Why is it common to mark record fields with UNPACK and strictness?

Question

I noticed this pattern is very common in Haskell libraries:

data Foo = Foo { field :: {-# UNPACK #-} !Sometype } 

e.g. UNPACKing a field's type and making it strict.

I understand what's the effect of the pragma and annotation but I don't understand why it is so pervasive: I have been programming in Haskell for 15 years and seldom used strictness annotation, and never UNPACK pragma.

If this idiom is so useful, why not make it less "ugly"?

Answer 1

The pragma may be a bit ugly, but it avoids a lot more ugliness elsewhere. When performance is critical, programmers often need to choose a particular shape for a data constructor. Suppose I have

data Point = Point Int Int
data Segment = Segment Point Point

That makes good logical sense, but it has a bunch of extra indirection: one Segment consists of seven heap objects. If I'm working with a lot of segments, that's pretty bad.

I could squash this flat by hand:

data Segment = Segment Int# Int# Int# Int#

but now I've lost the fact that the numbers represent points, and everything I do with a segment will have to involve rather inconvenient and weird unboxed operations.

Fortunately, there's a better way:

-- The small strict Int fields will be unpacked by default
-- with any reasonably recent GHC version.
data Point = Point !Int !Int

data Segment = Segment {-# UNPACK #-} !Point {-# UNPACK #-} !Point

This still gives me one heap object per segment, but I can use Points and Ints and (generally) rely on the compiler unboxing everything nicely.