Return value or false using the TryGetValue method

Question

I have a dictionary and get the value:

open System.Collections.Generic let price = Dictionary<string, int>() Array.iter price.Add [|"apple", 5; "orange", 10|] let buy key = price.TryGetValue(key) |> snd |> (<) printfn "%A" (buy "apple" 7) printfn "%A" (buy "orange" 7) printfn "%A" (buy "banana" 7) 

True

falsely

True

I need false in the 3rd call. How to get a value or false if the key is not found? The problem is that TryGetValue returns true or false depends on key or not, but the value is returned by reference.

Answer 1

This will simplify your life if you define an Adapter for TryGetValue , which is more like F #:

 let tryGetValue k (d : Dictionary<_, _>) = match d.TryGetValue k with | true, v -> Some v | _ -> None 

With this, you can now define the buy function as follows:

 let buy key limit = price |> tryGetValue key |> Option.map ((>=) limit) |> Option.exists id 

This gives the desired result:

 > buy "apple" 7;; val it : bool = true > buy "orange" 7;; val it : bool = false > buy "banana" 7;; val it : bool = false