CODEWITHSUNDEEP
haskellnotation
Pratik Deoghare
Pratik Deoghare

Reputation: 37172

What is ((->) t ) in haskell?

I am doing 20 intermediate Haskell exercises.

After finishing first 2 exercise there is this strange thing.

I would like to know what is ((->) t)? 

-- Exercise 3
-- Relative Difficulty: 5
instance Fluffy ((->) t) where
  furry = error "todo"

Thanks! :-)

Upvotes: 9

Views: 190

Answers (2)

viorior
viorior

Reputation: 1803

You should read prefix (->) t a as infix t -> a.

If we have 

instance Fluffy Maybe where

for Maybe a type (and * -> * kind), then 

instance Fluffy ((->) t) where

is for (->) t a == t -> a type (and * -> * kind) - for any function with 1 argument

Upvotes: 0

Lee
Lee

Reputation: 144126

(->) is the type constructor for functions which has kind * -> * -> * so it requires two type parameters - the input and result type of the function. ((->) t is a partial application of this constructor so it is functions with an argument type of t i.e. (t -> a) for some type a.

If you substitute that into the type of the furry function you get:

furry :: (a -> b) -> (t -> a) -> (t -> b)

Upvotes: 4

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