GHC 8.0 moves towards making Haskell programs more strict by introducing two new language pragmas that transform programs to programs that evaluate more like programs in a call-by-value language. One nice side-effect of having more strict programs is that we now need thunks less, and without thunks we don’t need boxing as much as before. GHC already has some compiler passes that optimize cases where we don’t need thunking and boxing, but those optimizations were only working on values with product types.

This talk will introduce “UnboxedSums”, a new GHC extension that adds a new kind of first-class value (anonymous unboxed sums) to the language that makes more efficient representation of strict sum types possible. This extension enables GHC to store strict sum types in “unboxed” form, and by extending existing “demand analysis” and “constructed product results analysis” passes it makes it possible for strict functions on sum types to use more efficient calling conventions (improving both argument passing and returning).

To our knowledge, this feature is novel. While it was always possible to do in languages with manual memory management, no other garbage-collected language has this feature and does the optimizations we do on sum types.