Depend on happy when installing haskell-src-exts

[ndmitchell]

See haskell-suite/haskell-src-exts#14. Stack should have some reasonable story for installing happy or ensuring it is available before building haskell-src-exts. I had a Hoogle user bump into this problem yesterday. The story may be that stack setup ensures happy.exe is present.

[snoyberg]

This hopefully already works, and better than you were expecting. stack automatically detects build tool dependencies and installs them on demand. So trying to install hoogle will detect the necessary happy build tool and install it. I checked on my machine with the following stack.yaml, and stack build --dry-run included happy:

resolver: lts-2.5
extra-deps:
- hoogle-4.2.41
packages: []

Note that I used lts-2.5 arbitrarily as a plan I hadn't built anything with yet.

[ndmitchell]

Awesome! In that case this ticket is a note to document it somewhere, and ideally add a test.

[snoyberg]

Good call, I just updated the README with a FAQ on build tools (though perhaps it should be elaborated upon), and added an item to our test suite issue (#26) about including this.