Belt.Result
Utilities for result data type.
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"
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);
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);
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;
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;
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