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Generic pattern-matching for tagged unions


By : D.shicheng
Date : September 14 2020, 07:00 PM
will help you I don't see anything wrong with your code; even the part where you have an "unnecessary" generic type parameter, since it helps the compiler infer what you want.
A possible modification to your approach, which you could take or leave, would be this:
code :
type Matcher<Obj extends { kind: string }> = {
    [K in Obj["kind"]]: (obj: Extract<Obj, { kind: K }>) => any;
};

function match<T extends { kind: string }, M extends Matcher<T>>(
    obj: T,
    matcher: M
): ReturnType<M[T["kind"]]> {
    const fn = matcher[obj.kind as T["kind"]];
    return fn(obj as Parameters<typeof fn>[0]);
}
type Square = { kind: "square"; side: number };
type Circle = { kind: "circle"; radius: number };
type Shape = Square | Circle;

const square = { kind: "square", side: 2 } as Shape;
const surface = match(square, {
    square: square => square.side ** 2,
    circle: circle => Math.PI * circle.radius ** 2
});

console.log(surface.toFixed()); // toFixed() doesn't work if not a number


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F# - weird behavior when pattern matching discriminated unions


By : Maxim Kharchenko
Date : March 29 2020, 07:55 AM
wish of those help Based on the comments, this is just a bad experience in the debugger (where the highlighting suggests that the control flow is going places it is not); the code does what you expect.
(There are a number of places where the F# compiler could improve its sequence-points generated into the pdbs, to improve the debugging experience; I think we'll be looking at this in a future release.)

Active Pattern Matching with Discriminated Unions


By : oszh
Date : March 29 2020, 07:55 AM
Hope that helps You only need to use of when declaring the type, so you can just construct values with the bar2 constructor like:
code :
bar2 a
let (|MatchFoo2|_|) aString =
  match aString with
  | "abcd" -> Some (bar2 a)
  | _ -> None

F# Pattern Matching Nested Discriminated Unions


By : Hadi Dahi
Date : March 29 2020, 07:55 AM
I wish did fix the issue. You won't be able to bypass constructor matching in that fashion, but you can remove some tedium by creating a function to pull the symbol out of a card:
code :
let symbol card =
  match card with
  | Heart(s) -> s
  | Diamond(s) -> s
  | Spade(s) -> s
  | Club(s) -> s

let GetValue (card : Card) =
  match symbol card with
  | Seven | Eight | Nine -> 0
  | Ten -> 10
  ...

Pattern matching tagged union types


By : Dcpapin
Date : March 29 2020, 07:55 AM
To fix this issue
Or is this a case of invalid use of union types and the model should be structured differently?
code :
type alias Comment =
    { id : Int
    , text : String
    , status : Status
    }


type Status
    = New
    | Edited
    | Flagged

Pattern matching the tag of a tagged union enum


By : Benjamin
Date : March 29 2020, 07:55 AM
should help you out You can use .. to ignore all fields of an enum, no matter how many there are, and you can import variants from inside the enums namespace with use. E.g
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