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:
library:
source-dirs: src
+ ghc-options:
+ # For details on warnings: https://downloads.haskell.org/~ghc/master/users-guide/using-warnings.html
+ # This list taken from https://medium.com/mercury-bank/enable-all-the-warnings-a0517bc081c3
+ # 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 https://ghc.haskell.org/trac/ghc/ticket/14794#ticket
+ - -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
exposed-modules:
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.
-Wmissing-import-lists:
-module Api.Search where
+module Api.Search
+ ( SearchAPI,
+ getResults,
+ )
+where
-Wimplicit-prelude:
import Servant ((:>), Get, Handler, JSON, QueryParam)
+import Prelude
-Wincomplete-patterns:
instance FromJSON Configuration where
parseJSON (Object x) = do
-- ...
+ parseJSON _ = fail "was expecting an object"
-Wmissing-exported-signatures:
+joinComments :: Query
joinComments = "LEFT JOIN comments ON posts.id = comments.post_id"
+textOrTitle :: Query
textOrTitle = "(posts.text ILIKE ? OR posts.title ILIKE ?)"
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.