f# - How do I copy a discriminated union case value? -

how copy discriminated union case value?

the following code has duplication:

let move (direction:direction) (checker:checker) =     match checker     | red   xy -> red   { xy x=2; y=2 }     | black xy -> black { xy x=2; y=2 } 

specifically, don't want specify type of checker set record value. hence, don't care if checker red or black. copy checker case value , update position.

i rather this:

let move (direction:direction) (checker:checker) =     match checker     | _ xy -> _ { xy x=2; y=2 } 

here's test:

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

appendix:

module test3  open nunit.framework open fsunit  type position = { x:int; y:int } type checker = | red of position | black of position  type direction =     | northeast     | northwest     | southeast     | southwest  (* functions *) let move (direction:direction) (checker:checker) =     match checker     | red   xy -> red   { xy x=2; y=2 }     | black xy -> black { xy x=2; y=2 }  [<test>] let ``move checker``() =     black { x=1; y=1 } |> move northeast                         |> should equal (black { x=2; y=2 }) 

the thing you're asking can't done in f# without reflection trickery.

here common sense explanation: rare union cases have exact same data under them. when case, indicates union cases "tags" used mark data, , should encoded "alongside" data, not "around" it. , since such union types rare exceptions, doesn't make sense invent language machinery support them.

this rationalization applies specific case nicely: color of checker "tag" should "next to" position, rather "wrapping" it. if define checker { pos: position; color: color }, you'll able update position without touching color.