7.8.1, template haskell, and dynamic libraries

Austin Seipp austin at well-typed.com
Sun Feb 9 21:14:02 UTC 2014

Actually, just to keep it even simpler, so nobody else is confused
further: Cabal will *also* properly turn on dynamic builds for regular
packages if GHC is dynamic, TemplateHaskell or not. So any library you
compile will still work in GHCi as expected.

So here's the breakdown:

  1) Cabal 1.18 will handle dynamical GHCi correctly, including
compiling things dynamically, however it must.
  2) Per #1, libraries are compiled dynamically. This means libraries
work in GHCi, just like they did.
  3) -Executables- are statically linked by default, still. (But
because of #1 and #2, it's very easy to turn on dynamic exes as well,
without needing to recompile a lot.)
  4) TemplateHaskell works as expected due to #1 and #2. But there is
one caveat for executables, noted separately below.

The caveat with TemplateHaskell is for executables: This is because if
you end up with an executable that needs TH and profiling, Cabal must
be aware of this. Why? Because GHCi cannot load *profiled* objects,
only normal ones. So we must compile twice: once without profiling,
and once with profiling. The second compilation will use the 'normal'
objects, even though the final executable will be profiled. Cabal
doesn't know to do this if it doesn't know TemplateHaskell is a

Does this clear things up? My last message might give the impression
some things aren't compiled dynamically, because I merely ambiguously
referred to 'packages'.

On Sun, Feb 9, 2014 at 2:37 PM, Austin Seipp <austin at well-typed.com> wrote:
> It is correct that Template Haskell now requires dynamic objects.
> However, GHC can produce static and dynamic objects at the same time,
> so you don't have to recompile a package twice (it's a big
> optimization, basically).
> Furthermore, if TemplateHaskell is enabled as a requirement for a
> package, and GHC is built dynamically, Cabal will do The Right Thing
> by building the shared objects for the dependencies as well. It will
> save time by doing so using -dynamic-too, if possible. This is because
> it queries GHC before compilation to figure this out (you can run 'ghc
> --info' with the 7.8.1 RC to see "GHC Dynamic" and "Supports
> dynamic-too" fields.)
> Finally, if you simply run 'ghc Foo.hs' on a file that requires
> TemplateHaskell, it will also switch on -dynamic-too for the needed
> objects in this simple case.
> There is one caveat, if I remember correctly: if a package uses
> TemplateHaskell, it must declare it as such in the Cabal file. This is
> because Cabal does not parse the source to detect if TemplateHaskell
> is needed in the dependency graph of the compiled modules. Only GHC
> can do this reliably. If you don't specify TemplateHaskell as an
> extension, Cabal might not do the right thing. This is noted in the
> release notes:
>> Note that Cabal will correctly handle -dynamic-too for you automatically, especially when -XTemplateHaskell is needed - but you *must* tell Cabal you are using the TemplateHaskell extension.
> However, based on the other suggestions in the thread and confusion
> around this, a big "Incompatible changes" section with this listed as
> the first thing with clear detail would be a good idea. I'll do so.
> If something else is going on, please file a bug.
> On Sun, Feb 9, 2014 at 1:37 PM, George Colpitts
> <george.colpitts at gmail.com> wrote:
>> Yes, in general I think the doc needs a section: Incompatible changes. The
>> hope is that you can take the release and just work as usual but when (for
>> good reasons as in this release) it is not true is is important to have such
>> a section. Another case that needs to be there is how to compile so you can
>> load compile object files into ghci as what you did in 7.6.3 won't work in
>> this release.
>> On Sun, Feb 9, 2014 at 1:11 PM, Carter Schonwald
>> <carter.schonwald at gmail.com> wrote:
>>> Indeed. The problem is that many folks might have cabal config files that
>>> explicitly disable shared.  (For the compile times!).  They might need clear
>>> information about wiping that field.
>>> On Sunday, February 9, 2014, Brandon Allbery <allbery.b at gmail.com> wrote:
>>>> On Sun, Feb 9, 2014 at 9:28 AM, Greg Horn <gregmainland at gmail.com> wrote:
>>>>> Is --enable-shared off by default?
>>>> It's supposed to be on by default in 7.8. That said, not sure how many
>>>> people have played with ~/.cabal/config....
>>>> --
>>>> brandon s allbery kf8nh                               sine nomine
>>>> associates
>>>> allbery.b at gmail.com
>>>> ballbery at sinenomine.net
>>>> unix, openafs, kerberos, infrastructure, xmonad
>>>> http://sinenomine.net
>>> _______________________________________________
>>> Glasgow-haskell-users mailing list
>>> Glasgow-haskell-users at haskell.org
>>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>> _______________________________________________
>> Glasgow-haskell-users mailing list
>> Glasgow-haskell-users at haskell.org
>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
> --
> Regards,
> Austin Seipp, Haskell Consultant
> Well-Typed LLP, http://www.well-typed.com/


Austin Seipp, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/

More information about the Glasgow-haskell-users mailing list