Nested UNPACKs in GHC

Question

I often gather multiple values in tuples, since I consider tuples to be the natural type for this. However, tuples are not strict. So consider

data A

data B =
    B !A

data C =
    C !(B, B)

data D =
    D !B !B

With -funbox-strict-fields and optimization, UNPACKs are nested. Will the tuple in C be unpacked into C A A as with D, or just C B B? Here is a reference in GHC-doc: 7.16. Pragmas.

(are non-empty tuples types? type-constructors?)

Answer 1

Consider:

data B = B !Int

data C = C !(B, B)

data D = D !B !B

b0 = B 7
b1 = B 4

c = C (b0, b1)

d = D b0 b1

where

data (,) a b = (a,b)

GHC will happily erase nested, strict constructors (!B), and it will make C strict in its first field, optimizing to:

b0 = B 7
b1 = B 4

c = C b0 b1

d = D 7 4

However, importantly, the fields of (,) itself are not strict -- so GHC can't unpack them. Furthermore, they're polymorphic, so it can't unpack them even if they are strict.

A work around for the first part is to use strict tuples. The workaround for the second part is to use self-specializing tuples (e.g. type families that specialize tuples).

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Note that using tuples for syntax in this style incurs overhead -- they introduce a level of indirection that must be optimized away. As such, it is somewhat unidiomatic.