Question

let sub (a:float[]) (b:float[])=  
    [| for i = 0 to Array.length a - 1 do yield a.[i]-b.[i] |]

let fx t = [0..t]
let x=sub (fx t) (fx t)

Above compiles OK.

But the same causes an error when I replace the function call with a method invocation:

type type_f = delegate of float -> float[]
let mutable f =new type_f(fun t->[|0..t|])
let m=f.Invoke(5) //float[]
let y=sub m f.Invoke(5)//error

How do I pass the result of a method invocation as a parameter to a function?

Answer 1

In F#, the line

let y = sub m f.Invoke(5)

ambiguously parses as

let y = sub   m   f.Invoke   (5)

(passing three arguments to sub).

To fix this, you need more parentheses:

let y = sub m (f.Invoke(5))

(Note that using delegate types is not idiomatic unless interoperating with .NET code in other languages. If f was a function, then

let y = sub m (f 5)

would be sufficient, as you have noted.)