Embrace Swift type inference

import SwiftUI

struct SmoothieList: View {
    var smoothies: [Smoothie]
    
    @State var searchPhrase = ""

    var body: some View {
        FilteredList(
            smoothies,
            filterBy: \.title,
            isIncluded: { title in title.hasSubstring(searchPhrase) }
        ) { smoothie in
            SmoothieRowView(smoothie: smoothie)
        }
    }
}

extension String {
    /// Returns `true` if this string contains the provided substring,
    /// or if the substring is empty. Otherwise, returns `false`.
    ///
    /// - Parameter substring: The substring to search for within
    ///   this string.
    func hasSubstring(_ substring: String) -> Bool {
        substring.isEmpty || contains(substring)
    }
}

import SwiftUI

public struct FilteredList<Element, FilterKey, RowContent>: View
        where Element: Identifiable, RowContent: View {

    private let data: [Element]
    private let filterKey: KeyPath<Element, FilterKey>
    private let isIncluded: (FilterKey) -> Bool
    private let rowContent: (Element) -> RowContent

    public init(
        _ data: [Element],
        filterBy key: KeyPath<Element, FilterKey>,
        isIncluded: @escaping (FilterKey) -> Bool,
        @ViewBuilder rowContent: @escaping (Element) -> RowContent
    ) {
        self.data = data
        self.filterKey = key
        self.isIncluded = isIncluded
        self.rowContent = rowContent
    }

    public var body: some View {
        let filteredData = data.filter {
            isIncluded($0[keyPath: filterKey])
        }

        return List(filteredData, rowContent: rowContent)
    }
}