Haskell define a non-empty list datatype

Question

I have defined a data type like this for representing a 2D point:

data Point = Point (Double, Double) deriving (Show)

And a data type Curve (curve is a list of points in 2D space that create a curve

data Curve  = Curve [Point] deriving (Show)

How can I set the Curve datatype to be nonempty? so that it always needs to have at least 1 Point in the list?

Answer 1

The Data.List.NonEmpty module, in the popular semigroups library, implements a non-empty list type, and utility functions for it.

The solution is effectively the same as Cactus' answer, a pair that contains an obligatory first element and then a list for the rest:

data NonEmpty a = a :| [a]

So Cactus' Curve type would be equivalent to NonEmpty Point .

Answer 2

列表[Point]将始终包含0或更多Point s，因此如果存储额外的一个，则得到1 +（0或更多）= 1或更多 Point s：

data Curve = Curve Point [Point]