swiftpm2nix: reduce duplication in generated files

pkgs/development/tools/swiftpm2nix/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, makeWrapper, jq, nix-prefetch-git }:
+{ lib, stdenv, callPackage, makeWrapper, jq, nix-prefetch-git }:
 
 stdenv.mkDerivation {
   name = "swiftpm2nix";
@@ -15,6 +15,8 @@ stdenv.mkDerivation {
 
   preferLocalBuild = true;
 
+  passthru = callPackage ./support.nix { };
+
   meta = {
     description = "Generate a Nix expression to fetch swiftpm dependencies";
     maintainers = with lib.maintainers; [ dtzWill trepetti dduan trundle stephank ];

pkgs/development/tools/swiftpm2nix/support.nix

@@ -0,0 +1,56 @@
+{ lib, fetchgit, formats }:
+with lib;
+let
+  json = formats.json { };
+in rec {
+
+  # Derive a pin file from workspace state.
+  mkPinFile = workspaceState:
+    assert workspaceState.version == 5;
+    json.generate "Package.resolved" {
+      version = 1;
+      object.pins = map (dep: {
+        package = dep.packageRef.name;
+        repositoryURL = dep.packageRef.location;
+        state = dep.state.checkoutState;
+      }) workspaceState.object.dependencies;
+    };
+
+  # Make packaging helpers from swiftpm2nix generated output.
+  helpers = generated: let
+    inherit (import generated) workspaceStateFile hashes;
+    workspaceState = builtins.fromJSON (builtins.readFile workspaceStateFile);
+    pinFile = mkPinFile workspaceState;
+  in rec {
+
+    # Create fetch expressions for dependencies.
+    sources = listToAttrs (
+      map (dep: nameValuePair dep.subpath (fetchgit {
+        url = dep.packageRef.location;
+        rev = dep.state.checkoutState.revision;
+        sha256 = hashes.${dep.subpath};
+      })) workspaceState.object.dependencies
+    );
+
+    # Configure phase snippet for use in packaging.
+    configure = ''
+      mkdir -p .build/checkouts
+      ln -sf ${pinFile} ./Package.resolved
+      install -m 0600 ${workspaceStateFile} ./.build/workspace-state.json
+    ''
+      + concatStrings (mapAttrsToList (name: src: ''
+        ln -s '${src}' '.build/checkouts/${name}'
+      '') sources)
+      + ''
+        # Helper that makes a swiftpm dependency mutable by copying the source.
+        swiftpmMakeMutable() {
+          local orig="$(readlink .build/checkouts/$1)"
+          rm .build/checkouts/$1
+          cp -r "$orig" .build/checkouts/$1
+          chmod -R u+w .build/checkouts/$1
+        }
+      '';
+
+  };
+
+}

pkgs/development/tools/swiftpm2nix/swiftpm2nix.sh

@@ -6,69 +6,39 @@
 set -eu -o pipefail
 shopt -s lastpipe
 
-pinFile="Package.resolved"
 stateFile=".build/workspace-state.json"
-
-for file in $pinFile $stateFile; do
-  if [[ ! -f "$file" ]]; then
-    echo >&2 "Missing $file. Run 'swift package resolve' first."
-    exit 1
-  fi
-done
-
-if [[ "$(jq .version $pinFile)" != "1" ]]; then
-  echo >&2 "Unsupported $pinFile version"
+if [[ ! -f "$stateFile" ]]; then
+  echo >&2 "Missing $stateFile. Run 'swift package resolve' first."
   exit 1
 fi
+
 if [[ "$(jq .version $stateFile)" != "5" ]]; then
   echo >&2 "Unsupported $stateFile version"
   exit 1
 fi
 
 # Iterate dependencies and prefetch.
-sources=""
+hashes=""
 jq -r '.object.dependencies[] | "\(.subpath) \(.packageRef.location) \(.state.checkoutState.revision)"' $stateFile \
 | while read -r name url rev; do
   echo >&2 "-- Fetching $name"
   sha256="$(nix-prefetch-git $url $rev | jq -r .sha256)"
-  sources+="
-    \"$name\" = fetchgit {
-      url = \"$url\";
-      rev = \"$rev\";
-      sha256 = \"$sha256\";
-    };"
+  hashes+="
+    \"$name\" = \"$sha256\";"
   echo >&2
 done
-sources+=$'\n'"  "
+hashes+=$'\n'"  "
 
 # Generate output.
 mkdir -p nix
-# Copy the pin file as-is.
-cp $pinFile nix/Package.resolved
 # Copy the workspace state, but clear 'artifacts'.
 jq '.object.artifacts = []' < $stateFile > nix/workspace-state.json
 # Build an expression for fetching sources, and preparing the working directory.
 cat > nix/default.nix << EOF
 # This file was generated by swiftpm2nix.
-{ lib, fetchgit }: rec {
-  sources = {$sources};
-  configure = ''
-    mkdir -p .build/checkouts
-    ln -sf \${./Package.resolved} ./Package.resolved
-    install -m 0600 \${./workspace-state.json} ./.build/workspace-state.json
-  ''
-    + lib.concatStrings (lib.mapAttrsToList (name: src: ''
-      ln -s '\${src}' '.build/checkouts/\${name}'
-    '') sources)
-    + ''
-      # Helper that makes a swiftpm dependency mutable by copying the source.
-      swiftpmMakeMutable() {
-        local orig="\$(readlink .build/checkouts/\$1)"
-        rm .build/checkouts/\$1
-        cp -r "\$orig" .build/checkouts/\$1
-        chmod -R u+w .build/checkouts/\$1
-      }
-    '';
+{
+  workspaceStateFile = ./workspace-state.json;
+  hashes = {$hashes};
 }
 EOF
 echo >&2 "-- Generated ./nix"