CODEWITHSUNDEEP
f#discriminated-union
Jamie Twells
Jamie Twells

Reputation: 2144

How to assign a specific type to a discriminated union type in F#

Suppose I have the following types:

type TypeA = { A: string }
type TypeB = { B: string }

and I have a union type for them both:

type MyUnionType = TypeA | TypeB

With a type just to contain the union type:

type MyContainer = { Union: MyUnionType }

now if I create an instance of one of the types in the union type:

let resultA = { A = "abc" }

when I try to assign that instance to the value in the container

let result = { Union = resultA }

the compiler complains, saying

Compilation error (line 10, col 24): This expression was expected to have type MyUnionType but here has type TypeA

but TypeA is one of the valid types specified in the union! How can I assign it to my Union property?

Here is an example of the program: https://dotnetfiddle.net/fgJKpM

Upvotes: 1

Views: 269

Answers (1)

Lee
Lee

Reputation: 144126

In type MyUnionType = TypeA | TypeB, TypeA and TypeB do not refer to the previous TypeA and TypeB records, but are nullary constructors for the MyUnionType type. If you want them to contain values of those types you need to include them in the definition e.g.

type MyUnionType = TypeA of TypeA | TypeB of TypeB

You might want to rename the constructors to avoid confusion between them and the contained types.

you then need to supply the corresponding instance to the constructor:

let resultA = TypeA { A = "abc" }

Upvotes: 6

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