stackage-diff: add a previous diff link

allowing navigating back to earlier consecutive diffs

Arguably we could also navigate name1 and name2 separately

Thanks to @chreekat for reviewing and improving the code (#340)

Author: juhp

src/Handler/StackageHome.hs

@@ -53,7 +53,8 @@ instance ToJSON SnapshotInfo where
 getStackageDiffR :: SnapName -> SnapName -> Handler TypedContent
 getStackageDiffR name1 name2 = track "Handler.StackageHome.getStackageDiffR" $ do
     cacheSeconds $ 60 * 60 * 48
-    Entity sid1 _ <- lookupSnapshot name1 >>= maybe notFound return
+    Entity sid1 prevSnap <- lookupSnapshot name1 >>= maybe notFound return
+    mprevprevSnapName <- map snd <$> snapshotBefore (snapshotName prevSnap)
     Entity sid2 _ <- lookupSnapshot name2 >>= maybe notFound return
     snapDiff <- getSnapshotDiff sid1 sid2
     selectRep $ do

templates/stackage-diff.hamlet

@@ -1,5 +1,10 @@
 <h1>Compare Stackage snapshots
 
+$maybe prevprev <- mprevprevSnapName
+  <div .snapshot-nav>
+    <a href=@{StackageDiffR prevprev name1}>
+      \< previous diff
+
 <div .container .content>
   <div .packages>
     <table .table>