CODEWITHSUNDEEP
f#
Paul Jurczak
Paul Jurczak

Reputation: 8127

Why there is a type inference error in this sample

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?

Upvotes: 3

Views: 239

Answers (1)

Be Brave Be Like Ukraine
Be Brave Be Like Ukraine

Reputation: 7735

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.

Upvotes: 6

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