Re: [Haskell-cafe] State monad is missing Applicative instance

2009/3/12 Peter Verswyvelen :

I think. Or is it defined in some other package?

Note that you can get an Applicative instance for "free" by using "WrapMonad" in Control.Applicative. For example, just today I was writing a quickcheck Arbitrary instance, and the Gen monad doesn't have an Applicative instance. No problem:

instance Gen MyDataType where arbitrary = MyDataConstructor <$> arbitrary <*> arbitrary

becomes

instance Gen MyDataType where arbitrary = unWrapMonad (MyDataConstructor <$> WrapMonad arbitrary <*> WrapMonad arbitrary)

This works because every monad induces an Applicative instance in a way I've ingested just enough wine to forget. =] Denis