Support hpack include directives

Add a `generateCabalFile` argument that allows us to pre-genereate the
cabal file, thus allowing us to create an environment where hpack can
successfully find included files.

We also just read the package.yaml file directly to get the name rather
than by calling readYAML, since include directives lead to unparseable
YAML.

Resolves cdepillabout#47

Author: TeofilC

nix/build-support/stacklock2nix/default.nix

@@ -133,7 +133,15 @@
   # defaultLocalPkgFilter: pkgName: path: type: true
   # ```
   localPkgFilter ?
-    defaultLocalPkgFilter: pkgName: path: type: defaultLocalPkgFilter path type
+    defaultLocalPkgFilter: pkgName: path: type: defaultLocalPkgFilter path type,
+  # A function to generate a cabal file for a local package.
+  # Normally this is handled by `cabal2nix`, but in special cases we might want
+  # to do this ourselves, eg, when using hpack `!include`s.
+  #
+  #
+  # generateCabalFile :: String -> String -> Path -> Path
+  generateCabalFile ?
+    pkgName: subdir: filteredSrc: filteredSrc
 }:
 
 # The stack.yaml path can be computed from the stack.yaml.lock path, or
@@ -558,15 +566,19 @@ let
       # Example: `"my-cool-pkg"`
       pkgNameFromPackageYaml =
         let
-          packageYaml = readYAML (justCabalFilePath + "/package.yaml");
-        in
-        if packageYaml ? name then
-          packageYaml.name
-        else
-          throw
-            ("Could not find read a .name field from the package.yaml file in package ${localPkgPathStr}.  " +
-             "This is unexpected.  All package.yaml files should have a top-level .name field. " +
-             "package.yaml file: ${builtins.toJSON packageYaml}");
+          packageYamlName = builtins.readFile
+            (runCommand "read-package-name" {}
+              ''
+              function fail {
+                echo "Could not find read a .name field from the package.yaml file in package ${localPkgPathStr}."
+                echo "This is unexpected.  All package.yaml files should have a top-level .name field."
+                echo "package.yaml file: "
+                cat package.yaml
+                exit 1
+              }
+              (sed -nr 's/name:\s*\"?(.*)\"?\s*/\1/p' ${justCabalFilePath + "/package.yaml"} | tr -d '\n' >$out) || fail''
+            );
+        in packageYamlName;
 
       # Whether or not this package has at least one .cabal file.
       #
@@ -626,12 +638,25 @@ let
 
       # Path to the Haskell package.
       #
-      # pkgPath :: Path
-      pkgPath = lib.cleanSourceWith {
+      # filteredPkgPath :: Path
+      filteredPkgPath = lib.cleanSourceWith {
         src = rawPkgPath;
         filter = path: type: localPkgFilter localPkgDefaultFilter pkgName path type;
         name = "stacklock2nix-pkg-sources-" + pkgName;
       };
+
+      # Path to the Haskell package,
+      # but if it doesn't contain a .cabal file
+      # then we call the `generateCabalFile` function to potentially pre-generate
+      # the .cabal file. This is useful if the `package.yaml` file needs special
+      # treatment, eg, because of includes.
+      #
+      # pkgPath :: Path
+      pkgPath =
+        if hasSingleCabalFile then
+          filteredPkgPath
+        else
+          generateCabalFile pkgName localPkgPathStr filteredPkgPath;
     in
     { inherit pkgPath pkgName; };