Data.HashMap: Strict or lazy by default?

[Jan-Willem Maessen]

NB I believe that it is possible to make a lazy-valued Map (IntMap,
HashMap, etc...) from a strict one via:

data Lazy a = Lazy a

newtype LazyMap k v = StrictMap k (Lazy v)

So in principle (and I believe this is only useful for key-strict
structures) the strict map is more "universal".  A separate lazy
implementation mostly saves us a layer of indirection.

-Jan-Willem Maessen