Use of unsafePerformIO

[throwaway1037]

https://github.com/jaspervdj/hakyll/blob/master/lib/Hakyll/Main.hs#L202
progName = unsafePerformIO getProgName

Is the use of unsafePerformIO necessary?
Is there a reason this cannot be done safely, or was this a post-hoc addition?

I think refactoring it to be a safe call to getProgName would require modifying the functions which use progName.

[Minoru]

Hi! Judging by git log --patch --follow lib/Hakyll/Main.hs, this used to be an implementation detail that became a part of the interface with #755. Now, we can't easily avoid unsafePerformIO because the users of the value don't use the IO monad.

I don't see much harm in this call though. It simply looks at the first element of argv, which is technically not a constant, but practically is.

I'll merge a PR that refactors this, preferably in a way that doesn't change the existing type signatures (so add a new function, deprecate the old one). But I honestly don't think that this is worth an effort, so I'll close this issue now.