A Result
is the result of a computation that may fail. This is a great
way to manage errors in Elm.
A Result
is either Ok
meaning the computation succeeded, or it is an
Err
meaning that there was some failure.
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
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.
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"