Belt_Result (melange.Belt

Belt.Result is a data type with two variants: Ok and Error. Each of these variants can contain data, and those two pieces of data need not have the same data type. Belt.Result is useful when you need to not only determine whether some data is valid or not (use Belt.Option for that), but also keep information about the invalid data.

In the examples, we presume the existence of two variables:

let good = Ok 42
let bad = Error "Invalid data"

type t('a, 'b) =

| Ok('a)
| Error('b)

;

let getExn: t('a, 'b) => 'a;

getExn res

when res is Ok n, returns n when res is Error m, raise an exception

getExn good = 42;;
getExn bad;; (* raises exception *)

let mapWithDefaultU: t('a, 'c) => 'b => Js.Fn.arity1(('a => 'b)) => 'b;

let mapWithDefault: t('a, 'c) => 'b => ('a => 'b) => 'b;

mapWithDefault res default f

When res is Ok n, returns f n, otherwise default.

mapWithDefault good 0 (fun x -> x / 2) = 21
mapWithDefault bad 0 (fun x -> x / 2) = 0

let mapU: t('a, 'c) => Js.Fn.arity1(('a => 'b)) => t('b, 'c);

let map: t('a, 'c) => ('a => 'b) => t('b, 'c);

map res f

When res is Ok n, returns Ok (f n). Otherwise returns res unchanged. Function f takes a value of the same type as n and returns an ordinary value.

let f x = sqrt (float_of_int x)
map (Ok 64) f = Ok 8.0
map (Error "Invalid data") f = Error "Invalid data"

let flatMapU: t('a, 'c) => Js.Fn.arity1(('a => t('b, 'c))) => t('b, 'c);

let flatMap: t('a, 'c) => ('a => t('b, 'c)) => t('b, 'c);

flatMap res f

When res is Ok n, returns f n. Otherwise, returns res unchanged. Function f takes a value of the same type as n and returns a Belt.Result.

let recip x = 
  if x != 0.0
  then
    Ok (1.0 /. x)
  else
    Error "Divide by zero"

    flatMap (Ok 2.0) recip = Ok 0.5
    flatMap (Ok 0.0) recip = Error "Divide by zero"
    flatMap (Error "Already bad") recip = Error "Already bad"

let getWithDefault: t('a, 'b) => 'a => 'a;

getWithDefault res defaultValue

if res is Ok n, returns n, otherwise default

getWithDefault (Ok 42) 0 = 42
getWithDefault (Error "Invalid Data") = 0

let isOk: t('a, 'b) => bool;

isOk res

Returns true if res is of the form Ok n, false if it is the Error e variant.

let isError: t('a, 'b) => bool;

isError res

Returns true if res is of the form Error e, false if it is the Ok n variant.

let eqU: t('a, 'c) => t('b, 'd) => Js.Fn.arity2(('a => 'b => bool)) => bool;

let eq: t('a, 'c) => t('b, 'd) => ('a => 'b => bool) => bool;

eq res1 res2 f

Determine if two Belt.Result variables are equal with respect to an equality function. If res1 and res2 are of the form Ok n and Ok m, return the result of f n m. If one of res1 and res2 are of the form Error e, return false If both res1 and res2 are of the form Error e, return true

let good1 = Ok 42
let good2 = Ok 32
let bad1 = Error "invalid"
let bad2 = Error "really invalid"

let mod10equal a b =
  a mod 10 == b mod 10
  
eq good1 good2 mod10equal = true
eq good1 bad1 mod10equal = false
eq bad2 good2 mod10equal = false
eq bad1 bad2 mod10equal = true

let cmpU: t('a, 'c) => t('b, 'd) => Js.Fn.arity2(('a => 'b => int)) => int;

let cmp: t('a, 'c) => t('b, 'd) => ('a => 'b => int) => int;

cmp res1 res2 f

Compare two Belt.Result variables with respect to a comparison function. The comparison function returns -1 if the first variable is "less than" the second, 0 if the two variables are equal, and 1 if the first is "greater than" the second.

If res1 and res2 are of the form Ok n and Ok m, return the result of f n m. If res1 is of the form Error e and res2 of the form Ok n, return -1 (nothing is less than something) If res1 is of the form Ok n and res2 of the form Error e, return 1 (something is greater than nothing) If both res1 and res2 are of the form Error e, return 0 (equal)

let good1 = Ok 59
let good2 = Ok 37
let bad1 = Error "invalid"
let bad2 = Error "really invalid"

let mod10cmp a b =
  Pervasives.compare (a mod 10) (b mod 10)
  
cmp (Ok 39) (Ok 57) mod10cmp = 1
cmp (Ok 57) (Ok 39) mod10cmp = -1
cmp (Ok 39) (Error "y") mod10cmp = 1
cmp (Error "x") (Ok 57) mod10cmp = -1
cmp (Error "x") (Error "y") mod10cmp = 0