Errors in Elm

In Elm, error handling is quite different from imperative languages. Elm uses a type system and pattern matching to handle errors, which eliminates runtime exceptions. Here’s how we can represent similar concepts:

import Html exposing (Html, div, text)
import Result exposing (Result)

-- In Elm, we use custom types to represent possible error states
type Error
    = CantWorkWith42
    | OutOfTea
    | NoPower

-- Functions that might fail return a Result type
f : Int -> Result Error Int
f arg =
    if arg == 42 then
        -- Instead of returning an error, we return Err with our custom type
        Err CantWorkWith42
    else
        -- Success case returns Ok with the value
        Ok (arg + 3)

-- Wrapping errors in Elm is done through custom types
type TeaError
    = TeaError Error
    | PowerError Error

makeTea : Int -> Result TeaError ()
makeTea arg =
    if arg == 2 then
        Err (TeaError OutOfTea)
    else if arg == 4 then
        -- We wrap the PowerError in our TeaError type
        Err (PowerError NoPower)
    else
        Ok ()

-- In Elm, we use case expressions for pattern matching
handleF : Int -> String
handleF i =
    case f i of
        Ok r ->
            "f worked: " ++ String.fromInt r

        Err CantWorkWith42 ->
            "f failed: can't work with 42"

handleTea : Int -> String
handleTea i =
    case makeTea i of
        Ok _ ->
            "Tea is ready!"

        Err (TeaError OutOfTea) ->
            "We should buy new tea!"

        Err (PowerError NoPower) ->
            "Now it is dark."

        Err _ ->
            "Unknown error"

-- Main function in Elm returns Html
main : Html msg
main =
    div []
        [ text (handleF 7)
        , text (handleF 42)
        , text (handleTea 0)
        , text (handleTea 1)
        , text (handleTea 2)
        , text (handleTea 3)
        , text (handleTea 4)
        ]

In Elm, we don’t have exceptions or nil values. Instead, we use the Result type to represent operations that might fail. The Result type has two constructors: Ok for success and Err for failure.

We define custom types (Error and TeaError) to represent different error states. This is similar to defining sentinel errors in other languages.

The f and makeTea functions return Result types. We use pattern matching with case expressions to handle these results, which is similar to checking for errors in other languages.

In Elm, there’s no direct equivalent to errors.Is. Instead, we use pattern matching to check for specific error types.

To run this Elm program, you would typically compile it to JavaScript and run it in a browser. The output would be displayed as HTML elements on the page, rather than printed to a console.

This Elm code demonstrates how to handle errors in a functional, strongly-typed language without exceptions or null values. It provides compile-time guarantees that all possible error states are handled.

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Errors in Elm