Question

In Haskell, lifted type products mean that there’s a semantic difference between (a,b,c) and (a, (b, c)).

If all pattern matches of all products was always irrefutable, then there would be no difference, and (a, b, c) could be syntactic sugar for (a, (b, c)).

Why did Haskell choose to lift type products?

Answer 1

One reason is that implementing seq for an unlifted product requires parallel/interleaved computation, since seq (a, b) True would be supposed to be True if and only if at least one of a and b is non-bottom. You might not find this reason terribly convincing, depending on how you feel about seq, but of course polymorphic seq is by definition a part of Haskell…