Defining a new monad in haskell raises no instance for Applicative
I am trying to define a new monad and I am getting a strange error
newmonad.hs
newtype Wrapped a = Wrap {unwrap :: a}
instance Monad Wrapped where
(>>=) (Wrap x) f = f x
return x = Wrap x
main = do
putStrLn "yay"
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.10.1
$ ghc newmonad.hs
[1 of 1] Compiling Main ( newmonad.hs, newmonad.o )
newmonad.hs:2:10:
No instance for (Applicative Wrapped)
arising from the superclasses of an instance declaration
In the instance declaration for ‘Monad Wrapped’
Why do I need to define an instance of Applicative ?
This is the Applicative Monad Proposal (AMP). Now whenever you declare something as Monad , you also have to declare it as Applicative (and therefore Functor ). Mathematically speaking, every monad is an applicative functor, so this makes sense.
You can do the following to remove the error:
instance Functor Wrap where
fmap f (Wrap x) = Wrap (f x)
instance Applicative Wrap where
pure = Wrap
Wrap f <*> Wrap x = Wrap (f x)
https://wiki.haskell.org/Functor-Applicative-Monad_Proposal
Edit: Maybe I should point out more clearly that this is a recent thing? The code you posted used to work before, but with recent versions of GHC you'll get an error. It's a breaking change.
Edit: The following declarations should work for any monad:
import Control.Applicative -- Otherwise you can't do the Applicative instance.
import Control.Monad (liftM, ap)
instance Functor ??? where
fmap = liftM
instance Applicative ??? where
pure = return
(<*>) = ap
Depending on the monad in question, there may be more efficient implementations possible, but this is a simple starting point.
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