Is it possible to define a function on only one variant of a type?

[rupert]

@mattpiz

it almost always makes the code clearer to extract those fields in common

See also this post discussing how record and custom types can be modelled using math (its not hard):

Mapping Tagged Union types to other languages?

As Math type Content = Text { uuid : String , title : String } | Markdown { uuid : String , markdown : String } | Figure { uuid : String , markdown : String , title : String , imageId : Maybe String } is equivalent to the expression: uuid * title + uuid * markdown + uuid * markdown * title * imageId taking the common factor = uuid * (title + markdown + markdown * title * imageId) Which converted back into Elm gives me an equivalent data model: type alias Content = { uuid : String …

Pulling out the common fields is akin to extracting a common factor from some expression. I tend to think of product-sum-product form with all common fields pulled to the outer product as what you are generally aiming for.

This is in product-sum-product form:

-- Outer product
type alias MyType =
  { common1 : ...
  , common2 : ...
  , specific : Specific
  }

-- Sum
type Specific
  = Variant1 { field1 : ..., field2 : ... } -- Inner Product
  | Variant2 { field3 : ... } -- Inner Product