type inference in F# starts from left to right and top to bottom. So if you do not use |> operator, then type annotations in lambda expression will be required.

TypeInferenceLambda.fs

let sizeOfFolder folder = 
    let allFiles = Directory.GetFiles(folder, "*.*", SearchOption.AllDirectories)
    let fileInfos = allFiles |> Array.map (fun x -> new FileInfo(x))
    let allSizes = fileInfos |> Array.map (fun x -> x.Length)
    Array.sum allSizes

///This version requires explicit type annotations in the lambda
let sizeOfFolder2 folder = 
    let allFiles  =  Directory.GetFiles(folder, "*.*", SearchOption.AllDirectories)
    let fileInfos =  Array.map (fun (x:string) -> new FileInfo(x)) allFiles
    let allSizes  =  Array.map (fun (x:FileInfo) -> x.Length) fileInfos
    Array.sum allSizes