(deftype (Result a b)
  (Success [a])
  (Error [b]))

(doc Result "is a data type for computations that might fail with an error. It
has two constructors, `(Success <value>)` and `(Error <value>)`, and provides
many functions for working with, combining, and wrapping and unwrapping
values.")
(defmodule Result
  (doc apply "takes a `Result` `a` and applies functions to them, one in the case that it is an `Error`, one in the case that it is a `Success`.")
  (defn apply [a success-f error-f]
    (match a
      (Success x) (Success (~success-f x))
      (Error x) (Error (~error-f x))))

  (doc map "takes a `Result` and applies a function `f` to it if it is a `Success` type, and wraps it back up. In the case that it is an `Error`, it is returned as is.")
  (defn map [a f]
    (match a
      (Success x) (Success (~f x))
      (Error x) (Error x)))

  (doc map-error "takes a `Result` and applies a function `f` to it if it is an `Error` type, and wraps it back up. In the case that it is a `Success`, it is returned as is.")
  (defn map-error [a f]
    (match a
      (Success x) (Success x)
      (Error x) (Error (~f x))))

  (doc and-then "takes a `Result` and applies a function `f` to it if it is a `Success` type. In the case that it is an `Error`, it is returned as is.

It is thus quite similar to [`map`](#map), but it will unwrap the value.")
  (defn and-then [a f]
    (match a
      (Success x) (~f x)
      (Error x) (Error x)))

  (doc unwrap-or-zero "takes a `Result` and either unwraps it if it is a `Success`, or calls `zero`. `zero` must be defined on the `Success` type.")
  (defn unwrap-or-zero [a]
    (match a
      (Success x) x
      (Error _) (zero)))

  (doc or-else "takes a `Result` and applies a function `f` to it if it is an `Error` type. In the case that it is a `Success`, it is returned as is.

It is the inverse of [`and-then`](#and-then).")
  (defn or-else [a f]
    (match a
      (Success x) (Success x)
      (Error x) (~f x)))

  (doc unwrap-or-else "takes a `Result` and either unwraps it if it is a `Success`, or calls a function `f` on the value contained in the `Error`.")
  (defn unwrap-or-else [a f]
    (match a
      (Success x) x
      (Error x) (~f x)))

  (doc unsafe-from-success "is an unsafe unwrapper that will get the value from a `Success`. If `a` is an `Error`, a runtime error will be generated.")
  (defn unsafe-from-success [a]
    (match a
      (Success x) x))

  (doc from-success "is an unwrapper that will get the value from a `Success`. If `a` is an `Error`, a default value will be returned.")
  (defn from-success [a dflt]
    (match a
      (Error _) dflt
      (Success x) x))

  (doc unsafe-from-error "is an unsafe unwrapper that will get the value from an `Error`. If `a` is a `Success`, a runtime error will be generated.")
  (defn unsafe-from-error [a]
    (match a
      (Error x) x))

  (doc from-error "is an unwrapper that will get the value from an `Error`. If `a` is a `Success`, a default value will be returned.")
  (defn from-error [a dflt]
    (match a
      (Success _) dflt
      (Error x) x))

  (doc to-maybe "is a function that will convert a `Result` to a `Maybe`, where a `Success` becomes a `Just` and an `Error` becomes a `Nothing`.

The error value is thrown away.")
  (defn to-maybe [a]
    (match a
      (Success x) (Maybe.Just x)
      (Error _) (Maybe.Nothing)))

  (doc success? "checks whether the given value `a` is a `Success`.

It is the inverse of [`error?`](#error?).")
  (defn success? [a]
    (match-ref a
      (Error _) false
      (Success _) true))

  (doc error? "checks whether the given value `a` is an `Error`.

It is the inverse of [`success?`](#success?).")
  (defn error? [a]
    (match-ref a
      (Error _) true
      (Success _) false))

  (doc = "equality is defined as the equality of both the constructor and the contained value, i.e. `(Success 1)` and `(Success 1)` are the same, but `(Success 1)` and `(Success 2)` are not.")
  (defn = [a b]
    (match-ref a
      (Success x)
        (match-ref b
          (Error _) false
          (Success y) (= x y))
      (Error x)
        (match-ref b
          (Success _) false
          (Error y) (= x y))))
  (implements = Result.=)
)

(defmodule Maybe
  (doc to-result "is a function that will convert a `Maybe` to a `Result`, where a `Just` becomes a `Success` and a `Nothing` becomes an `Error`.

You’ll also need to provide an error message `error` that is given to the `Error` constructor if necessary.")
  (defn to-result [a error]
    (match a
      (Nothing) (Result.Error error)
      (Just x) (Result.Success x)))
)