haskellalgebraic-data-types

How to list all values of sum type in Haskell

Question:

I would like to generate a HTML form from types in Haskell.

Color = Green | Yellow | Red | ....

Fruit = Apple Color Int
      | Banana Color Int String
      | ....

User shall pick a dropdown list with option of ("Apple","Banana") or ("Green","Yellow","Red")

Question:

But how to get ALL values of type Fruit/Color values to HTML ? something like:

[ toHtml v <-  Fruit.values() ]
-- render "Apple" to html
-- render "Banana" to html

expected:

<dropdown> : values :apple/banana...
<dropdown> : values of green/yellow...
<input text> : for Int
<input text> : for String(if Banana was selected
Solution

  • The Data class helps here, e.g

    data Color = Green | Yellow | Red deriving Data

colorStrings :: [String]
colorStrings = map show $ dataTypeConstrs (dataTypeOf Green :: DataType)

    To get Fruit where data constructors also have fields you'd have to work harder but the required info should be available within the DataType data type.