compose.swift

import Cocoa

precedencegroup ForwardApplication {
    associativity: left
}

precedencegroup ForwardComposition {
    associativity: left
    higherThan: ForwardApplication
}

infix operator |>: ForwardApplication
func |> <A, B>(a: A, f: (A) throws -> B) throws -> B {
    return try f(a)
}

infix operator >>>: ForwardComposition
func >>> <A, B, C>(f: @escaping (A) throws -> B, g: @escaping (B) throws -> C) -> ((A) throws -> C) {
    return { a in
        try g(f(a))
    }
}

typealias EmojiMapping = [String: URL]

func loadJSON<T: Decodable>(file: URL) throws -> T {
    let decoder = JSONDecoder()
    let content = try Data(contentsOf: file)

    return try decoder.decode(T.self, from: content)
}

func makeReplacer(maps: EmojiMapping) -> (String) -> NSAttributedString? {
    return { name in
        guard let url = maps[name] else {
            return nil
        }

        let icon = NSTextAttachment()
        icon.image = NSImage(contentsOf: url)

        return NSAttributedString(attachment: icon)
    }
}

let replace = try URL(fileURLWithPath: "emojis.json")
    |> loadJSON
    >>> makeReplacer

let result = replace("poop")

print("Result = \(result)")