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How to check if a string contains only alphabets in F #

I have a string that should only contain alphabets. To check that I have written the following code

let isCorrectString(str:string) = 
 let isInRange x = (x > 64 && x < 91 ) || (x > 96 && x < 123) || x = 32
 not (str.Any(fun ch -> not (isInRange (ch :?> int)) ))

I am obviously using LINQ's extension method `Any '. Is there a better way to write the above code?

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If you just want to make sure this is correct, you can do:

let isCorrectString(str:string) = 
    let isInRange x = (x > 64 && x < 91 ) || (x > 96 && x < 123) || x = 32
    let bad = 
        str
        |> Seq.map (fun c -> isInRange(int c))
        |> Seq.exists (fun b -> b = false)
    not bad

Note that this might be a simpler alternative:



let isCorrectString(str:string) = 
    str
    |> Seq.forall (fun c -> System.Char.IsLetter(c) || c = ' ')

Or, if you prefer:

let isCorrectString(str:string) = 
    str
    |> Seq.tryFind (fun c -> not(System.Char.IsLetter(c) || c = ' '))
    |> Option.isNone
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