Thanks for teaching me about this function!

fixed

Already raised as issue

Nice catch! Fixed.

λ: solution "CDLXI" -- must be: 461
661

λ: solution "MMMCMXV" -- should be 3915
4115

λ: solution "MMMCMXV" -- should be 3915
4115

λ: solution "MMMCMXXIX" -- should be 3929
4131

λ: solution "MCXLI" -- should be 1141
1161

λ: solution "MCXLI" -- should be 1141
1161

FYI: Tests are incomplete, sorry. Your code is not OK.

λ: solution "CMVIII"    -- must be 908
1108
λ: solution "XIV"       -- must be 14
16

Hi! FYI: Your solution function pass fawlty tests, but it's wrong.

λ: solution "MCCXXXIV"   -- It must be 1234
1236
λ: solution "MMMCCXLI"   -- It must be 3241
3261

HASKELL:

In order to prevent people faking a solution by writing a tricky Num instance, proposed tests must change.
This modified specs will do the thing:

spec :: Spec
spec =
  describe "solution" $ do
    it "testing 'XXI'"     $ (solution "XXI")     `shouldBe` ( 21 :: Integral a => a )
    it "testing 'I'"       $ (solution "I")       `shouldBe` 1
    it "testing 'IV'"      $ (solution "IV")      `shouldBe` 4
    it "testing 'MMVIII'"  $ (solution "MMVIII")  `shouldBe` 2008
    it "testing 'MDCLXVI'" $ (solution "MDCLXVI") `shouldBe` 1666

Current tests are:

spec :: Spec
spec =
  describe "solution" $ do
    it "testing 'XXI'"     $ (solution "XXI")     `shouldBe` 21
    it "testing 'I'"       $ (solution "I")       `shouldBe` 1
    it "testing 'IV'"      $ (solution "IV")      `shouldBe` 4
    it "testing 'MMVIII'"  $ (solution "MMVIII")  `shouldBe` 2008
    it "testing 'MDCLXVI'" $ (solution "MDCLXVI") `shouldBe` 1666