On 23/01/14 03:52, John Lato wrote:
However, these are all rather obviously fixable as part of the build
system. For me, the worst problems have to do with cleaning. If you're
using a Makefile, typically you want to leave intermediate object files
around and only rebuild them when the sources have changed. However,
there are various issues with ghc batch-mode that make this difficult
(e.g. https://ghc.haskell.org/trac/ghc/ticket/8029 ). The workarounds
to deal with this are not as straightforward. The alternative is to
live with the occasional build error that can only be fixed by blowing
away the entire build dir (a remedy that I often need with ghc's source
tree, as even make maintainer-clean doesn't always cut it. Hopefully my
experience here is unique, but I do not believe it is).
You said "various issues", but you've only mentioned *one* specific
issue so far: #8029, and we concluded that was not a bug, although I do
see how it could require manually deleting a .hi file if you have a
module that shadows a package module and then remove it. This seems a
rare occurrence to me, but perhaps it is something you do often. If it
really hurts, then you could have a way to tell your build system about
a file when it is removed from the project, so that it can delete the
build artifacts that go with it.
Anyway, are there other problems you'd like to bring to our attention?
Cheers,
Simon
Also, the most common use case seems to be for parallel building of
modules. As ghc-7.8 provides this with --make, I'd expect the demand
for ghc -M will be greatly reduced. That's why I'm not certain it's
worth the time it would take to resolve these issues.
Cheers,
John
Cheers,
Simon
For an example of some of the extra steps necessary to make
something like this work, see e.g.
https://github.com/nh2/__multishake
<https://github.com/nh2/multishake> (which is admittedly for a more
complicated setup, and also has some issues). The especially
frustrating part is, just when you think you have everything
working,
someone wants to add some other tool to a workflow (hsc2hs, .cmm
files,
etc), and your build system doesn't support it.
ghc --make doesn't allow building several binaries in one run,
however
if you use cabal all the separate runs will use a shared build
directory, so subsequent builds will be able to take advantage
of the
intermediate output of the first build. Of course you could do
the same
without cabal, but it's a convenient way to create a common build
directory and manage multiple targets. This is the approach I would
take to building multiple executables from the same source files.
ghc doesn't do any locking of build files AFAIK. Running
parallel ghc
commands for two main modules that have the same import, using
the same
working directory, is not safe. In pathological cases the two
different
main modules may even generate different code *for the imported
module*.
This sort of situation can arise with the IncoherentInstances
extension, for example.
The obvious approach is of course to make a library out of your
common
files. This has the downsides of requiring a bit more work on the
developer's part, but if the common files are relatively stable
it'll
probably lead to the fastest builds of your executables. Also
in this
case you could run multiple `ghc --make`s in parallel, using
different
build directories, since they won't be rebuilding any common code.
John L.
On Sun, Jan 5, 2014 at 1:47 PM, Sami Liedes <sami.lie...@iki.fi
<mailto:sami.lie...@iki.fi>
<mailto:sami.lie...@iki.fi <mailto:sami.lie...@iki.fi>>> wrote:
Hi,
I have a Haskell project where a number of executables are
produced
from mostly the same modules. I'm using a Makefile to
enable parallel
builds. I received advice[1] that ghc -M is broken, but
that there
is parallel ghc --make in HEAD.
As far as I can tell, ghc --make does not allow building
several
binaries in one run, so I think it may not still be a full
replacement
for Makefiles.
However I have a question about ghc --make that is also
relevant
without parallel ghc --make:
If I have two main modules, prog1.hs and prog2.hs, which
have mutual
dependencies (for example, both import A from A.hs), is it
safe to run
"ghc --make prog1" in parallel with "ghc --make prog2"?
IOW, is there
some kind of locking to prevent both from building module A
at the
same time and interfering with each other?
Is there a good way (either in current releases or HEAD) to
build
multiple binaries partially from the same sources in parallel?
Sami
[1]
http://stackoverflow.com/__questions/20938894/generating-__correct-link-dependencies-for-__ghc-and-makefile-style-builds
<http://stackoverflow.com/questions/20938894/generating-correct-link-dependencies-for-ghc-and-makefile-style-builds>
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