Re: [Haskell-cafe] What can be UNPACK'ed?

On Mon, Jul 31, 2017 at 5:58 PM, Tom Ellis < tom-lists-haskell-cafe-2013@jaguarpaw.co.uk> wrote:

On Mon, Jul 31, 2017 at 11:44:20PM +0200, Nick Smallbone wrote:

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Tom Ellis writes:

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Thanks, that's helpful info, but what I'm really after is a definitive condition under which fields can be UNPACK'ed. The docs are very vague:

https://downloads.haskell.org/~ghc/7.0.2/docs/html/users_ guide/pragmas.html

As far as I know, the restrictions for UNPACKing a field are as follows:

Ah, very nice, thanks Nick.

I think this might be in the Commentary somewhere, as I've seen essentially that list.

* The type must not be just a type variable

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(i.e., the constructor must be known at compile time)

Eventually I'd like to see if we can relax this condition but first ...

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* The field's type must have only one constructor

Do you know the rationale for this? It seems like there should be no problem unpacking a constructor tag plus payload, even if the payload size varies, as long as the maximum payload size in known.

UNPACK means unbox, and the constructor tag is part of the box. I suspect something (gc?) assumes that a constructor tag is always the first thing in a memory chunk, so having a constructor tag unpacked into the middle of something else causes problems (e.g. it will try to gc the middle of the containing memory object). -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net