DEV Community

Riccardo Odone
Riccardo Odone

Posted on • Updated on • Originally published at

Rewriting to Haskell–Making GHC More Nitpicky

You can keep reading here or jump to my blog to get the full experience, including the wonderful pink, blue and white palette.

This is part of a series:

GHC by default compiles with some warnings enabled. But there's a ton more. Thus, we decided to turn on several. Also, we have the compiler treat them as errors with -Wall. Luckily, Max Tagher did all the work for us and prepared a "Copy-Pastable List" with nice explanations for us lazy developers. Thanks Max!

This is what we threw in package.yaml:

   source-dirs: src
+  ghc-options:
+    # For details on warnings:
+    # This list taken from
+    # Enable all warnings with -Weverything, then disable the ones we don’t care about
+    - -Weverything
+    - -Werror
+    - -Wno-missing-exported-signatures # missing-exported-signatures turns off the more strict -Wmissing-signatures. See
+    - -Wno-missing-import-lists # Requires explicit imports of _every_ function (e.g. ‘$’); too strict
+    - -Wno-missed-specialisations # When GHC can’t specialize a polymorphic function. No big deal and requires fixing underlying libraries to solve.
+    - -Wno-all-missed-specialisations # See missed-specialisations
+    - -Wno-unsafe # Don’t use Safe Haskell warnings
+    - -Wno-safe # Don’t use Safe Haskell warnings
+    - -Wno-missing-local-signatures # Warning for polymorphic local bindings; nothing wrong with those.
+    - -Wno-monomorphism-restriction # Don’t warn if the monomorphism restriction is used
Enter fullscreen mode Exit fullscreen mode

Turns out we have been disciplined enough to just need a few edits to fix the new errors. What follows is a non-comprehensive list of the changes we performed together with the warning that was triggered.


-module Api.Search where
+module Api.Search
+  ( SearchAPI,
+    getResults,
+  )
Enter fullscreen mode Exit fullscreen mode


 import Servant ((:>), Get, Handler, JSON, QueryParam)
+import Prelude
Enter fullscreen mode Exit fullscreen mode


 instance FromJSON Configuration where
   parseJSON (Object x) = do
     -- ...
+  parseJSON _ = fail "was expecting an object"
Enter fullscreen mode Exit fullscreen mode


+joinComments :: Query
 joinComments = "LEFT JOIN comments ON = comments.post_id"

+textOrTitle :: Query
 textOrTitle = "(posts.text ILIKE ? OR posts.title ILIKE ?)"
Enter fullscreen mode Exit fullscreen mode

Some warnings can be annoying or even unneeded depending on the use-case, your mileage may vary. Feel free to treat this as a starting point and tweak the options down the line.

Get the latest content via email from me personally. Reply with your thoughts. Let's learn from each other. Subscribe to my PinkLetter!

Top comments (0)