Compiling projects with dependencies using GHC HEAD

Ben Gamari ben at
Tue Oct 1 23:56:11 UTC 2019

Alexis King <lexi.lambda at> writes:

>> On Oct 1, 2019, at 18:19, Ben Gamari <ben at> wrote:
>> Your question is a very good one which, until quite recently, there was
>> not a particularly good answer to. However, in the past months we have
>> been working on infrastructure (in the form of head.hackage [0]) to make
>> GHC more testable in pre-release form.
>> I have a draft blog post explaining the currently state-of-play
>> here [1,2]. Unfortunately between ICFP, release things, vacation, and bug
>> fixing I've lacked the time to finish these off (the tutorial in particular).
> Hi Ben,
> Many thanks for your response—this infrastructure looks incredibly helpful! I’ll give it a try. One question I do have just from giving it a look involves the cabal.constraints file available here:
> <>
> A number of packages appear to be listed twice, and some of them (but
> not all of them) are pinned to different versions. Is that
> intentional? Or is it just a part of still being a bit of a work in
> progress?
Indeed it's just a function of this being a bit work in progress; we
only started allowing multiple patch versions relatively recently and it
looks like the constraint file generation hasn't been updated to account
for this yet. I'll try to fix that tonight.

> In any case, this gets me quite a bit further, and though I’m not sure
> yet if my project will actually build, I can now at least construct a
> valid build plan. Thanks again!

- Ben

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