Unpack pragma requires strictness

Created: 2023-01-11 09:52

The UNPACK pragma instruct the compiler to unwrap, or unbox, a value so that the actual value would be used instead of a pointer to it.

See the definition of Data.Map for example:

data Map k a
  = Tip 
  | Bin 
      {-# UNPACK #-} !Size -- size, unpacked Int
      !k                   -- key
      a                    -- value
      !(Map k a)           -- left side tree
      !(Map k a)           -- right side tree
 
type Size = Int

Using the unpacked Int here means the compiler saves the actual machine Int instead of a pointer. But it wasn’t strict the thunk to the Int would be what’s unpacked.