What does the UNPACK pragma do, in this case?

Question

I have trouble understanding how UNPACK works in Haskell. Consider, for example, the following data declarations:

data P a b = P !a !b
data T = T {-# UNPACK #-} !(P Int Int)

How will datatype T be unpacked? Will it be equivalent to

data T' = T' !Int !Int

or will the Ints be further unpacked:

data T'' = T'' Int# Int#

? What about

data U = U {-# UNPACK #-} !(P Int (P Int Int))

?

Answer 1

The GHC documentation describes the UNPACK pragma as follows:

The UNPACK indicates to the compiler that it should unpack the contents of a constructor field into the constructor itself, removing a level of indirection.

---

How will datatype T be unpacked?

data T = T (P Int Int) corresponds to

Therefore, data T = T {-# UNPACK #-} !(P Int Int) corresponds to

In plain English, UNPACK has unpacked the contents of constructor P into the field of constructor T, removing one level of indirection and one constructor header (P).

data T = T {-# UNPACK #-} !(P Int Int) isn't as "compact" as data T'' = T'' Int# Int#:

---

What about

data U = U {-# UNPACK #-} !(P Int (P Int Int))

?

Similarly, data U = U (P Int (P Int Int)) corresponds to

and data U = U {-# UNPACK #-} !(P Int (P Int Int)) corresponds to

In plain English, UNPACK has unpacked the contents of constructor P into the field of constructor U, removing one level of indirection and one constructor header (P).

Resources

• GHC manual (section 7.22)
• Johann Tibell's ZuriHac 2015 slides