| Copyright | (c) Sergey Vinokurov 2019 |
|---|---|
| Safe Haskell | Safe-Inferred |
| Language | Haskell2010 |
FastTags.LensBlaze
Description
Minimal implementation of lenses. The aim is to get lenses that allow to work with parts of an Int as another Int.
E.g. to be able to treat upper 32 bits and lower 32 bits of an integer as two different integers, read them independently and set back.
This way values that have limited range will not occupy whole pointer when stored in a struct, but rather will all be tightly packed in a single integer "store" field that can be {--}'ed.
E.g. suppose we define 2d points that will be drawn on a computer screen. We could start with
data Point = Point
{ getX :: {-# UNPACK #-} !Int
, getY :: {-# UNPACK #-} !Int
}Now, this is not too bad but but observe that screens don't typically require 64 bits of precision - 32 would be enough! If only we could pack two Int32's into single Int64, that would occupy twice as less space. But we don't want to have bit masks and shifts all over the place in client code - that's error-prone. This is where this module comes in. Instead of the above, we can do:
newtype Point = Point { unPoint :: Int64 }
pointL :: Lens Point Int64
pointL = lens unPoint (\x _ -> Point x)
xCoordL, yCoordL :: Lens Point Int32
xCoordL = pointL . int32L 0
yCoordL = pointL . int32L 32Now, all the accesses to x and y coordinate have to go through lenses and we lose record update syntax. But that's a manageable price and we get 2x savings on memory traffic.