存在量化类型的内存占用和相关的优化技术

Question

Consider the following data model utilizing an existential:

data Node a = Node a (Map TypeRep AnyNode)
data AnyNode = forall a. Show a => AnyNode a

The rules about memory footprint of standard types have been explained previously . Now, what are the rules for existential types, like AnyNode ?

Are there any optimization techniques, eg some workarounds using unsafeCoerce making it possible to elude the existential declaration? I'm asking this because a type similar to Node is going to be placed in a cost centre of a highly memory-intensive lib, so memory footprint is all, that's why the most dirty hacks are welcome.

Answer 1

The ghc-datasize package may be of help here:

{-# LANGUAGE RankNTypes, GADTs #-}

import GHC.DataSize

data Node = forall a. Show a => Node a 

main = do
    s <- closureSize $ Node 0 
    print s -- 24 bytes on my 64-bit system

So, it seems that Node takes one extra word compared to the plain unary data constructor, presumably because of the Show class dictionary pointer. Also, I tried adding more class constraints to Node , and each of them takes one extra word of space.

I don't know for sure whether it is possible to magic away the dictionary pointer in specific circumstances. I think it isn't possible if you'd like to keep the existential type.