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Order of fileds in record definition
f#compiler

Hi All,
I'm writing an image processing app.

I declared a record type ImageRegion as follows:

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type ImageRegion =
{ Name : string; Membership : float*float -> bool; Center : float*float; }

If I define a value as follows:

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let nwCorner =
   { Name = "NW"; Center = (0.25, 0.25); Membership = fun (x, y) -> x + y <= 0.5; }

all works fine. But if I change the order of the fields to be the same as in the type declaration, e.g.:

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let nwCorner =
    { Name = "NW"; Membership = fun (x, y) -> x + y <= 0.5; Center = (0.25, 0.25); }   ,

I get a compiler error: "the value or constructor "Center" is not defined. I tried getting rid of the last semicolon to no avail.

Does anybody know what's going on here? It compiles OK now, but I'd like to know what's happening.

Cheers
Mau

.

Hi Mau,

this error is caused by the syntax of anonymous function declaration. The semicolon after fun is interpreted as a sequencing operation and in this context 

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Center = (0.25, 0.25)

would become a boolean expression (using structural equality) that was part of the lambda.

You can fix this by inserting parentheses at the correct location so:

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let nwCorner =
    { Name = "NW"; Membership = (fun (x, y) -> x + y <= 0.5); Center = (0.25, 0.25); }

or use #light syntax so:

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let nwCorner =
    {   Name = "NW"; 
        Membership = fun (x, y) -> x + y <= 0.5;
        Center = (0.25, 0.25); }

regards,

Danny

By on 12/22/2008 4:20 AM ()

Oh, I see!
Thanks a lot Danny!

Mau

By on 12/22/2008 4:36 PM ()

Ehm, sorry for the typo in the subject.
Mau

By on 12/22/2008 2:58 AM ()

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