Question

I’m looking for a way to reduce this list to a boolean. Here is the original:

let ones = [1;1;1;1]
let twos = [2;2;2;2]
let bad = [1;2;3]

let isAllOnes = List.forall (fun op -> op = 1)
let isAllTwos = List.forall (fun op -> op = 2)  

let isOneOrTwo ops = isAllOnes ops || isAllTwos ops

isOneOrTwo ones |> should be True
isOneOrTwo twos |> should be True
isOneOrTwo bad |> should be False

I’m trying to refactor this using a kind of reduce. Something like this:

let isOneOrTwo ops = [isAllOnes; isAllTwos] |> List.tryFind (fun acc -> acc ops) 

(isOneOrTwo ones).IsSome |> should be True
(isOneOrTwo twos).IsSome |> should be True
(isOneOrTwo bad).IsSome |> should be False

I don’t like how isOneOrTwo reduces to an Option. I really would like to reduce the list to a boolean so that my assertions can look like this:

isOneOrTwo ones |> should be True
isOneOrTwo twos |> should be True
isOneOrTwo bad |> should be False

Anyone know how to make this happen? List.reduce didn’t work because the types were different.

Answer 1

Replace List.tryFind with List.exists

let ones = [1;1;1;1]
let twos = [2;2;2;2]
let bad = [1;2;3]

let allOnes = List.forall ((=) 1)
let allTwos = List.forall ((=) 2)

let isOneOrTwo l = [allOnes; allTwos] |> List.exists (fun f -> f l)

printfn "%A " (isOneOrTwo ones) // true
printfn "%A " (isOneOrTwo twos) // true
printfn "%A " (isOneOrTwo bad)  // false