1-hmAt.hs

import qualified Data.HashMap as HM

hmAt :: (Functor f, Ord t, Hashable t) =>  t -> (Maybe a -> f b) -> HM.Map t a -> f t1
hmAt k = lens g s
  where
  s v = HM.insert k v
  g = HM.lookup k

2-console output.hs

                src/Test/Hspec.hs:75: 
    2) ! eff
         uncaught exception: TypeError (/path/to/spec/Spec.hs:1353:10: error:
             • Couldn't match type ‘b’ with ‘HM.Map t a0’
               ‘b’ is a rigid type variable bound by
                 the type signature for:
                   hmAt :: forall (f :: * -> *) t a b t1.
                           (Functor f, Ord t, Hashable t) =>
                           t -> (Maybe a -> f b) -> HM.Map t a -> f t1
                 at /path/to/spec/Spec.hs:1352:9
               Expected type: (Maybe a -> f b) -> HM.Map t a -> f t1
                 Actual type: (Maybe a -> f (HM.Map t a0))
                              -> HM.Map t a -> f (HM.Map t a0)
             • In the expression: lens g s
               In an equation for ‘hmAt’:
                   hmAt k
                     = lens g s
                     where
                         s (Just v) = HM.insert k v
                         g = HM.lookup k
             • Relevant bindings include
                 s :: forall a. Maybe a -> HM.Map t a -> HM.Map t a
                   (bound at /path/to/spec/Spec.hs:1355:3)
                 g :: forall a. HM.Map t a -> Maybe a
                   (bound at /path/to/spec/Spec.hs:1356:3)
                 k :: t
                   (bound at /path/to/spec/Spec.hs:1353:6)
                 hmAt :: t -> (Maybe a -> f b) -> HM.Map t a -> f t1
                   (bound at /path/to/spec/Spec.hs:1353:1)
         (deferred type error))