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Forward Declarations
f#compiler

A few of us are working on learning F# at my workplace. The subject of compilation order came up today and I mentioned that one of the designers of the language said part of the reason for that requirement is avoiding circular dependencies--which seems a very valid reason.

However, some of the developers in the room inquired why it is that they couldn't allow forward declarations of symbols. Like this in C++:

class MyClass;

//code using MyClass

class MyClass

{

//etc.

};

I'm sure there are valid design reasons to avoid this approach--maybe this would make the F# compiler design orders of magnitude more complex. I am curious about why forward declarations are not allowed. Or are they (I'm still learning the language)?

.

Hi,

AFAIK

and I'm happy to be corrected (as a newbie to F#), what you want is perfectly possible. Create a signature of the class in a file compiled before the use and then provide the implementation later.

Something like

1
2
3

myClass.fsi
useMyClass.fs
myClass.fs

See [link:msdn.microsoft.com] for details

Regards
Joe

By on 3/20/2010 12:25 PM ()

Thanks for the thought Joe but that's not exactly what I'm talking about.

--

Onorio

By on 3/20/2010 8:00 PM ()

AFAIK and I'm happy to be corrected (as a newbie to F#), what you want is perfectly possible. Create a signature of the class in a file compiled before the use and then provide the implementation later.

No, that is not how signature files work. Signature files provide a succinct summary of a file and enable portions to be hidden (e.g. control accessibility). They don't interact with F# "ordering" rules.

In F#, to use an entity, it must already be defined above (or be co-defined as part of a recursive group, e.g. "let rec f1 ... and f2 ..." or "type T1 ... and T2 ...").

By on 3/20/2010 1:19 PM ()

No, that is not how signature files work. Signature files provide a succinct summary of a file and enable portions to be hidden (e.g. control accessibility). They don't interact with F# "ordering" rules.

My mistake. Thanks for the correction.

By on 3/20/2010 2:44 PM ()

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