Add command-line options --hash and --no-hash

[sol]

This is an alternative proposal for #381. It adds two new command-line flags, --no-hash and --hash. A brief description of the semantics follows.

Conceptually, there are four different modes, of which hpack exposes three to the user.

ForceNoHash

(enabled with --no-hash, implies --force)

• No hash will be included in the header comment of the generated cabal file

ForceHash

(enabled with --hash, implies --force)

• A hash will be included in the header comment of the generated cabal file

PreferNoHash

(the default behavior for hpack)

• When there is no existing cabal file it behaves like ForceNoHash
• When there is an existing cabal file the convention from that file is followed

PreferHash

(not exposed, but available through the API)

• When there is no existing cabal file it behaves like ForceHash
• When there is an existing cabal file the convention from that file is followed

[sol]

@snoyberg what behavior do you think will be appropriate for stack? Given that the hashes are a pain to work with when you put .cabal under revision control, I'm tempted to remove support for them entirely. Would that work for you?

[snoyberg]

I honestly don't remember the discussion that led to hashes being included in the first place, or what problems the hashes were intended to solve. I can't think of a way this would break Stack, but I may not be remembering a corner case. Do you remember where we discussed this before?

[sol]

@snoyberg took me a while to dig it up, commercialhaskell/stack#3383 (comment).

[snoyberg]

It looks like the original issue the hashes are meant to solve is: user edits cabal file instead of hpack file. There's a pretty clear warning at the top of the generated cabal file that it's autogenerated, and hpack files are probably better understood now than when that original issue opened. I wouldn't advocate for removing the hash, but I'm also not opposed to it. Entirely your call :)

[snoyberg]

And thanks for doing the digging on finding that, and pinging me on the topic in the first place

[Javran]

For people spending time reading every single line of this PR and yet have no clue how to disable existing hash: just delete your *.cabal file and let it regenerate.

[sol]

@Javran this may not be obvious, but hpack --force --no-hash will do what you want.