.8.7/elm-format-0.8.7-linux-x64.tgz",
+    "config//os:macos": select({
+        "config//cpu:arm64": "https://github.com/avh4/elm-format/releases/download/0.8.7/elm-format-0.8.7-mac-arm64.tgz",
+        "config//cpu:x86_64": "https://github.com/avh4/elm-format/releases/download/0.8.7/elm-format-0.8.7-mac-x64.tgz",
+    }),
+})
+
+ELM_FORMAT_SHA256 = select({
+    "config//os:linux": "44344c7b6f838dc5d9495dfe4253280a698c2251ee8cfa29b6d1a032b6efb13b",
+    "config//os:macos": select({
+        "config//cpu:arm64": "d8f898be599fa767d3b6607256e273dd4f62ea7abc41369a068e903159787098",
+        "config//cpu:x86_64": "064102cd471550beb43ff7eb3dd6ac7c2a1946cf038dbde389873384f62cbdc4",
+    }),
+})
+
+http_archive(
+    name = "elm_format_archive",
+    urls = [ELM_FORMAT_URL],
+    sha256 = ELM_FORMAT_SHA256,
+)
+
+genrule(
+    name = "elm_format_binary",
+    out = "elm-format",
+    cmd = "cp $(location :elm_format_archive)/elm-format $OUT",
+    executable = True,
+    visibility = ["PUBLIC"],
+)