You’re almost there. Parentheses are required in order that the compiler interprets a let-bound as pattern matching: let (OrderId id) = orderId If orderId is a parameter of a function, you can also use pattern matching directly there: let extractId (OrderId id) = id
Like many good solutions in programming, you achieve this by adding a layer of indirection. Specifically, what we can do here is add a table between action tags (i.e. “Example” and “Another”) and their respective payloads. type ActionPayloadTable = { “Example”: { example: true }, “Another”: { another: true }, } then what we can … Read more
You can use conditional type for distributing over the members of the union type (conditional type always takes only one branch and is used only for its distributive property, I learned this method from this answer) type A = ‘one’ | ‘two’ | ‘three’; type Distribute<U> = U extends any ? {type: U} : never; … Read more
Updated for TS3.5+ on 2019-06-20 Issue #1: K extends string | number for the index signature parameter: Yeah, this can’t be done in a very satisfying way. There are a few issues. The first is that TypeScript only recognizes two direct index signature types: [k: string], and [k: number]. That’s it. You can’t do (edit … Read more
Yes, F# has it’s own reflection layer build on top of .NET’s reflection to help you make sense of types that are specific to F#, like discriminating unions. Here’s the code that will let you enumerate a union’s cases: open Microsoft.FSharp.Reflection type MyDU = | One | Two | Three let cases = FSharpType.GetUnionCases typeof<MyDU> … Read more
Kotlin’s sealed class approach to that problem is extremely similar to the Scala sealed class and sealed trait. Example (taken from the linked Kotlin article): sealed class Expr { class Const(val number: Double) : Expr() class Sum(val e1: Expr, val e2: Expr) : Expr() object NotANumber : Expr() }
I don’t really like the type-checking and type-casting solutions provided above, so here’s 100% type-safe union which will throw compilation errors if you attempt to use the wrong datatype: using System; namespace Juliet { class Program { static void Main(string[] args) { Union3<int, char, string>[] unions = new Union3<int,char,string>[] { new Union3<int, char, string>.Case1(5), new … Read more