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monolog about class let, val, null, this, DefaultValue ...
f#

English is not my native, sorry for my poor english ^^;

I'm translating ASP.NET MVC Web Site written in C# to F#.

I have studied F# just 3 week, and I love it very much.

But, there are some weird problems.

If thingking in F# style, I understand that decison.

But, translating C# code or using C# framwork, some uglyness unaviodable,

Here is, I'm defining a class.

[method 1]

type PostController() =

inherit Controller()

let mutable PostThread : IEnumerable = null ### OK, C# class

let mutable ThreadPager : Pager = null ### Error, F# class

[method 2]

type PostController() =

inherit Controller()

[DefaultValue] val mutable PostThread : IEnumerable ### val can't be inited

[DefaultValue] val mutable ThreadPager : Pager

[method 1]

pros: need not ugly attribute, can define value, no need 'this.' in member to access

cons: can't assign null to F# class object, only private, naver public, need annoying public propery to access

[method 2]

pros: can use null for F# class object, public, need not propery to access

cons: need guly attribute, can't define value, need 'this.' in member to access,

I'm sad. T.T

I need not null in 98% of my code, but definitely need null in 2%,

option/Some/None is not perfect solution, because this properies used in C# module. T.T

I have long list of value member definitions, constructor or other init method make code too long unnecessarily.

Record type does not provide inheritance. T.T

I love F#, F# exception, F# curry function, F# event, F# option, let, naver null, ...

But on actual work ground, there are tons of C# people, tons of C# framework, and we need communicate with them smoothly. T.T

.

option/Some/None is not perfect solution, because this properies used in C# module.

I'm not sure I understand... aren't let-bindings in a class private? If so, how are you going to use them in C# context, anyway?

By on 6/25/2009 2:54 PM ()

Oh, one other thing, regarding attributes. Sometimes, you find yourself adding a lot of attributes, and it looks really ugly having big long names. In this case, you can use F#'s type aliases:

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type DV = DefaultValueAttribute
type Bla = 
    [<DV>]
    val mutable id : int


That reduces the visual noise a bit.

By on 6/25/2009 1:56 PM ()

// MichaelGG

thank you for the helpful tips,

they rescue me a lot :)

// int19h

yes, let is private,

to access let definition from C# or outside of class,

bunch of annoying properties are needed :(

but, I does not complain this,

let is private, it's good, it's right,

We just need some helper like C# automatic property,

or DefaultValue-attr-less, initializeable val,

it will be enough,

By on 6/26/2009 12:14 AM ()

Hi!

Yes, sometimes F# interop can require a bit more work to figure out. We are also using ASP.NET MVC, and there were a few things we had to deal with. However, I think you will see that overall, F# is a major win, even with a bit of extra work for certain interop. The amount of extra code you type is less than all the extra type definitions you'd need in C# anyways ;).

Looking at your code, it seems you have a problem with setting an F# class to null. I would recommend not setting it to null, but if you must, then what you want is this:

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 let mutable SomeFSharpValue = Unchecked.defaultof<_>

Notice you don't always need to specify the type. It will figure it out from constraints later on (like, if you pass the value to a function).

But, I agree that it can be a pain to declare a bunch of "C#-normal", default value properties. C# 3 has automatic properties -- I think F# would benefit from something like that to let us quickly make class types that fit a certain schema.

On my site I just wrote about how to make ASP.NET MVC bind to F# records: [link:www.atrevido.net] -- Maybe that will help you in other places.

By on 6/25/2009 1:52 PM ()

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