Re: heads-up: Haskell updates

[Ricardo Wurmus]

Mark H Weaver <address@hidden> writes:

> Given that it's using #ifdef to detect this, I'd expect the result to
> depend only on the _headers_ being used to compile, and not the actual
> kernel running the build.  Is that right?  If so, this doesn't explain
> why it works for Ricardo but not for Andreas and Hydra.
>
> Note that on our 'master' branch we are using Linux-libre-4.4 kernel
> headers, but on 'core-updates' we're using 4.9 kernel headers.

FWIW, I only tried to build ghc-resourcet on the master branch, not on
core-updates.

> I suppose if one reads the error message literally:
>
>   Control/Monad/Trans/Resource/Internal.hs:302:10: error:
>       • Could not deduce (MonadBase IO (Strict.StateT s m))
>           arising from the superclasses of an instance declaration
>         from the context: MonadResource m
>           bound by the instance declaration
>           at Control/Monad/Trans/Resource/Internal.hs:302:10-63
>       • In the instance declaration for
>           ‘MonadResource (Strict.StateT s m)’
>
> "Could not deduce" might be interpreted to include the case where we
> failed for lack of sufficient memory.  They might have written the code
> to catch any error at all, including out of memory errors, and report
> the failure with this error message.

I think that’s a misunderstanding.  The cause for the error is earlier
when it complains that some packages depend on different versions of the
“transformers” package.  “StateT” is a monad transformer.

I think that “ghc-pkg” fails, which leads to some other version of the
transformers package (maybe a default included with GHC) to be used.  In
the case where ghc-pkg does not fail (as on my laptop) the proper
version is used, so there are no problems finding the expected
instances.

--
Ricardo

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