Runtime error when comparing nested Html of different depths

[lpil]

Hello!

Today while trying to do some testing I encounted a runtime error. Here's a recreation.

> import Html exposing (..)
> (div [] []) == (div [] [])
-- True : Bool

> (div [] [ text "Hi" ]) == (div [] [ text "Hello" ])
-- False : Bool

> (div [] [ text "Hi" ]) == (div [] [])
-- TypeError: Cannot read property 'type' of undefined

Elm version 0.17.0, installed via the elm package on npm.

Debian Jessie Linux OS.

Running Elm in node v4.1.1 via the REPL.

Here are the entries from my elm-stuff/exact-dependencies.json.

    "elm-lang/html": "1.1.0",
    "elm-lang/core": "4.0.4"

I was pointed over here from elm/compiler#1465

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[evancz]

Equality is not defined on virtual DOM values, so the fix is going to be modifying the compiler to detect when folks try to use equality in invalid cases (like on functions) and making that an error at compile time. This is already planned, so I'd rather not track the implications of that change here.

[evancz]

Equality is not defined on virtual DOM values, so the fix is going to be modifying the compiler to detect when folks try to use equality in invalid cases (like on functions) and making that an error at compile time. This is already planned, so I'd rather not track the implications of that change here.

[jvoigtlaender]

But maybe at least mentioning this in the core documentation would be good? Currently, http://package.elm-lang.org/packages/elm-lang/core/4.0.5/Basics#== carries this note:

Note: Equality (in the Elm sense) is not possible for certain types. For example, the functions (\n -> n + 1) and (\n -> 1 + n) are “the same” but detecting this in general is undecidable. In a future release, the compiler will detect when (==) is used with problematic types and provide a helpful error message. This will require quite serious infrastructure work that makes sense to batch with another big project, so the stopgap is to crash as quickly as possible. Problematic types include functions and JavaScript values like Json.Encode.Value which could contain functions if passed through a port.

That html values are also of "problematic type" seems important information for users, since trying to test views by comparing html values via == is something that is quite tempting. Currently neither the documentation for core nor for the html package tell users that using == on html values is not okay.

[jvoigtlaender]

But maybe at least mentioning this in the core documentation would be good? Currently, http://package.elm-lang.org/packages/elm-lang/core/4.0.5/Basics#== carries this note:

Note: Equality (in the Elm sense) is not possible for certain types. For example, the functions (\n -> n + 1) and (\n -> 1 + n) are “the same” but detecting this in general is undecidable. In a future release, the compiler will detect when (==) is used with problematic types and provide a helpful error message. This will require quite serious infrastructure work that makes sense to batch with another big project, so the stopgap is to crash as quickly as possible. Problematic types include functions and JavaScript values like Json.Encode.Value which could contain functions if passed through a port.

That html values are also of "problematic type" seems important information for users, since trying to test views by comparing html values via == is something that is quite tempting. Currently neither the documentation for core nor for the html package tell users that using == on html values is not okay.

[lpil]

A compiler error and improved documentation would be wonderful, thanks all.

Given we cannot test equality on html is there another method for testing views? Often I have business logic that results in html that I would love to be able to test.

[lpil]

A compiler error and improved documentation would be wonderful, thanks all.

Given we cannot test equality on html is there another method for testing views? Often I have business logic that results in html that I would love to be able to test.