MIN_VERSION macros

[Reid Barton]

On Fri, Sep 25, 2015 at 12:18 PM, Eric Seidel <eric at seidel.io> wrote:

> I've been meaning to ask about this as well. It also forces tools like
> ghc-mod and hdevtools to be cabal-aware, which is an unnecessary source
> of complexity IMO.
>

This would certainly be nice, but...

GHC certainly has enough information to generate these macros, as it
> knows which packages (and versions) it's compiling against.
>

It knows at some point, but it doesn't necessarily know before parsing the
module, at which point it is too late. I can have two versions of a package
A, and two other packages B and C that depend on different versions of A,
and depending on whether a module M uses package B or package C, M will see
different versions of package A automatically. This is all slightly
magical, and I have to say I don't entirely understand how GHC decides
which versions to expose in general, but that's how GHC works today and
it's quite convenient.

GHC could provide MIN_VERSION_* macros for packages that have had their
versions specified with -package or similar flags (which is how Cabal
invokes GHC). That would go only a small way towards the original goals
though.

(Also, I wonder how MIN_VERSION_* fits into a Backpack world...)

Regards,
Reid Barton
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