Implicit reboxing of unboxed tuple in let-patterns
Carter Schonwald
carter.schonwald at gmail.com
Fri Aug 28 13:20:04 UTC 2020
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 <arnaud.spiwack at 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.html#extension-UnboxedTuples>
> :
>
>
>
>
> 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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