Custom Errors in Elm

module Main exposing (main)

import Html exposing (Html, text)


type alias ArgError =
    { arg : Int
    , message : String
    }


type Error
    = CustomError ArgError
    | OtherError String


f : Int -> Result Error Int
f arg =
    if arg == 42 then
        Err (CustomError { arg = arg, message = "can't work with it" })

    else
        Ok (arg + 3)


main : Html msg
main =
    case f 42 of
        Err (CustomError ae) ->
            text <| String.fromInt ae.arg ++ " - " ++ ae.message

        Err (OtherError msg) ->
            text msg

        Ok _ ->
            text "No error occurred"

In Elm, we don’t have the concept of “custom errors” in the same way as Go, but we can achieve similar functionality using the Result type and custom types for errors.

We define a custom type ArgError to represent our specific error case:

type alias ArgError =
    { arg : Int
    , message : String
    }

We then create a union type Error that can represent either our custom ArgError or other types of errors:

type Error
    = CustomError ArgError
    | OtherError String

The f function returns a Result Error Int, which is Elm’s way of handling errors:

f : Int -> Result Error Int
f arg =
    if arg == 42 then
        Err (CustomError { arg = arg, message = "can't work with it" })
    else
        Ok (arg + 3)

In the main function, we use pattern matching to handle the different possible outcomes:

main : Html msg
main =
    case f 42 of
        Err (CustomError ae) ->
            text <| String.fromInt ae.arg ++ " - " ++ ae.message
        Err (OtherError msg) ->
            text msg
        Ok _ ->
            text "No error occurred"

This approach allows us to handle our custom error type specifically, while still accommodating other potential error types.

To run this Elm program, you would typically compile it to JavaScript and run it in a browser. The output would be:

42 - can't work with it

This example demonstrates how to implement custom error handling in Elm, which provides a type-safe way to deal with different error scenarios.

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Custom Errors in Elm