Re: Implicit reboxing of unboxed tuple in let-patterns

I haven't used unboxed tuples enough to perhaps feel the pain, but on paper the current design makes sense to me. The laziness of the binding is suppose to have to do with the runtime rep of the binding itself, not any enclosing pattern. For example take     {-# LANGUAGE ScopedTypeVariables #-}     {-# LANGUAGE MagicHash #-}     import GHC.Base     data Foo = Foo Int# Int#     main = pure ()         where Foo x y = Foo undefined undefined This program will fail even though x and y are unused. While this principle may not match how this stuff is used in practice, the alternative of making the strictness of the bindings depend on more than their runtime reps seems less-local / more ad-hoc to me. John On 8/31/20 10:34 AM, Spiwack, Arnaud wrote:

I’ve been pointed to https://github.com/ghc-proposals/ghc-proposals/pull/35 where this was debated a few years ago. With much of the same arguments as today.

Simon Marlow said

making an unboxed tuple binding lazy by default seems to be intuitively the wrong choice. I guarantee I would get tripped up by this! Giving unboxed tuples an implicit bang seems reasonable to me.

I can share that I got tripped by it. And so were other members of my team.

That being said, Richard seemed to feel rather strongly about this one. Richard, do you still agree with your then position that |let (#x, y#) = …| being a lazy pattern (hence implicitly boxes the pair) is the right semantics?

On Fri, Aug 28, 2020 at 8:26 PM chessai mailto:chessai1996@gmail.com> wrote:

Arnaud,

I have dealt with this in the past and find the laziness extremely counterintuitive and never wanted. Every time I have let-bound an unboxed tuple, I have never wanted that boxing to occur. Perhaps there is a good reason this is the case but I wish it would change.

On Fri, Aug 28, 2020, 08:26 Spiwack, Arnaud mailto:arnaud.spiwack@tweag.io> wrote:

Hi Carter,

We are using |let !(#x,y#) = …| actually. Having the strict behaviour is not particularly difficult. You can even use |case … of (#x, y#) ->…| directly, it’s not too bad. My complaint, as it were, is solely about the potential for mistakes.

On Fri, Aug 28, 2020 at 3:20 PM Carter Schonwald mailto:carter.schonwald@gmail.com> wrote:

Have you tried using do notation for bindings you want to keep strict, with Eg the identity monad?  That doesn’t address  the design critique but gives you a path forward ?

I do agree that the semantics / default recursivity Of let bindings  can be inappropriate for non recursive code , but would any other non uniform semantics or optimization be safe?

On Fri, Aug 28, 2020 at 9:05 AM Spiwack, Arnaud mailto:arnaud.spiwack@tweag.io> wrote:

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 = hx in ..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!

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