Initial Haskell support for allowed tokens

This is based off of tags generated by Hasktags:

https://hackage.haskell.org/package/hasktags

src/Unused/LikelihoodCalculator.hs

@@ -3,7 +3,7 @@ module Unused.LikelihoodCalculator
     ) where
 
 import Unused.Types
-import Unused.ResponseFilter (railsSingleOkay, elixirSingleOkay)
+import Unused.ResponseFilter (railsSingleOkay, elixirSingleOkay, haskellSingleOkay)
 
 calculateLikelihood :: TermResults -> TermResults
 calculateLikelihood r =
@@ -14,6 +14,7 @@ calculateLikelihood r =
     newLikelihood
         | railsSingleOkay r = (Low, "a class, module, or migration that often occurs in only one file")
         | elixirSingleOkay r = (Low, "a class, module, or migration that often occurs in only one file")
+        | haskellSingleOkay r = (Low, "a module, function, or special case that often occurs in only one file")
         | singleNonTestUsage r && testsExist r = (High, "only the definition and corresponding tests exist")
         | doubleNonTestUsage r && testsExist r = (Medium, "only the definition and one other use, along with tests, exists")
         | totalScore < 2 = (High, "used once")

src/Unused/ResponseFilter.hs

@@ -6,6 +6,7 @@ module Unused.ResponseFilter
     , isClassOrModule
     , railsSingleOkay
     , elixirSingleOkay
+    , haskellSingleOkay
     , updateMatches
     ) where
 
@@ -57,6 +58,14 @@ elixirSingleOkay r =
     allowedTerms = ["Mixfile", "__using__"]
     paths = tmPath <$> trMatches r
 
+haskellSingleOkay :: TermResults -> Bool
+haskellSingleOkay r =
+    isAllowedTerm r allowedTerms || cabalFile
+  where
+    allowedTerms = ["instance"]
+    cabalFile = any (matchRegex "^*.cabal$") paths
+    paths = tmPath <$> trMatches r
+
 updateMatches :: ([TermMatch] -> [TermMatch]) -> TermMatchSet -> TermMatchSet
 updateMatches fm =
     Map.map (updateMatchesWith $ fm . trMatches)

test/Unused/ResponseFilterSpec.hs

@@ -102,3 +102,16 @@ spec = parallel $ do
             let result = resultsFromMatches [appToken, testToken]
 
             elixirSingleOkay result `shouldBe` True
+
+    describe "haskellSingleOkay" $ do
+        it "allows instance" $ do
+            let match = TermMatch "instance" "src/Lib/Types.hs" 1
+            let result = resultsFromMatches [match]
+
+            haskellSingleOkay result `shouldBe` True
+
+        it "allows items in the *.cabal file" $ do
+            let match = TermMatch "Lib.SomethingSpec" "lib.cabal" 1
+            let result = resultsFromMatches [match]
+
+            haskellSingleOkay result `shouldBe` True