how to define a class of boolean functions with infix operators in F#?

Maybe it's already implemented in F# ?

basically i would like to define a class of generic filter functions with infix operators, so something which would look like

type Filter<'T> = ('T -> bool) with 
     static member (|*) (f:Filter<'T>) (g:Filter<'T>) = (fun x -> (f x) || 
     (g x)) // OR operator

but this is not the right syntax it seems

Stopped due to error System.Exception: Operation could not be completed due to earlier error Type abbreviations cannot have augmentations at 2,5 Type abbreviations cannot have members at 3,19

thanks

Solution

What you are defining there is a type abbreviation, which, as the errors will indicate, can neither have augmentations nor members. You could fix that by using a single case discriminated union:

type Filter<'T> = Filter of ('T -> bool) with 
     static member (|* ) (Filter f, Filter g) =
        Filter(fun x -> f x || g x) // OR operator

Of course, you now need to unwrap the predicate function prior to the boolean operation, and wrap the composed function again afterwards. A simple test...

let odd x = x % 2 <> 0
let big x = x > 10
let (Filter f) = Filter odd |* Filter big in [8..12] |> List.filter f
// val it : int list = [9; 11; 12]