Fetch .tar.gz using nix-prefetch-url --unpack

Fetching `tar.gz` files is common enough of a use-case that warrants
treating them specially by bypassing other fetching options and using
`nix-prefetch-url --unpack` directly. This speeds up large tarballs as
`cabal2nix` currently downloads them twice as the first fetch assumes an
unpacked url and thus will fail.

Author: pwm

cabal2nix/src/Distribution/Nixpkgs/Fetch.hs

@@ -12,6 +12,7 @@ module Distribution.Nixpkgs.Fetch
   , derivKindFunction
   , FetchSubmodules(..)
   , UnpackArchive(..)
+  , fetchTarball
   , fetch
   , fetchWith
   ) where
@@ -115,6 +116,9 @@ fromDerivationSource DerivationSource{..} =
     sourceCabalDir = "."
   }
 
+fetchTarball :: (String -> MaybeT IO a) -> Source -> IO (Maybe (DerivationSource, a))
+fetchTarball f = runMaybeT . (fetchWith (False, DerivKindUrl UnpackArchive) >=> \(derivSource, file) -> (,) derivSource <$> f file)
+
 -- | Fetch a source, trying any of the various nix-prefetch-* scripts.
 fetch :: forall a.
          FetchSubmodules

cabal2nix/src/Distribution/Nixpkgs/Haskell/PackageSourceSpec.hs

@@ -95,7 +95,11 @@ fetchOrFromDB optHpack optSubmodules hackageDB src
       (msrc, pkgDesc) <- fromDB hackageDB . drop (length "cabal://") $ sourceUrl src
       return (msrc, False, pkgDesc)
   | otherwise                             = do
-    r <- fetch optSubmodules (\dir -> cabalFromPath optHpack (dir </> sourceCabalDir src)) src
+    let fetcher =
+          if "tar.gz" `isSuffixOf` sourceUrl src
+            then fetchTarball
+            else fetch optSubmodules
+    r <- fetcher (\dir -> cabalFromPath optHpack (dir </> sourceCabalDir src)) src
     case r of
       Nothing -> fail $ "Failed to fetch source. Does this source exist? " ++ show src
       Just (derivSource, (externalSource, ranHpack, pkgDesc)) ->