Defining a new monad in haskell raises no instance for Applicative -

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i trying define new monad , getting strange error

newmonad.hs

 newtype wrapped = wrap {unwrap :: a} instance monad wrapped   (>>=) (wrap x) f =  f x   return x = wrap x  main =   putstrln "yay" 
 $ ghc --version 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 (applicative wrapped)       arising superclasses of instance declaration     in instance declaration ‘monad wrapped’

why need define instance of applicative?

this applicative monad proposal (amp). whenever declare monad, have declare applicative (and therefore functor). mathematically speaking, every monad is applicative functor, makes sense.

you can following remove error:

instance functor wrap   fmap f (wrap x) = wrap (f x)  instance applicative wrap   pure = wrap   wrap f <*> wrap x = wrap (f x)

https://wiki.haskell.org/functor-applicative-monad_proposal

edit: maybe should point out more recent thing? code posted used work before, recent versions of ghc you'll error. it's breaking change.

edit: following declarations should work any monad:

import control.applicative -- otherwise can't applicative instance. import control.monad (liftm, ap)  instance functor ???   fmap = liftm  instance applicative ???   pure  = return   (<*>) = ap

depending on monad in question, there may more efficient implementations possible, simple starting point.


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