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Re: Dependency cycle issues when using a Gexp-based snippet
From: |
Ludovic Courtès |
Subject: |
Re: Dependency cycle issues when using a Gexp-based snippet |
Date: |
Wed, 16 Sep 2020 12:08:08 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) |
Hi,
Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:
>>> What is the difference between delayed and thunked? Would a thunked
>>> capture the closure of its environment while delayed not? Is the
>>> closure useful to access record-bound values such as the version field
>>> of a package?
>>
>> ‘Thunk’ uses an actual thunk (zero-argument procedure) that’s called
>> each time the field is accessed; ‘delayed’ uses a promise, which is
>> similar except that the result is memoized (info "(guile) Delayed
>> Evaluation").
>
> Thanks for the explanation! Now I wonder why delayed should not always
> be preferable to thunks? Is there a reason to offer thunked as well?
Thunked fields were introduced pretty much as a hack so one could look
up dynamic bindings ‘%current-system’ and ‘%current-target-system’ and
produce a result that’s a function of that. (See
<https://hal.inria.fr/hal-00824004/en>.)
>> What would be interesting is a comparison of the performance of
>> ‘package-derivation’, which can be done with something like:
>>
>> time guix build -d --no-grafts libreoffice pandoc
>>
>> For memory consumption, try:
>>
>> GUIX_PROFILING=gc guix build -d --no-grafts libreoffice pandoc
>
> Thanks for these "benchmarking" tips :-). Unfortunately, making the
> 'snippet' field either thunked or delayed causes 'guix build' to stop
> working entirely, peaking the CPU and slowy eating RAM away (looks like
> a typical dependency cycle).
Hmm. In packages.scm, there’s a couple of places where <origin> are
matched directly, which means that you get to see the raw
thunk/promise. The solution is to explicitly call the thunk/force the
promise there, like we do for the ‘patches’ field in this clause:
($ <origin> uri method hash name (= force (patches ...)) snippet
(flags ...) inputs (modules ...) guile-for-build)
If that still doesn’t help, please send a patch! :-)
Ludo’.