Hi Matthew You'll have to give `a` a concrete type, as `singleton` is overloaded, GHC cannot infer a type. The TrieMap type is quite complicated as it allows different representations of (finite) Maps within it. There are three different internal maps you can use (the three pre-defined instances of the Map class - Data.Map, WrappedIntMap and AList) - I'd go for Data.Map as it will be faster than AList and I think you are using Char for key so you can't use an IntMap. I don't have the package `list-tries` installed but I'd guess at one of these two for the concrete type signature. Assuming you have imported Data.Map as import qualified Data.Map as Map (Probably this...) a :: TrieMap Map.Map [Char] Int (Possibly this...) a :: TrieMap (Map.Map Char Int) [Char] Int