Back to Questions
896
views1
comment
F# Currying and winforms...
f#

In the process of trying to simplify my code I have tried to curry a function, to reduce text. However the curreid function returns a type error in some cases where the use of underlying function (passing all the params) doesn't.

Key Snippets from my project are

<quote> let handler (wp:RichTextBox) (v) = let translate t = match box t with | :? int as i -> i.ToString() | :? string as s -> s | _ as t -> t.ToString() in (fun _ -> (wp.Text <- (wp.Text ^ (translate v)))) let showForm = let form = new Form() let windowPane = new RichTextBox() form.Controls.Add windowPane let wp_handler = handler windowPane position_button form (create_button "1" (wp_handler 1)) (3,1) position_button form (create_button "2" (wp_handler 2)) (3,2) position_button form (create_button "0" (wp_handler 0)) (4,1) position_button form (create_button "." (wp_handler <i>"."<i>)) (4,3)</quote> Which yields <quote> C:\workspace\neuCalculator\neuCalculator\form.fs(66,60): error: FS0001: This expression has type string but is here used with type int</quote> However this works fine (uncurried): <quote> . . . position_button form (create_button "1" (handler windowPane 1)) (3,1) position_button form (create_button "2" (handler windowPane 2)) (3,2) position_button form (create_button "3" (handler windowPane 3)) (3,3) position_button form (create_button "." (handler windowPane ".")) (4,3)</quote> in FSI I run the key snippets and it reveals some details to me which am not yet able to decipher meaningfully: <quote> val handler : RichTextBox -> 'a -> ('b -> unit) > let handler (wp:RichTextBox) (v) = - let translate t = - match box t with - | :? int as i -> i.ToString() - | :? string as s -> s - | _ as t -> t.ToString() in - (fun _ -> (wp.Text <- (wp.Text ^ (translate v))));; val handler : RichTextBox -> 'a -> ('b -> unit) > let windowPane = new RichTextBox() - ;; val windowPane : RichTextBox > let wp_handler = handler windowPane - ;; let wp_handler = handler windowPane ----^^^^^^^^^^^ stdin(109,4): error: FS0030: Value restriction. Type inference has inferred the signature val wp_handler : ('_a -> '_b -> unit) Either define 'wp_handler' as a syntactic function, or add a type constraint to instantiate the type parameters. stopped due to error</quote> There must be a way of typing the parameters in the curried function or somthing... Still even if I try to call handler from FSI I get a very similar error: <quote> > let x = handler windowPane 1 ;; let x = handler windowPane 1 ;; ----^^ stdin(140,4): error: FS0030: Value restriction. Type inference has inferred the signature val x : ('_a -> unit) Either define 'x' as a syntactic function, or add a type constraint to instantiate the type parameters. stopped due to error ></quote> Ugh. I am hacking away trying to fuigure this out. But any help would be wonderful at this point. Thank you all... Todd

.

It has to do with the fact that F# can't tell what the type of the arguments are. Obviously, because you are boxing, you want the function to be as generic as possible. In addition, you completely ignore your third parameter (as seen in 

1

fun _ -> (wp.Text <- (wp.Text ^ (translate v)))

). So, basically, when the new function is produced as a result of currying, you need to tell F# what types to expect (whether polymorphic or constrained). I don't know if this explanation is 100% technically accurate, but it is basically what is going on.

One way to do that is to declare wp_handler as 

1

let wp_handler (x: 'a) (y: 'a) = handler windowPane x y

. This says that the last 2 parameters (though the last will be discarded) will be of the same type.

If you just want the last parameter as a means of triggering the handler's effect, you can change the final line of handler to 

1

fun () -> (wp.Text <- (wp.Text ^ (translate v)))

. That way, wp_handler can be defined as 

1

let wp_handler (x: 'a) = handler windowPane x

since F# already knows that the last parameter to handler is of type unit.

Hope this helps,

z.

By on 12/20/2007 5:20 PM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper