If Map contains key return value, otherwise return 0-CodePudding

I am new to F# and wanna do as the title says, but I keep getting these type errors.

I know this works:

> let rec validString2 =
  function
  | a -> Map.find a
> validString2 "x" (Map.ofList [("x", 5)]);;
>val it: int = 5

But I wanna return Map.find a only if a is in the map, otherwist I wanna return 0. When I try this:

let rec validString =
  function
  | a -> if (Map.containsKey a) then Map.find a else 0

The errors message I get (and red underline under Map.containsKey a):

This expression was expected to have type
    'bool'    
but here has type
    'Map<'a,'b> -> bool'

The same happens if I tried:

let validString =
  function
  | Some x -> x
  | None -> 0

let rec evalValidString =
  function
  | V a -> validString (Map.tryFind a)

It seems I'm misunderstanding how types with maps work. I don't understand how bool is different from Map<'a, 'b> -> bool. The map function returns a bool, I should be able to evaluate it.

CodePudding user response：

I think the problem here is that you have to specify the map you want to search. In the function below, I've named it myMap:

let myLookup (key : string) myMap =
    myMap
        |> Map.tryFind key         // does the key exist in myMap?
        |> Option.defaultValue 0   // if not, return 0

Note that myMap is a value of type Map<string, int>. You could then use this function as follows:

let aMap = Map.ofList [("x", 5)]
let xValue = myLookup "x" aMap
let yValue = myLookup "y" aMap
printfn "%A" (xValue, yValue)   // 5, 0