Cannot install ghc-mod due to 'happy' cannot be installed?

Question

On Mac OS X, I'm not able to install ghc-mod due to that a dependency called 'happy' cannot be installed:

bash> cabal install ghc-mod                                                                                           
Resolving dependencies...
Configuring haskell-src-exts-1.17.1...
Failed to install haskell-src-exts-1.17.1
Build log ( /Users/meng/.cabal/logs/haskell-src-exts-1.17.1.log ):
Configuring haskell-src-exts-1.17.1...
setup-Simple-Cabal-1.22.5.0-x86_64-osx-ghc-7.10.3: The program 'happy' version
>=1.19 is required but it could not be found.
cabal: Error: some packages failed to install:
ghc-mod-5.5.0.0 depends on haskell-src-exts-1.17.1 which failed to install.
haskell-src-exts-1.17.1 failed during the configure step. The exception was:
ExitFailure 1
hlint-1.9.31 depends on haskell-src-exts-1.17.1 which failed to install.

Any idea?

Answer 1

For some reason, cabal doesn't install executable dependencies, but you can get around this by installing by hand.

cabal install happy

then proceed with the original installation.

---

This problem generally doesn't go away with stack either, but the solution is similar

stack install happy

then continue with the installation.

Answer 2

it seems the program happy is not installed in your system, therefore the package haskell-src-exts-1.17.1 (a dependency of a dependency of ghc-mod) which depends on it fails to install.

The common wisdom today, especially for beginners, is to not use the tool cabal-install, but to use stack for everything you need instead. including installing ghc itself. after downloading stack I think all you have to do is run stack install ghc-mod to install ghc-mod.