More failure

Andrey Mokhov andrey.mokhov at
Tue Dec 10 00:57:37 UTC 2019

Hi Simon,

(Re-sending from the email address that's allowed on the mailing list.)

> Ugh.  That's not a very happy state of affairs, is it?  It didn't happen with 'make'.
> Is it a fundamental problem, or just not yet fixed?

I think this is not a fundamental problem, but the problem of getting dependencies right.

In this case, the complexity comes from the fact that a single invocation of GHC produces a set of files, and which set depends on the command line flags, which are in turn determined dynamically by reading environment settings (specifically, `platformSupportsSharedLibs`).

Such rules are hard to describe precisely, because build systems are tuned to the typical case where we statically know, for every output file, which rule produces it -- recall the Tasks = k -> Maybe Task function from our paper. In this case, we deal with something like k -> f (Maybe Task) instead, i.e. with `f` around the Maybe.

The Make build system happens to do the right thing, somehow. I believe we should be able to express the same logic in Shake, but it's not easy.

(I never really had a chance to look at dynamic builds, since they are not supported on Windows. I guess I should finally find a Linux box for Hadrian.)

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