Question

In F# why does my add function not add two floats

let add a b = a+b

(add 3 4) //returns 7
(add 3.5 5.5) //error

also please explain how type inference works in F#.

Thanks.

Answer 1

You have to make it inline.

let inline add a b = a+b

The problem is that + is an inline operator, so if your function add is not inline it will take the default overload which is for int.

Have a look at this answer Use of `inline` in F#

When the function is declared inline, type inference will infer the static type constraints.

val inline add :
   ^a ->  ^b ->  ^c
    when ( ^a or  ^b) : (static member ( + ) :  ^a *  ^b ->  ^c)

So now a and b could be any type that implement the static member (+) with that signature.