Haskell: Problems using Happy 1.19.5 with GHC 7.10.3

1
votes

the latest version of Happy gives the following error message with GHC 7.10.4

No instance for (Applicative HappyIdentity) arising from the superclasses of an instance declaration

I think I should nof mess up Happy, but how can I solve my problem?

1
haskellhappy
happy 1.19.5 actually compiles fine for me with GHC 7.10.3. Are you sure you were trying to compile 1.19.5 and not an older version? – Xandaros

1 Answers

1
votes

Begining with GHC 7.10 the "Applicative Monad Proposal" has been implemented. This is a breaking change and the Happy library has not yet been updated to address this.

There is a migration guide in the haskell wiki, which addresses exactly this problem.

Basically since Applicative and Functor are now superclasses of Monad you need to add instance declarations. In this case HappyIdentity is an instance of Monad so you need to add instances Applicative HappyIdentity and Functor HappyIdentity. In addition the Monad instance can be removed since it is now redundant.

Without modifying the library yourself, the only option is to donwgrade to an earlier GHC release or wait until the library maintainer updates the library.