Extending Elm records

elm_extend_records.elm

-- Extensible Records

-- given a record
x = { age = 42, name = "Boke" }

-- you can clone and remove a field in the process
{ x - age } -- { name = "Boke" }

-- you can also add a field
{ x | species = "Jade Dragon" } -- { age = 42, name = "Boke", species = "Jade Dragon" }

-- you can mix them up
{ x - age | species = "Jade Dragon" } -- { name = "Boke", species = "Jade Dragon" }


-- Composible Record Types

-- given these type aliases for extensible records
type Named   a = { a | name    : String }
type Aged    a = { a | age     : Int }
type Species a = { a | species : String }

-- them can be composed together
boke : Named(Aged(Species {}))
boke = { name = "Boke", age = 42, species = "Jade Dragon" }

-- same goes to functions
aboutMe : Named(Aged(Species {})) -> String
aboutMe { age, name, species } = name ++ " is a " ++ species ++ " at " ++ (show age) ++ " years of age"

aboutMe boke -- Boke is a Jade Dragon at 42 years of age

-- type aliases defined using this syntax can still be composed as the inner most part
type NPC = { name : String, age : int }
spyrion : Species(NPC)
spyrion = { name = "Spyrion", age = 42, species = "Ruby Dragon" }

-- similarly you can just defined bespoke labels too
shady : Named(Species { age : Int })
shady = { name = "Shady", age = 21, species = "Goblin" }


-- Structural Typing

-- this function can take any record that has a name field
whoAmI { name } = "My name is " ++ name

Thanks for the gist.

For future readings, I can say that with Elm .019 you need to explicitly list out all the fields, and cannot append/extend them like the above.