Dear all, I discovered the hard way, yesterday, that lazy let pattern matching is allowed on unboxed tuples. And that it implicitly reboxes the pattern. Here is how the manual describes it, from the relevant section https://downloads.haskell.org/ghc/latest/docs/html/users_guide/glasgow_exts.... : You can have an unboxed tuple in a pattern binding, thus f x = let (# p,q #) = h x in ..body.. If the types of p and q are not unboxed, the resulting binding is lazy like any other Haskell pattern binding. The above example desugars like this: f x = let t = case h x of { (# p,q #) -> (p,q) } p = fst t q = snd t in ..body.. Indeed, the bindings can even be recursive. Notice how h x is lazily bound, hence won’t necessarily be run when body is forced. as opposed to if I had written, for instance, let u = hxin ..body.. My question is: are we happy with this? I did find this extremely surprising. If I’m using unboxed tuples, it’s because I want to guarantee to myself a strict, unboxed behaviour. But a very subtle syntactic detail seems to break this expectation for me. My expectation would be that I would need to explicitly rebox things before they get lazy again. I find that this behaviour invites trouble. But you may disagree. Let me know!