Copying a union case but with a different meaning in F #
In F #, I want to construct an instance (correct terminology?) Of a discriminatory join based on an existing instance. Example:
type union Currency =
| Dollar of int
| Euro of int
let lowPrice = Dollar 100 (* or, it could be *) let lowPrice = Euro 100
let highPrice = (* of the same union case as lowPrice but with value 200 *)
What code can be inserted instead of a comment to create this effect?
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You could do
let highPrice =
let n = 200
match lowPrice with
| Dollar _ -> Dollar n
| Euro _ -> Euro n
but units are probably better.
EDIT
Alternatively, perhaps you want
type MoneyType = Dollar | Euro
type Currency = Currency of MoneyType * int
let lowPrice = Currency(Dollar, 100)
let highPrice =
match lowPrice with
| Currency(kind, _) -> Currency(kind, 200)
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I think it makes more sense to use units for this kind of problem - somthing like
[<Measure>] type Dollar
[<Measure>] type Euro
let lowprice = 100<Dollar>
let inline _highprice (newv:int) (oldv:int<'t>) : int<'t> =
LanguagePrimitives.Int32WithMeasure newv
let highprice = _highprice 200 lowprice
the transform function is a little tricky but will do what you want
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You can introduce new case union values based on the existing value of the same case union using Reflection. To achieve this, simply add a member instance Same
to your discriminated union, which first deduces the specific case of the union from the instance self
, and then creates a new instance with the same case of the union, but is now populated with newVal
:
open Microsoft.FSharp.Reflection
type Currency =
| Dollar of int
| Euro of int
member self.Same newVal : Currency =
FSharpValue.MakeUnion(fst (FSharpValue.GetUnionFields(self,
typeof<Currency>)), [|newVal|])
|> unbox
Now applying it to lowPrice
below value
let lowPrice = Euro(100)
let highPrice = lowPrice.Same 200
You'll get highPrice : Currency = Euro 200
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