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 <chessai1...@gmail.com
<mailto:chessai1...@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
<arnaud.spiw...@tweag.io <mailto:arnaud.spiw...@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
<carter.schonw...@gmail.com
<mailto:carter.schonw...@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
<arnaud.spiw...@tweag.io <mailto:arnaud.spiw...@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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