F# Match with discriminated unions

Question

I define 2 discriminated unions: "Direction" and "TurnCommand":

type Direction = 
| South of string
| East of string
| North of string
| West of string

type TurnCommand = 
| Left of string
| Right of string

Then I define function to have new Direction after making TurnCommand:

type Turn = Direction -> TurnCommand -> Direction

Here is not working implementation of this function:

let Do:Turn = fun(startDirection) (turn) -> 
    match startDirection, turn with
    | South, Left  -> East 
    | East,  Left  -> North
    | North, Left  -> West
    | West,  Left  -> South
    | South, Right -> West 
    | East,  Right -> South
    | North, Right -> East
    | West,  Right -> North

There is error: " The constructor applied to 0 arguments but expects 1 ". I understand that it expects string value but I need to match types here. Thanks!

Answer 1

When you defined your constructors (eg South of string ), you said that they required a string argument. When pattern matching on those constructors, you have to use a variable pattern to store the value given to the constructor (or _ to ignore it) and when constructing a value you have to provide a string too: South s1, Left _ -> East "a string" . If you don't need any kind of value associated with your constructors just drop the of string part from their definition.