optimization - 解包严格字段如何与多态性结合在一起？

Question

{-# UNPACK #-} pragma 告诉编译器消除多余的构造函数。报价 Haskell wiki :

For example, given this:

data T = T {-# UNPACK #-} !(Int,Float)

GHC will represent the type T like this:

data T = T Int Float

eliminating the tuple. This is commonly used to put unboxed Ints directly in a constructor:

data T = T {-# UNPACK #-} !Int

will be represented as

data T = T Int#

我想知道， 当要解包的字段是多态的时，这是否也有效？ 例如，如果我定义

data S' a   = S String {-# UNPACK #-} !a

type S1     = S' Int
newtype S2  = S2 (S' Int)

将使用 S1 的函数或 S2被优化？

Answer 1

引自唐的blog关于开箱

It does not make sense for polymorphic fields, since they have variable size, and the compiler does not (except in very specific circumstances) have access to information about what types are in use.

您也可以阅读 Tibell 的 reply到邮件列表。

Unfortunately unpacking doesn't work for polymorphic fields (the new warning for ineffective unpack pragmas in GHC head should warn about this) ...