类型中的类型与值构造函数

Question

I am learning Haskell and am confused by this example .

Consider the following:

class Tofu t where
    tofu :: j a -> t a j

data Frank a b = Frank {frankField :: b a} deriving (Show)

instance Tofu Frank where
    tofu x = Frank x

Why is it that when making Frank an instance of Tofu , we provide (as far as I understand), a type constructor Frank x , instead of a value constructor, ie tofu x = Frank {frankField = x} ?

Answer 1

You're still using the value constructor. You can use types that have record fields without having to use the record syntax to construct them, so Frank x does the same thing as Frank { frankField = x } .

If you have more than one record field, eg:

data Frank a b = Frank { frankA :: a, frankB :: b } 

... then the fields appear in the order you have written them in, so you can do:

-- With x :: a, y :: b
Frank x y