You are introducing an Eq a constraint with your use of elem. Don't use elem ( or elemIndex ). find and findIndex do not introduce such a constraint.

The given signature is [[a]] -> [a]. You can't just go around adding constraints to given signatures; you're not implementing the spec.

Same problem.

multiple numbers should sum to correct total
Expected 1927.2828119841463, but instead got 1927.2828119841465
multiple numbers with negatives should sum to correct total
Expected -5147.9807804336315, but instead got -5147.980780433631

Got these errors in testing upon submission:
multiple numbers should sum to correct total
Expected 3921.1946752712815, but instead got 3921.194675271282

multiple numbers with negatives should sum to correct total
Expected -2474.1512793585343, but instead got -2474.151279358535