Haskell: Data type containing other Data Types

Question

if I have two data structures

data Tri = Tri {a :: Int, b :: Int , c :: Int} deriving Show data Quad = Quad {w :: Int, x :: Int, y :: Int, z :: Int} deriving Show

how do I create another data structure made up of those two? ie something alon the lines of:

data Shape = Tri | Quad derivng Show

Something as simple as type Shape = Either Tri Quad might suffice. — chepner
– chepner, Commented Jan 30, 2019 at 12:37

Will Ness · Accepted Answer · 2019-02-02 15:36:06Z

10

You have to give names to data constructors:

data Shape = ShapeTri { shapeTri :: Tri } | ShapeQuad { shapeQuad :: Quad } deriving Show

edited Feb 2, 2019 at 15:36

Will Ness

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answered Jan 30, 2019 at 7:27

talex

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Comments

dopamane · Accepted Answer · 2019-01-30 07:34:52Z

4

@talex's answer is correct. Here are some variations (mainly just different syntax).

Without record syntax:

data Shape = ShapeTri Tri | ShapeQuad Quad deriving Show

It may make more sense to combine Shape, Tri, and Quad:

data Shape = Tri {a :: Int, b :: Int , c :: Int} | Quad {w :: Int, x :: Int, y :: Int, z :: Int} deriving Show

answered Jan 30, 2019 at 7:34

dopamane

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4 Comments

matt Over a year ago

The reason I wanted to keep them separate is because I wanted to write different instances for them. Is there a better way for that?

dopamane Over a year ago

Might I ask what instances you are writing for Tri and Quad?

matt Over a year ago

to be honest that is not the actual code, I just used those as an example. The last example you wrote seems the easiest to use

luqui Over a year ago

I definitely recommend against combining them. All the record accessors a, b, ... become partial functions that blow up if you use them on the wrong constructor

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