The containers package has Data.Map.Strict and Data.IntMap.Strict, offering functions for handling maps that are strict in the values stored. But these modules are lies, as comments in the source explain: -- API of this module is strict in the keys, but lazy in the values. -- If you need value-strict maps, use "Data.Map.Strict" instead. -- The 'Map' type itself is shared between the lazy and strict modules, -- meaning that the same 'Map' value can be passed to functions in -- both modules (although that is rarely needed). -- Be aware that the 'Functor', 'Traversable' and 'Data' instances -- are the same as for the "Data.Map.Lazy" module, so if they are used -- on strict maps, the resulting maps will be lazy. Yuck. This is not nice. Clearly, the current interfaces need to stay around for compatibility, but I think we should add some unbroken ones too. There are two and a half sane ways to do this: 1. Make strict types that just wrap the lazy ones in newtype wrappers. The benefits: a) They can reuse all the code from Data.Map.Strict or Data.IntMap.Strict, producing tiny little wrapper modules. b) They can support cheap strictToLazy and unsafeLazyToStrict operations (newtype unwrappers/wrappers). c) They support lazyToStrict operations with no significant new code (reusing the NFData code). 2. Make entirely separate strict types. The benefits: a) The code implementing them does not have to be careful to force everything as it goes in, so the strictness is maintained automatically; changes to the code don't need careful strictness auditing. b) Values only need to be forced on insertion, and GHC should be able to recognize that returned values will have been forced. This should speed things up when lookups are common and updates are rare. c) There may (I'm not sure) be more opportunities for GHC to unbox values when their types are known. 2.5. Do both of the above.