How to create function with bools?

Firstly, to address your question directly, consider the code below:

var fishStock: [String: Bool] = [
        "Salmon" : true,
        "Tuna": true,
        "Shrimp": false
]

func inStock(fish: String) -> Bool {
    // Statically configured to use fishStock dictionary.

    // Using if let to verify key exists and return value.
    if let result = fishStock[fish] {
        return result
    } else {
        return false
    }
}

let haveSalmon = inStock("Salmon")  // true
let haveShrimp = inStock("Shrimp")  // false
let haveShark = inStock("Shark")  // false

You will note that we are taking care to use an if let as the dictionary returns an optional when you are requesting a key with subscript. It returns nil if the key does not exist trigging the else where I return false (as with the haveShark).

See how if a key exists it will return an optional and if it does not it returns a nil. So perhaps your dictionary would be better suited for stock levels rather than bools. You can still return true or false based upon if the key exists, but use the key value to store stock levels instead. Consider the following code.

var fishStock: [String: Int] = [
    "Salmon" : 3,
    "Tuna": 1,
    "Shrimp": 0
]

func inStock(fish: String, stock: [String: Int]) -> Bool {
    // Using if let to verify key exists and value > 0 and return true.
    if let result = stock[fish] {
        if result > 0 {
            return true
        } else {
            return false
        }
    } else {
        return false
    }
}

func stockLevel(fish: String, stock: [String: Int]) -> Int {
    // Using if let to verify key exists and return value.
    if let result = stock[fish] {
        return result
    } else {
        return 0
    }
}

let haveSalmon = inStock("Salmon", stock: fishStock)  // true
let haveShrimp = inStock("Shrimp", stock: fishStock)  // false
let haveShark = inStock("Shark", stock: fishStock)  // false

let salmonStock = stockLevel("Salmon", stock: fishStock)  // 3
let shrimpStock = stockLevel("Shrimp", stock: fishStock)  // 0
let sharkStock = stockLevel("Shark", stock: fishStock)  // 0

You will note that we are still taking care to use an if let when accessing the dictionary keys. In the additional function (stockLevel) we are returning the value of the key (the stock level Int in this case) and if the key does not exist we are returning 0 (as with the sharkStock). Additionally you will see we are now passing in the dictionary along with the fish rather than statically coding it to the functions. This is better coding because it means the function is not tied to a single dictionary.

Thank you Nathan for such detailed answer.

The only part I am still trying to make sense of is " if let result = stock[fish]"

It is the first time I see such a use of an IF statement and don't understand what exactly is a constant doing there assigned to "result" and how how does it work.

Oh it seems "if let" is part of Optionals in Swift - And that is a topic I still haven't covered (just finished Functions).

Is there a way to solve my question using a function with a Switch or IF statement, without using Optionals?