CODEWITHSUNDEEP
haskell
Gilgamesz
Gilgamesz

Reputation: 5073

One argument insted of two. Either

instance Monad (Either e) where
        return = Right
        Right m >>= k = k m
        Left e  >>= _ = Left e

I found a such piece of code. It is not understandable for me because, after all, Either constructor type gets two arguments ( Either a b ) and here took one. I don't understandy why it is possible.

Upvotes: 0

Views: 71

Answers (1)

Sebastian Redl
Sebastian Redl

Reputation: 71899

Type constructors, like normal functions, are curried in Haskell. That means you can give a constructor that takes two arguments, such as Either, one argument, and the result is a type constructor that needs only one argument to construct a full Either.

The Monad typeclass is not for simple types, but for type constructors taking one argument. For example, you have instance Monad Maybe, but Maybe is not a type, it's a type constructor taking one argument. So you can't make Either an instance of Monad, since it takes two arguments, not one, and you can't make Either a b an instance, since it takes no arguments. You can make Either a an instance of monad.

In Haskell, we say that a simple type has kind *, a type constructor taking one argument like Maybe has kind * -> *, and Either has kind * -> * -> *. Monad wants something of kind * -> *, and by supplying one type to Either, that's what you get.

Upvotes: 8

Related Questions