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Newbie question about non-nullable strings..
f#c#

I am newbie in F# (with c# background). One of the features in F# is non-nullable types (by default). What is somewhat confuses me is that strings are still nullable. Yes, I know that they just System.String, but.. May be F# has something to replace or mark them as non-null?

.

The spec has quite a nice section on nullness:

[link:research.microsoft.com]

In a nutshell, F# still has to deal with null since any framework method could return null. Strings are part of the CLR's BCL so can be null, F#'s native types, such as tuples, records and unions cannot be null, most other types can.

Cheers,

Rob

By on 1/12/2009 4:16 AM ()

The spec has quite a nice section on nullness:

[link:research.microsoft.com]

In a nutshell, F# still has to deal with null since any framework method could return null. Strings are part of the CLR's BCL so can be null, F#'s native types, such as tuples, records and unions cannot be null, most other types can.

Cheers,

Rob

No plans for implementing native strings or char types? I hate when types are nullable by default!

By on 1/13/2009 2:25 PM ()

I hate when types are nullable by default!

Yeh, me too. Actually Spec# has solution for this. They just use special symbol (similar with Nullable<T> - "?").

By on 1/13/2009 2:51 PM ()

Thank you, a bit strange behaviour...

Sorry, but then I have another question.

Is there any way to improve this code

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type Content =
    | Html of string

to restrict Html to be non null?

By on 1/12/2009 4:53 AM ()

Here's an improved version that uses a constructed type where the non-nullable constraint is enforced in the constructor. This is much safer such that any time you have an instance of the NonNull class, you are guaranteed to have a non-null value:

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#light

type NonNull<'a>(x : 'a) =
    do if box x = null
        then failwith "null is not a valid value"
    
    member this.Value = x
    
    override this.ToString() = x.ToString()

let (!!) (x : NonNull<'a>) = x.Value

let nn1 = NonNull("a string")
printfn "%A" !!nn1 // "a string"

try
    let nn2 = NonNull(null)
    ()
with ex -> printfn "Error: %s" ex.Message // Error: null is not a valid value

You can use this type with your Union type like so:

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type Content = 
    | Html of NonNull<string>

let c1 = Html(NonNull("a string"))
printfn "%A" c1 // Html a string

try
    let c2 = Html(NonNull(null))
    ()
with ex -> printfn "Error: %s" ex.Message // Error: null is not a valid value
By on 1/13/2009 5:42 AM ()

Here's an improved version that uses a constructed type....

Thank you very much, but this is common solution for languages with nullable types. For example you can find one of the implementations here - [link:msmvps.com]. Beside noise in syntax it move checks at the runtime, which non-nullable types could overcome.

By on 1/13/2009 2:46 PM ()

You can pretty easily create a type to enforce non-nullable values of any reference type. Here's a pretty simple version of one as a script:

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#light

type NonNull<'a when 'a : null> =
    { Value : 'a }

let nnull x = 
    if x = null
        then failwith "null is not a valid value"
        else { Value = x }

type Content = 
    | Html of NonNull<string>

let c1 = Html(nnull "a string")
printfn "%A" c1 // Html {Value = "a string";}

try
    let c2 = Html(nnull null)
    ()
with ex -> printfn "Error: %s" ex.Message // Error: null is not a valid value

The big shortcoming here, of course, is that the non-null constraint is only enforced through the constructor function.

By on 1/13/2009 5:24 AM ()

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