Why there is a type inference error in this sample

Question

I'm getting "Lookup on object of indeterminate type based on information prior to this program point" error on this snippet:

let a = [|"a"; "bb"|]
let n = Array.mapi (fun i x -> (i * x.Length)) a

What is wrong with it? Visual Studio F# Interactive correctly shows the type of x as string when I hover the cursor above it. Why do I have to write:

let a = [|"a"; "bb"|]
let n = Array.mapi (fun i (x:string) -> (i * x.Length)) a

to compile successfully?

bytebuster bytebuster · Accepted Answer · 2013-03-01T08:53:55

The type checker works from left to right. Due to this reason, the compiler doesn't have enough information to infer a proper type for x.

The easiest way to overcome this problem is to place a in the beginning:

let n = a |> Array.mapi (fun i x -> (i * x.Length))

The compiler will know that a has type of string [], and therefore, x is of string.

Another alternative is using static functions:

let n = Array.mapi (fun i x -> (i * String.length x)) a

String.length takes a string, and so string becomes an inferred type for x.