Result

A Result is the result of a computation that may fail. This is a great way to manage errors in Gren.

Type and Constructors

type Result
= Ok value
| Err error

A Result is either Ok meaning the computation succeeded, or it is an Err meaning that there was some failure.

Mapping

map : (a -> value) -> Result x a -> Result x value

Apply a function to a result. If the result is Ok, it will be converted. If the result is an Err, the same error value will propagate through.

map sqrt (Ok 4.0) == Ok 2.0

map sqrt (Err "bad input") == Err "bad input"
map2 : (a -> b -> value) -> Result x a -> Result x b -> Result x value

Apply a function if both results are Ok. If not, the first Err will propagate through.

map2 max (Ok 42) (Ok 13) == Ok 42

map2 max (Err "x") (Ok 13) == Err "x"

map2 max (Ok 42) (Err "y") == Err "y"

map2 max (Err "x") (Err "y") == Err "x"

This can be useful if you have two computations that may fail, and you want to put them together quickly.

map3 : (a -> b -> c -> value) -> Result x a -> Result x b -> Result x c -> Result x value
map4 : (a -> b -> c -> d -> value) -> Result x a -> Result x b -> Result x c -> Result x d -> Result x value
map5 : (a -> b -> c -> d -> e -> value) -> Result x a -> Result x b -> Result x c -> Result x d -> Result x e -> Result x value

Chaining

andThen : (a -> Result x b) -> Result x a -> Result x b

Chain together a sequence of computations that may fail. It is helpful to see its definition:

andThen : (a -> Result e b) -> Result e a -> Result e b
andThen callback result =
    case result of
        Ok value ->
            callback value

        Err msg ->
            Err msg

This means we only continue with the callback if things are going well. For example, say you need to use (toInt : String -> Result String Int) to parse a month and make sure it is between 1 and 12:

toValidMonth : Int -> Result String Int
toValidMonth month =
    if month >= 1 && month <= 12 then
        Ok month

    else
        Err "months must be between 1 and 12"

toMonth : String -> Result String Int
toMonth rawString =
    toInt rawString
        |> andThen toValidMonth

-- toMonth "4" == Ok 4
-- toMonth "9" == Ok 9
-- toMonth "a" == Err "cannot parse to an Int"
-- toMonth "0" == Err "months must be between 1 and 12"

This allows us to come out of a chain of operations with quite a specific error message. It is often best to create a custom type that explicitly represents the exact ways your computation may fail. This way it is easy to handle in your code.

Handling Errors

withDefault : a -> Result x a -> a

If the result is Ok return the value, but if the result is an Err then return a given default value. The following examples try to parse integers.

Result.withDefault 0 (Ok 123) == 123

Result.withDefault 0 (Err "no") == 0
toMaybe : Result x a -> Maybe a

Convert to a simpler Maybe if the actual error message is not needed or you need to interact with some code that primarily uses maybes.

parseInt : String -> Result ParseError Int

maybeParseInt : String -> Maybe Int
maybeParseInt string =
    toMaybe (parseInt string)
fromMaybe : x -> Maybe a -> Result x a

Convert from a simple Maybe to interact with some code that primarily uses Results.

parseInt : String -> Maybe Int

resultParseInt : String -> Result String Int
resultParseInt string =
    fromMaybe ("error parsing string: " ++ toString string) (parseInt string)
mapError : (x -> y) -> Result x a -> Result y a

Transform an Err value. For example, say the errors we get have too much information:

parseInt : String -> Result ParseError Int

type alias ParseError =
    { message : String
    , code : Int
    , position : (Int,Int)
    }

mapError .message (parseInt "123") == Ok 123
mapError .message (parseInt "abc") == Err "char 'a' is not a number"