Haskell 多重声明错误类型“形状”

Question

I trying to create an sum type named Shape and later on another sum type named PositionedShape with the property of Shape init

data Shape = Circle Float |
             Rectangle Float Float

area :: Shape -> Float
area (Circle r)      = pi * r * r
area (Rectangle h w) = h * w

data Point = Point Float Float
             deriving (Eq,Show )


data PositionedShape = Circle Point Float |
                       Rectangle Point Float Float
                       deriving (Eq,Show)

From me, it seem correct but it kept saying multiple declaration.

Week7.hs:92:24: error:
    Multiple declarations of ‘Circle’
    Declared at: Week7.hs:26:14
                 Week7.hs:92:24

   |
92 | data PositionedShape = Circle Point Float |
   |                        ^^^^^^^^^^^^^^^^^^

Week7.hs:93:24: error:
    Multiple declarations of ‘Rectangle’
    Declared at: Week7.hs:27:14
                 Week7.hs:93:24
   |
93 |                        Rectangle Point Float Float
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Answer 1

As the error message says, you have declared Circle and Rectangle multiple times. In Haskell, data constructors must have unique names, at least within the same module.

You can resolve the issue by giving the data constructors unambiguous names, eg:

data PositionedShape = PositionedCircle Point Float |
                       PositionedRectangle Point Float Float
                       deriving (Eq,Show)

The data constructors can also be used as functions. For example, you can view PositionedCircle as a function:

Prelude> :t PositionedCircle
PositionedCircle :: Point -> Float -> PositionedShape

You can call it with a Point and a Float value:

Prelude> PositionedCircle (Point 1 2) 3
PositionedCircle (Point 1.0 2.0) 3.0

Function names must be unique, because otherwise the compiler don't know which one you mean. That's the reason that data constructors must have unambiguous names.

---

It seems that the problem that you're really trying to solve is how to associate a position with a shape, which might be better done like this:

data PositionedShape = PositionedShape Point Shape deriving (Eq, Show)

You could use this alternative like this:

Prelude> PositionedShape (Point 1 2) (Circle 3)
PositionedShape (Point 1.0 2.0) (Circle 3.0)