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Records with default values
f#

Let's take this example record:

type Person =

{ First : string;

Last : string }

I think it would be useful if you could define default values for it, e.g.

type Person =

{ First : string = "John" ;

Last : string = "Doe" }

Currently we can use Person as a sort of named arguments

let fullName (p: Person) = p.First + " " + p.Last

let male = fullName {First = "John"; Last = "Doe"}

If records support default values we could also have optional arguments like this

let female = fullName {First = "Jane"}

and

let defaultName = fullName {new Person}

.

From 1.9.2.7 you can use the following:

type Person =
{ First: string; Last: string }
static member make (?first, ?last) =
{ First = defaultArg first "John"; Last = defaultArg last "Doe" }

By on 9/3/2008 5:41 PM ()

I like felixmar's suggestion. It's a fairly obvious extension to suggest that type inference could reduce this to the beautifully succinct:

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type Person =
    { First : "John" ;
      Last  : "Doe" }

I remember Don mentioning that there were discussions about unifying syntax for records and classes. I would love to see a property syntax for classes something like:

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type Person =
    First : mutable "John" 
    Last  : mutable "Doe" 

    override x.ToString() = x.First + " " + x.Second

I do hate superfluous brackets and braces! (One of the many reasons I love F#.)

Currently this class would look so:

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 type Person(?first, ?second) =
    let mutable first = defaultArg first "John"
    let mutable second = defaultArg second "Doe"
    member this.First with get () = first and set v = first <- v
    member this.Second with get () = second and set v = second <- v
    override x.ToString() = first + " " + second

which somewhat obscures the nature of the class.

regards,

Danny

By on 9/4/2008 3:57 AM ()

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