Putting a ()->int into an F# type

Question

I have an application that uses a (generic) service to perform IO actions. I want to aggregate the usual IO functions (Save, SaveAs, etc.) into an F# type but the compiler seems to dislike this notation:

type InputService<'a> = { // Fine SomeFunc : 'a -> Option<'a> // Error (VS2012): "Anonymous type variables are not permitted in this declaration" Save : 'a -> () // Error (see above) Load : () -> 'a }

I'm aware that stateful functions like this may not be idiomatic. In reality, I plan on currying in UI prompts, file paths, etc but is it possible to define that function signature in my type?

John Palmer · Accepted Answer · 2013-09-17 07:16:35Z

8

It appears that in this notation you need to write

type InputService<'a> = { SomeFunc : 'a -> Option<'a> Save : 'a -> unit Load : unit -> 'a }

i.e. write unit instead of ()

You can see a simpler example here

let t : () = ();;

produces the same error message, but writing unit works fine.

The reason for these error messages is that () is a constant like 1. Obviously you can't write

let t : 1 = 1;;

So the same applies to ()

edited Sep 17, 2013 at 7:16

answered Sep 15, 2013 at 10:41

John Palmer

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3 Comments

Adam Kewley Over a year ago

Thanks for the additional information also, I'm relatively new to F# and come from an OO background. Little things like this can be a real grind!

John Palmer Over a year ago

Glad to see you appreciate the info. I think though for your example it might be better to use an interface rather than the record type.

Adam Kewley Over a year ago

Hey thanks again. I've been using a set of interfaces for the service layer in C# (an 'in' IInput and 'out' IOutput composed into an invariant IIOInterface) and they've been working great and interchangeably. However, with discriminated unions around I've created seperate service records with different type signatures (e.g. Load : string->('a * FileStream)) which may allow consumers to more adequately handle slight different requirements. However, I'll likely subclass these from a generic interface incase it becomes too heavyweight. I find juggling these new powers a big responsibility though.

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