f# - How do I specify a type that shares a type definition with another type? -


how specify type shares type definition type?

the following code not compile:

[<test>] let ``move checker``() =     { position={ x=1; y=1 } } |> moveblack northeast                                |> should equal { position={ x=2; y=2 } }

this because record i'm passing moveblack function mapped redchecker instead of blackchecker.

type mismatch. expecting redchecker -> 'a given blackchecker -> blackchecker type 'redchecker' not match type 'blackchecker'

more likely, error occurs because last type have definition redchecker:

type blackchecker = { position:position } type redchecker =   { position:position }

i thought specify black checker doing this:

(blackchecker:{ position={ x=1; y=1 } })

and have:

[<test>] let ``move checker``() =     (blackchecker:{ { position={ x=1; y=1 } }) |> moveblack northeast                                |> should equal (blackchecker:{ { position={ x=2; y=2 } })

however, above code doesn't compile.

here's rest of code:

(* types *) type color = | red | black  type north = northeast | northwest type south = southeast | southwest  type position =     { x:int; y:int } type blackchecker = { position:position } type redchecker =   { position:position }  (* functions *) let moveblack (direction:north) (checker:blackchecker) =     match direction     | northeast -> { checker position= { x=2; y=2 } }     | northwest -> { checker position= { x=1; y=2 } }  (* tests *) [<test>] let ``move checker``() =     { position={ x=1; y=1 } } |> moveblack northeast                                |> should equal { position={ x=2; y=2 } }

the labels of declared type take precedence on of declared type. should able this:

({ blackchecker.position={ x=1; y=1 } })

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