data-forced is a tiny library to remove references to unintended values on long lived data structures. It targets liveness leaks on the classification done on this blog post.

You should consider using it on data structures that are long lived. These are structured that are either:

• Stored and retrieved via get/put on a state monad close to main.
• Stored on IORef, MVar, TVar or related mutable locations.
• Passed recursively as an argument on a event loop.

The big idea

We should not throw the information that a value was forced to WHNF. We neither should be required to remember to put a seq or BangPattern at the use site to maintain this invariant.

We accomplish this using two newtypes:

• newtype ForcedWHNF a = ForcedWHNF a
• newtype ForcedNF a = ForcedNF a

that don't have their constructors exported. Only unidirectional patterns are provided so you can de-structure them.

The only way to construct values of these types is via two CBVs functions:

• demandWHNF :: forall a. a -> Demand (ForcedWHNF a)
• demandNF :: forall a. NFData a => a -> Demand (ForcedNF a)

The Demand data type is inspired by its analog Strict on data-elevator. It is a UnliftedDatatype, which means it cannot contain bottoms. The dynamic semantics of bound values of unlifted data types is that before being bound, these are evaluated to WHNF. It is a BangPattern you cannot forget!

Once you obtain the Demand (ForcedWHNF a) value, you have to extract it on IO to obtain the lifted ForceWHNF a and store that. You are guaranteed that the references necessary to force it to WHNF are now free. Hopefully you will store it on a data structure that requires values to have been forced before like

    import Data.Map.Lazy

    type Accum1 = Map Char (ForcedWHNF Int)
    type Accum2 = Map Char (ForcedNF (Maybe Text))

Acknowledgment

Sebastian Graf (sgraf812) should take all the credit. I was inspired by his data-elevator package that solves a sightly different problem. He infact pushed the proposal for unlifted data types and made the work to integrate them to GHC.