Add Suffix newtype and use in PPSuffixHandler

[sheaf]

This commit adds a Suffix newtype to describe suffixes as handled by suffix handlers & preprocessors, and changes the PPSuffixHandler type synonym to use it.

It also moves some type definitions from Distribution.Simple.PreProcess to the new module Distribution.Simple.PreProcess.Types.

As this commit changes the definition of PPSuffixHandler, it will break custom Setup scripts which use the hookedPreProcessors functionality. As such, this PR should at least make mention of the breakage in a changelog; but as there isn't all that much payoff to the change it might be best to hold off entirely until we can get rid of Custom setups.

[andreabedini]

What is the point of a newtype wrapper if we export the constructor?

If the breaking change is

type PPSuffixHandler =
-  (String, BuildInfo -> LocalBuildInfo -> ComponentLocalBuildInfo -> PreProcessor)
+  (Suffix, BuildInfo -> LocalBuildInfo -> ComponentLocalBuildInfo -> PreProcessor)

I would expect Suffix to be significantly different from a String.

Actually, maybe the breaking change can be avoided entirely, the field

hookedPreProcessors :: [PPSuffixHandler]

gets passed to Cabal. If I am not mistaken, all we would need to do to keep compatibility is converting that String to a Suffix as we receive it.

[sheaf]

The intent was more of documentation than anything else:

• using a different type rather than String makes it a lot easier to understand type signatures, especially those which have other String arguments,
• using a newtype rather than a type synonym makes it so that one is less likely to make a silly mistake, and can help enforce the invariant that a suffix is something like "hs" and not ".hs".

However, I don't particular mind making it a type synonym if that's what we need to do to avoid breaking people's custom Setup scripts.

NB: I have a few additional changes involving Suffix that I hope to push to this MR.

[sheaf]

I would of course like to use a different type than String (which IMO should basically never be used), but Cabal needs to change the representation of filepaths that it uses for that change to make sense.

[philderbeast]

However, I don't particular mind making it a type synonym if that's what we need to do to avoid breaking people's custom Setup scripts.

From the docs on Custom setup scripts, these are compiled aren't they (I don't use them)? The change to a Suffix newtype would be flagged by a build failure but limited to packages with custom setups doing something with suffixes, wouldn't it? Do we have a build failure report for one of our test suites hitting this type mismatch already?

[andreabedini]

Ok, I am ok with a newtype. The breaking change is minimal after all.

[sheaf]

Do we have a build failure report for one of our test suites hitting this type mismatch already?

Yes, AutogenModulesToggling and CustomPreProcess ran into type-checking failures, which I solved without introducing CPP by enabling OverloadedStrings.

[Mikolaj]

@Kleidukos: I know you are super-busy. Shall I dismiss your blocking review, optimistically assuming all concerns have been addressed?

[Kleidukos]

[alt-romes]

I addressed @Kleidukos' review.

[Kleidukos]

@sheaf any reason why you closed the PR?

[sheaf]

This PR was already merged when merging #9543.

[Kleidukos]