Tips from NDC Oslo 2019–F# Adding overloads to a discriminated union

A neat trick I noticed while watching the Dungeons, Dragons and Functions talk by Mathias Brandewinder is adding overloads to a discriminated union.

In his talk, he is modelling Dungeons and Dragons using F#. One of the things he needs to model is a dice roll. In D&D you have uncommon dice shapes and inside the rule book you’ll find dice rules like:  4d6+2d10+8.

What does this formula means? 4 roles of a 6-sided dice + 2 roles of a 10-sided dice+ 8

To model this in F#, Mathias created the following discriminated union:

	type Roll =
	| Roll of int * Dice
	| Value of int
	| Add of Roll list

view raw Roll.fs hosted with ❤ by GitHub

You can then use this Roll type to create the formula above:

let example = Add [ Roll(4, D 6); Roll(2, D 10); Value 8 ]

view raw RollFormula.fs hosted with ❤ by GitHub

This works but is not very readible. To fix this, Mathias introduced some overloads on the Roll type:

	type Roll =
	| Roll of int * Dice
	| Value of int
	| Add of Roll list
	// omitted for brevity
	static member (+) (v1:Roll,v2:Roll) =
	match v1,v2 with
	| Add(rolls1), Add(rolls2) -> Add(rolls1 @ rolls2)
	| Add(rolls1), roll2 -> Add(rolls1 @ [ roll2 ])
	| roll1, Add(rolls2) -> Add(roll1 :: rolls2)
	| roll1, roll2 -> Add [ roll1 ; roll2 ]
	static member (+) (roll:Roll,num:int) = roll + Value num
	static member (+) (num:int,roll:Roll) = Value num + roll

view raw RollUpdated.fs hosted with ❤ by GitHub

This nicely cleans up the formula to:

let example=Roll(2, D 6) + 10 + Roll(4, D 10)

view raw RollFormulaUpdated.fs hosted with ❤ by GitHub

More information can be found in Mathias blog post here: https://brandewinder.com/2018/07/31/give-me-monsters-part-3/