better nix-shell support via shellHook (#3243)

nix/buildIdris.nix

@@ -13,7 +13,7 @@
 { src
 , ipkgName
 , version ? "unversioned"
-, idrisLibraries
+, idrisLibraries # Other libraries built with buildIdris
 , ... }@attrs:
 
 let
@@ -29,7 +29,8 @@ let
     "idrisLibraries"
   ];
 
-  sharedAttrs = drvAttrs // {
+  derivation = stdenv.mkDerivation (finalAttrs:
+    drvAttrs // {
       pname = ipkgName;
       inherit version;
       src = src;
@@ -47,11 +48,16 @@ let
       passthru = {
         inherit propagatedIdrisLibraries;
       };
-  };
+
+      shellHook = ''
+        export IDRIS2_PACKAGE_PATH="${finalAttrs.IDRIS2_PACKAGE_PATH}"
+      '';
+    }
+  );
 
 in rec {
-  executable = stdenv.mkDerivation (sharedAttrs //
-    { installPhase = ''
+  executable = derivation.overrideAttrs {
+    installPhase = ''
       runHook preInstall
       mkdir -p $out/bin
       scheme_app="$(find ./build/exec -name '*_app')"
@@ -73,21 +79,19 @@ in rec {
       fi
       runHook postInstall
     '';
-    }
-  );
+  };
 
   library = { withSource ? false }:
     let installCmd = if withSource then "--install-with-src" else "--install";
-    in stdenv.mkDerivation (sharedAttrs //
-      { installPhase = ''
+    in derivation.overrideAttrs {
+      installPhase = ''
         runHook preInstall
         mkdir -p $out/${libSuffix}
         export IDRIS2_PREFIX=$out/lib
         idris2 ${installCmd} ${ipkgFileName}
         runHook postInstall
       '';
-      }
-    );
+    };
   # deprecated aliases:
   build = lib.warn "build is a deprecated alias for 'executable'." executable;
   installLibrary = lib.warn "installLibrary is a deprecated alias for 'library { }'." (library { });