Haskell：了解自定义数据类型

Question

I am trying to make my own custom data type in Haskell.

I have the following data types:

type Length = Integer 
type Rotation = Integer 
data Colour = Colour { red, green, blue, alpha :: Int }
            deriving (Show, Eq)

I am trying to make a custom data type that can be either one of the data types above. I have the following:

data Special 
  = L Length 
  | R Rotation 
  | Col Colour  
  deriving (Show, Eq) 

However, I would like to be able to extract the Length , Rotation and Colour value if I have an instance of the Special data type.

If I had:

L length

Would length here be of type Special or of type Length ? If length is of type Special is there any way to extract it so it's of type Length ?

For example, is the following code valid?

takeL (x:xs)
      | x == (L length) = length

Any insights are appreciated.

Answer 1

For the expression L length to be valid, length would have to be a Length (because L :: Length -> Special ).

takeL (x:xs)
      | x == (L length) = length

is not valid. Unless you've redefined length somewhere, length is a function [a] -> Int from the standard library, so L length is a type error.

I think what you're trying to do here is just pattern matching:

takeL (L length : xs) = length

Answer 2

The data type definition

data Special  = 

reads: Special is a new type such that to create a value of type Special ,

                  L Length 

• call L l where l is a value of type Length ; or

    | R Rotation 

• call R r where r is a value of type Rotation ; or

    | Col Colour 

• call Col c where c is a value of type Colour .

---

To analyze a value of type Special , there are three cases to consider:

foo :: Special -> ...
foo val = 
    case val of
        L   l -> ...  

• l is a value of type Length , in the case val was actually L l ; or

    R r -> ... 

• r is a value of type Rotation , in the case val was actually R r ; or

    Col c -> ... 

• c is a value of type Colour , in the case val was actually Col c .

The case syntax in function definitions can also be expressed with the pattern based clauses:

foo :: Special -> ...
foo (L   l) = ...
foo (R   r) = ...
foo (Col c) = ...