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type ('a, 'e) t = ('a, 'e) result
let return (a: 'a): ('a, 'e) t =
Ok a
let fail (e: 'e): ('a, 'e) t =
Error e
let to_option (r: ('a, _) t): 'a option =
match r with
| Ok a ->
Some a
| Error _ ->
None
let (>>=) (m: ('a, 'e) t) (f: 'a -> ('b, 'e) t): ('b, 'e) t =
match m with
| Ok a ->
f a
| Error e ->
Error e
let ( let* ) = (>>=)
let map (f: 'a -> 'b): ('a, 'e) t -> ('b, 'e) t = function
| Ok a ->
Ok (f a)
| Error e ->
Error e
let map_error (f: 'e -> 'f): ('a, 'e) t -> ('a, 'f) t = function
| Ok a ->
Ok a
| Error e ->
Error (f e)
let get: ('a, Void.t) t -> 'a = function
| Ok a ->
a
| Error e ->
Void.absurd e
module Monad (E: Interfaces.ANY) =
struct
type 'a t = ('a, E.t) result
let return = return
let fail = fail
let to_option = to_option
let (>>=) = (>>=)
let ( let* ) = (>>=)
end
type 'a r = ('a, string) result
let add (a: int r) (b: int r): int r =
let* x = a in
let* y = b in
Ok (x + y)
let divide (a: int r) (b: int r): int r =
let* x = a in
let* y = b in
if y = 0 then
Error "Division by Zero"
else
Ok (x / y)
let%test _ =
add (Ok 1) (divide (Ok 2) (Ok 0))
=
Error "Division by Zero"
let%test _ =
add (Ok 1) (divide (Ok 10) (Ok 2))
=
Ok 6