Module `Tablecloth.Result`

Functions for working with computations which may fail.

A Result is used to represent a computation which may fail.

A Result is a variant, which has a constructor for successful results (Ok 'ok), and one for unsuccessful results ((Error 'error)).

  type ('ok, 'error) t =
    | Ok of 'ok
    | Error of 'error

Here is how you would annotate a Result variable whose Ok variant is an integer and whose Error variant is a string:

let ok: (int, string) Result.t = Ok 3

let error: (int, string) Result.t = Error "This computation failed!"

Note The 'error case can be of any type and while string is very common you could also use:

string List.t to allow errors to be accumulated
exn, in which case the result type just makes exceptions explicit in the return type
A variant or polymorphic variant, with one case per possible error. This is means each error can be dealt with explicitly. See this excellent article for mnore information on this approach.

If the function you are writing can only fail in a single obvious way, maybe you want an Option instead.

type ('ok, 'error) t = ('ok, 'error) result

Create

val ok : 'ok -> ('ok, 'error) t

A function alternative to the Ok constructor which can be used in places where the constructor isn't permitted such as at the of a Fun.(|>) or functions like List.map.

Examples

String.reverse "desserts" |> Result.ok = Ok "stressed"

List.map [1; 2; 3] ~f:Result.ok = [Ok 1; Ok 2; Ok 3]

val error : 'error -> ('ok, 'error) t

A function alternative to the Error constructor which can be used in places where the constructor isn't permitted such as at the of a Fun.pipe or functions like List.map.

Note

When targetting the Bucklescript compiler you can use constructors with the fast pipe.

5 |. Ok = (Ok 5)

See the Reason docs for more.

Examples

Int.negate 3 |> Result.error 3 = Error (-3)

List.map [1; 2; 3] ~f:Result.error = [Error 1; Error 2; Error 3]

val attempt : (unit -> 'ok) -> ('ok, exn) t

Run the provided function and wrap the returned value in a Result, catching any exceptions raised.

Examples

Result.attempt (fun () -> 5 / 0) = Error Division_by_zero

  let numbers = [|1,2,3|] in
  Result.attempt (fun () -> numbers.(3)) =
    Error (Invalid_argument "index out of bounds")

val fromOption : 'ok option -> error:'error -> ('ok, 'error) t

Convert an Option to a Result where a (Some value) becomes (Ok value) and a None becomes (Error error).

Examples

Result.fromOption (Some 84) ~error:"Greater than 100" = Ok 8

  Result.fromOption None ~error:"Greater than 100" =
    Error "Greater than 100"

val from_option : 'ok option -> error:'error -> ('ok, 'error) t

val isOk : (_, _) t -> bool

Check if a Result is an Ok.

Useful when you want to perform some side affect based on the presence of an Ok like logging.

Note if you need access to the contained value rather than doing Result.isOk followed by Result.unwrapUnsafe its safer and just as convenient to use pattern matching directly or use one of Result.andThen or Result.map

Examples

Result.isOk (Ok 3) = true

Result.isOk (Error 3) = false

val is_ok : (_, _) t -> bool

val isError : (_, _) t -> bool

Check if a Result is an Error.

Useful when you want to perform some side affect based on the presence of an Error like logging.

Examples

Result.isError (Ok 3) = false

Result.isError (Error 3) = true

val is_error : (_, _) t -> bool

val and_ : ('ok, 'error) t -> ('ok, 'error) t -> ('ok, 'error) t

Returns the first argument if it isError, otherwise return the second argument.

Unlike the Bool.(&&) operator, the and_ function does not short-circuit. When you call and_, both arguments are evaluated before being passed to the function.

Examples

Result.and_ (Ok "Antelope") (Ok "Salmon") = Ok "Salmon"

  Result.and_
    (Error (`UnexpectedBird "Finch"))
    (Ok "Salmon")
    = Error (`UnexpectedBird "Finch")

  Result.and_
    (Ok "Antelope")
    (Error (`UnexpectedBird "Finch"))
      = Error (`UnexpectedBird "Finch")

  Result.and_
    (Error (`UnexpectedInvertabrate "Honey bee"))
    (Error (`UnexpectedBird "Finch"))
      = Error (`UnexpectedBird "Honey Bee")

val or_ : ('ok, 'error) t -> ('ok, 'error) t -> ('ok, 'error) t

Return the first argument if it isOk, otherwise return the second.

Unlike the built in || operator, the or_ function does not short-circuit. When you call or_, both arguments are evaluated before being passed to the function.

Examples

Result.or_ (Ok "Boar") (Ok "Gecko") = (Ok "Boar")

Result.or_ (Error (`UnexpectedInvertabrate "Periwinkle")) (Ok "Gecko") = (Ok "Gecko")

Result.or_ (Ok "Boar") (Error (`UnexpectedInvertabrate "Periwinkle")) = (Ok "Boar")

Result.or_ (Error (`UnexpectedInvertabrate "Periwinkle")) (Error (`UnexpectedBird "Robin")) = (Error (`UnexpectedBird "Robin"))

val orElse : ('ok, 'error) t -> ('ok, 'error) t -> ('ok, 'error) t

Return the second argument if it isOk, otherwise return the first.

Like or_ but in reverse. Useful when using the |> operator

Examples

Result.orElse (Ok "Boar") (Ok "Gecko") = (Ok "Gecko")

Result.orElse (Error (`UnexpectedInvertabrate "Periwinkle")) (Ok "Gecko") = (Ok "Gecko")

Result.orElse (Ok "Boar") (Error (`UnexpectedInvertabrate "Periwinkle")) = (Ok "Boar")

Result.orElse (Error (`UnexpectedInvertabrate "Periwinkle")) (Error (`UnexpectedBird "Robin")) = (Error (`UnexpectedInvertabrate "Periwinkle"))

val or_else : ('ok, 'error) t -> ('ok, 'error) t -> ('ok, 'error) t

val both : ('a, 'error) t -> ('b, 'error) t -> ('a * 'b, 'error) t

Combine two results, if both are Ok returns an Ok containing a Tuple of the values.

If either is an Error, returns the Error.

The same as writing Result.map2 ~f:Tuple.make

Examples

Result.both (Ok "Badger") (Ok "Rhino") = Ok ("Dog", "Rhino")

  Result.both (Error (`UnexpectedBird "Flamingo")) (Ok "Rhino") =
    (Error (`UnexpectedBird "Flamingo"))

  Result.both
    (Ok "Badger")
    (Error (`UnexpectedInvertabrate "Blue ringed octopus")) =
      (Error (`UnexpectedInvertabrate "Blue ringed octopus"))

  Result.both
    (Error (`UnexpectedBird "Flamingo"))
    (Error (`UnexpectedInvertabrate "Blue ringed octopus")) =
      (Error (`UnexpectedBird "Flamingo"))

val flatten : (('ok, 'error) t, 'error) t -> ('ok, 'error) t

Collapse a nested result, removing one layer of nesting.

Examples

Result.flatten (Ok (Ok 2)) = Ok 2

  Result.flatten (Ok (Error (`UnexpectedBird "Peregrin falcon"))) =
    (Error (`UnexpectedBird "Peregrin falcon"))

  Result.flatten (Error (`UnexpectedInvertabrate "Woodlouse")) =
    (Error (`UnexpectedInvertabrate "Woodlouse"))

val unwrap : ('ok, 'error) t -> default:'ok -> 'ok

Unwrap a Result using the ~default value in case of an Error

Examples

Result.unwrap ~default:0 (Ok 12) = 12

Result.unwrap ~default:0 ((Error (`UnexpectedBird "Ostrich"))) = 0

val unwrapLazy : ('ok, 'error) t -> default:'ok Lazy.t -> 'ok

Unwrap a Result using the Lazy.force default value in case of an Error

Examples

Result.unwrap ~default:(lazy 0) (Ok 12) = 12

Result.unwrap ~default:(lazy 0) ((Error (`UnexpectedBird "Ostrich"))) = 0

val unwrapUnsafe : ('ok, _) t -> 'ok

Unwrap a Result, raising an exception in case of an Error

Exceptions

Raises an Invalid_argument "Result.unwrapUnsafe called with an Error" exception.

Examples

Result.unwrapUnsafe (Ok 12) = 12

Result.unwrapUnsafe (Error "bad")

val unwrap_unsafe : ('ok, _) t -> 'ok

val unwrapError : ('ok, 'error) t -> default:'error -> 'error

Like Result.unwrap but unwraps an Error value instead

Examples

  Result.unwrapError
    (Error (`UnexpectedBird "Swallow"))
    ~default:(`UnexpectedInvertabrate "Ladybird") =
      `UnexpectedBird "Swallow"

  Result.unwrapError
    (Ok 5)
    ~default:(`UnexpectedInvertabrate "Ladybird") =
      `UnexpectedInvertabrate "Ladybird"

val unwrap_error : ('ok, 'error) t -> default:'error -> 'error

val map2 : 
  ('a, 'error) t ->
  ('b, 'error) t ->
  f:('a -> 'b -> 'c) ->
  ('c, 'error) t

Combine two results

If one of the results is an Error, that becomes the return result.

If both are Error values, returns its first.

Examples

Result.map2 (Ok 7) (Ok 3) ~f:Int.add = Ok 10

Result.map2 (Error "A") (Ok 3) ~f:Int.add = Error "A"

Result.map2 (Ok 7) (Error "B") ~f:Int.add = Error "B"

Result.map2 (Error "A") (Error "B") ~f:Int.add = Error "A"

val values : ('ok, 'error) t list -> ('ok list, 'error) t

If all of the elements of a list are Ok, returns an Ok of the the list of unwrapped values.

If any of the elements are an Error, the first one encountered is returned.

Examples

Result.values [Ok 1; Ok 2; Ok 3; Ok 4] = Ok [1; 2; 3; 4]

Result.values [Ok 1; Error "two"; Ok 3; Error "four"] = Error "two"

val combine : ('ok, 'error) result list -> ('ok list, 'error) result

Result.combine results takes a list of Result values. If all the elements in results are of the form Ok x, then Result.combine creates a list xs of all the values extracted from their Oks, and returns Ok xs If any of the elements in results are of the form Error err, the first of them is returned as the result of Result.combine.

Result.combine [Ok 1; Ok 2; Ok 3; Ok 4] = Ok [1; 2; 3; 4]
Result.combine [Ok 1; Error "two"; Ok 3; Error "four"] = Error "two"

val map : ('a, 'error) t -> f:('a -> 'b) -> ('b, 'error) t

Transforms the 'ok in a result using f. Leaves the 'error untouched.

Examples

Result.map (Ok 3) ~f:(Int.add 1) = Ok 9

Result.map (Error "three") ~f:(Int.add 1) = Error "three"

val mapError : ('ok, 'a) t -> f:('a -> 'b) -> ('ok, 'b) t

Transforms the value in an Error using f. Leaves an Ok untouched.

Examples

Result.mapError (Ok 3) ~f:String.reverse = Ok 3

Result.mapError (Error "bad") ~f:(Int.add 1)  = Error "bad"

val map_error : ('ok, 'a) t -> f:('a -> 'b) -> ('ok, 'b) t

val andThen : ('a, 'error) t -> f:('a -> ('b, 'error) t) -> ('b, 'error) t

Run a function which may fail on a result.

Short-circuits of called with an Error.

Examples

  let reciprical (x:float) : (string, float) Standard.Result.t = (
    if (x = 0.0) then
      Error "Divide by zero"
    else
      Ok (1.0 /. x)
  )

  let root (x:float) : (string, float) Standard.Result.t = (
    if (x < 0.0) then
      Error "Cannot be negative"
    else
      Ok (Float.squareRoot x)
  )

Examples

Result.andThen ~f:reciprical (Ok 4.0) = Ok 0.25

Result.andThen ~f:reciprical (Error "Missing number!") = Error "Missing number!"

Result.andThen ~f:reciprical (Ok 0.0) = Error "Divide by zero"

Result.andThen (Ok 4.0) ~f:root  |> Result.andThen ~f:reciprical = Ok 0.5

Result.andThen (Ok -2.0) ~f:root |> Result.andThen ~f:reciprical = Error "Cannot be negative"

Result.andThen (Ok 0.0) ~f:root |> Result.andThen ~f:reciprical = Error "Divide by zero"

val and_then : ('a, 'error) t -> f:('a -> ('b, 'error) t) -> ('b, 'error) t

val tap : ('ok, _) t -> f:('ok -> unit) -> unit

Run a function against an (Ok value), ignores Errors.

Examples

  Result.tap (Ok "Dog") ~f:print_endline
  (* prints "Dog" *)

Convert

val toOption : ('ok, _) t -> 'ok option

Convert a Result to an Option.

An Ok x becomes Some x

An Error _ becomes None

Examples

Result.toOption (Ok 42) = Some 42

Result.toOption (Error "Missing number!") = None

val to_option : ('ok, _) t -> 'ok option

Compare

val equal : 
  ('ok -> 'ok -> bool) ->
  ('error -> 'error -> bool) ->
  ('ok, 'error) t ->
  ('ok, 'error) t ->
  bool

Test two results for equality using the provided functions.

Examples

Result.equal String.equal Int.equal (Ok 3) (Ok 3) = true

Result.equal String.equal Int.equal (Ok 3) (Ok 4) = false

Result.equal String.equal Int.equal (Error "Fail") (Error "Fail") = true

Result.equal String.equal Int.equal (Error "Expected error") (Error "Unexpected error") = false

Result.equal String.equal Int.equal (Error "Fail") (Ok 4) = false

val compare : 
  ('ok -> 'ok -> int) ->
  ('error -> 'error -> int) ->
  ('ok, 'error) t ->
  ('ok, 'error) t ->
  int

Compare results for using the provided functions.

In the case when one of the results is an Error and one is Ok, Errors are considered 'less' then Oks

Examples

Result.compare String.compare Int.compare (Ok 3) (Ok 3) = 0

Result.compare String.compare Int.compare (Ok 3) (Ok 4) = -1

Result.compare String.compare Int.compare (Error "Fail") (Error "Fail") = 0

Result.compare String.compare Int.compare (Error "Fail") (Ok 4) = -1

Result.compare String.compare Int.compare (Ok 4) (Error "Fail") = 1

Result.compare String.compare Int.compare (Error "Expected error") (Error "Unexpected error") = -1

Operators

In functions that make heavy use of Results operators can make code significantly more concise at the expense of placing a greater cognitive burden on future readers.

val (|?) : ('a, 'error) t -> 'a -> 'a

An operator version of Result.unwrap where the default value goes to the right of the operator.

Examples

The following eamples assume open Result.Infix is in scope.

Ok 4 |? 8 = 4

Error "Missing number!" |? 8 = 8

val (>>=) : ('ok, 'error) t -> ('ok -> ('b, 'error) t) -> ('b, 'error) t

An operator version of andThen

Examples

The following examples assume

  open Result.Infix

  let reciprical (x:float) : (string, float) Standard.Result.t =
    if (x = 0.0) then
      Error "Divide by zero"
    else
      Ok (1.0 /. x)

Is in scope.

Ok 4. >>= reciprical = Ok 0.25

Error "Missing number!" >>= reciprical = Error "Missing number!"

Ok 0. >>= reciprical = Error "Divide by zero"

val (>>|) : ('a, 'error) t -> ('a -> 'b) -> ('b, 'error) t

An operator version of map

Examples

The following examples assume open Result.Infix is in scope.

Ok 4 >>| Int.add(1) = Ok 5

Error "Its gone bad" >>| Int.add(1) = Error "Its gone bad"

val pp : 
  (Format.formatter -> 'ok -> unit) ->
  (Format.formatter -> 'error -> unit) ->
  Format.formatter ->
  ('ok, 'error) t ->
  unit

Result.pp errFormat okFormat destFormat result "pretty-prints" the result, using errFormat if the result is an Error value or okFormat if the result is an Ok value. destFormat is a formatter that tells where to send the output.

  let good: (int, string) Result.t = Ok 42 in
  let not_good: (int, string) Tablecloth.Result.t = Error "bad" in
  Result.pp Format.pp_print_int Format.pp_print_string Format.std_formatter good;
  Result.pp Format.pp_print_int Format.pp_print_string Format.std_formatter not_good;
  Format.pp_print_newline Format.std_formatter ();
  (* prints <ok: 42><error: bad>*)

Install

dune-project
Dependency

Authors

Maintainers

Sources

doc/tablecloth-native/Tablecloth/Result/index.html

Module `Tablecloth.Result`

Create

Convert

Compare

Operators

package tablecloth-native

Install

dune-project Dependency

Authors

Maintainers

Sources

doc/tablecloth-native/Tablecloth/Result/index.html

Module Tablecloth.Result

Create

Convert

Compare

Operators

dune-project
Dependency

Module `Tablecloth.Result`