diff --git a/CHANGES.txt b/CHANGES.txt
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,5 +1,9 @@
 Changelog for HLint (* = breaking change)
 
+2.1.22, released 2019-05-25
+    #634, suggest modifyIORef ==> writeIORef when applicable
+    #642, suggest null in more places
+    #640, reenable GHC parsing
 2.1.21, released 2019-05-19
     #637, temporary workaround for GHC parser segfaults
 2.1.20, released 2019-05-15
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -245,7 +245,7 @@
 
 For `ANN` pragmas it is important to put them _after_ any `import` statements. If you have the `OverloadedStrings` extension enabled you will need to give an explicit type to the annotation, e.g. `{-# ANN myFunction ("HLint: ignore" :: String) #-}`. The `ANN` pragmas can also increase compile times or cause more recompilation than otherwise required, since they are evaluated by `TemplateHaskell`.
 
-For `{-# HLINT #-}` pragmas GHC may give a warning about an unrecognised pragma, which can be supressed with `-Wno-unrecognised-pragmas`.
+For `{-# HLINT #-}` pragmas GHC may give a warning about an unrecognised pragma, which can be suppressed with `-Wno-unrecognised-pragmas`.
 
 For `{- HLINT -}` comments they are likely to be treated as comments in syntax highlighting, which can lead to them being overlooked.
 
diff --git a/data/hlint.yaml b/data/hlint.yaml
--- a/data/hlint.yaml
+++ b/data/hlint.yaml
@@ -155,6 +155,8 @@
     - warn: {lhs: "zipWith (,)", rhs: zip}
     - warn: {lhs: "zipWith3 (,,)", rhs: zip3}
     - hint: {lhs: length x == 0, rhs: null x, note: IncreasesLaziness}
+    - hint: {lhs: length x < 1, rhs: null x, note: IncreasesLaziness}
+    - hint: {lhs: length x <= 0, rhs: null x, note: IncreasesLaziness}
     - hint: {lhs: "x == []", rhs: null x}
     - hint: {lhs: length x /= 0, rhs: not (null x), note: IncreasesLaziness, name: Use null}
     - hint: {lhs: "\\x -> [x]", rhs: "(:[])", name: "Use :"}
@@ -535,6 +537,11 @@
     - hint: {lhs: throw (ErrorCall a), rhs: error a}
     - warn: {lhs: toException NonTermination, rhs: nonTermination}
     - warn: {lhs: toException NestedAtomically, rhs: nestedAtomically}
+
+    # IOREF
+
+    - hint: {lhs: modifyIORef r (const x), rhs: writeIORef r x}
+    - hint: {lhs: modifyIORef r (\v -> x), rhs: writeIORef r x}
 
     # STOREABLE/PTR
 
diff --git a/hlint.cabal b/hlint.cabal
--- a/hlint.cabal
+++ b/hlint.cabal
@@ -1,7 +1,7 @@
 cabal-version:      >= 1.18
 build-type:         Simple
 name:               hlint
-version:            2.1.21
+version:            2.1.22
 license:            BSD3
 license-file:       LICENSE
 category:           Development
@@ -60,7 +60,7 @@
         extra >= 1.6.6,
         refact >= 0.3,
         aeson >= 1.1.2.0,
-        ghc-lib-parser >= 0.20190423
+        ghc-lib-parser >= 0.20190523
 
     if flag(gpl)
         build-depends: hscolour >= 1.21
diff --git a/src/HSE/All.hs b/src/HSE/All.hs
--- a/src/HSE/All.hs
+++ b/src/HSE/All.hs
@@ -225,13 +225,10 @@
             (ParseFailed sl msg, pfailed) ->
                 failOpParseModuleEx ppstr flags file str sl msg $ fromPFailed pfailed
     where
-        -- TEMPORARY HACK TO AVOID EVALUATING parseFileGhcLib
-        -- see https://github.com/ndmitchell/hlint/issues/637
-        -- fromPFailed (PFailed x) = Just x
+        fromPFailed (PFailed x) = Just x
         fromPFailed _ = Nothing
 
-        -- TEMPORARY HACK
-        -- fromPOk (POk _ x) = Just x
+        fromPOk (POk _ x) = Just x
         fromPOk _ = Nothing
 
         fixity = fromMaybe [] $ fixities $ hseFlags flags