Introducing the `stack hoogle` feature!

With the release of hoogle5, we can now hoogle all packages referenced
by our packages in our `stack.yaml`. This feature is on the master
branch of stack, but is not yet on a stack release. We'd like you to
try it out before we do!

To upgrade to the latest stack from git, use:

    $ stack upgrade --git

Now, in any stack project, run:

    $ stack hoogle map

After a bunch of setup work (see below), you'll get something like

    bash-3.2$ stack hoogle map
    map :: (a -> b) -> [a] -> [b]
    map :: (Word8 -> Word8) -> ByteString -> ByteString
    [..]

Pass `-i` to show documentation:

    bash-3.2$ stack hoogle -- -i map
    map :: (a -> b) -> [a] -> [b]
    base Prelude
    map f xs is the list obtained by applying f
    to each element of xs, i.e.,


    map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn]
    map f [x1, x2, ...] == [f x1, f x2, ...]

Each project has its own installation of hoogle:

    bash-3.2$ stack exec which hoogle
    /Users/chris/Work/hs-popen/.stack-work/install/x86_64-osx/lts-5.3/7.10.3/bin/hoogle

Unless you install it globally, in which case it'll use the one in
your PATH if it's >=5.0. So if you get sick of it installing copies of
hoogle, do `stack install hoogle-5.0`.

Please give it a try!

---

Hoogle5 is probably not installed on your system, so you'll see this
message:

> Hoogle isn't installed or is too old. Automatically installing (use --no-setup to disable) ...
> Minimum version is hoogle-5.0. Found acceptable hoogle-5.0 in your index, installing it.

After installing Hoogle5 it'll say:

> No Hoogle database yet. Automatically building haddocks and hoogle
> database (use --no-setup to disable) ...

Then it'll run `stack haddock` and then `stack hoogle -- generate
--local`.

If you see a message like

> Packages not found: abstract-par aeson aeson-compat [..]

Don't worry about it; stack only generates haddocks for the things
you're using in your project, whereas hoogle is looking at everything
in the snapshot database.

Relatedly, if you add a dependency to your project, run `stack hoogle
--rebuild`, and that'll generate the haddocks and the hoogle database
again.