Record syntax default value for accessor

Question

As I was writing up an answer just now, I ran across an interesting problem:

data Gender = Male | Female
            deriving (Eq, Show)

data Age = Baby | C         


        

Answer 1

Yes there is a tension between types and data... which by the way shows how thin is the line.

The pratical answer is to use a default instance as indicated in the Haskell Wiki. It does answer your exact question since you must give up direct constructor use.

Thus for your example,

data Age = Baby | Child | PreTeen | Adult | NoAge
data Clothing = Pants {gender :: Gender, age :: Age}
              | Shirt {gender :: Gender, age :: Age}
              | Skirt {gender :: Gender, age :: Age}
              deriving (Show, Eq)

skirt = Skirt { gender=Female, age=NoAge }

then developpers can create new instances with default values, using the copy-and-update facility of the record syntax

newSkirt = skirt { age=Adult }

and gender newSkirt evaluates to Female

I want to stress that this approach leads you to define default values at the type level, which I think is a Good Thing (of course the NoAge constructor is the Nothing of a Maybe Age type).