# Elm-common-tests library + test results across the ecosystem

**URL:** <https://discourse.elm-lang.org/t/elm-common-tests-library-test-results-across-the-ecosystem/8828>\
**Category:** Show and Tell\
**Created:** [December 17, 2022, 11:05pm UTC](https://discourse.elm-lang.org/t/elm-common-tests-library-test-results-across-the-ecosystem/8828 "2022-12-17T23:05:48Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Janiczek](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/janiczek/32/4516_2.png) [@Janiczek](https://discourse.elm-lang.org/u/Janiczek)\
**Post date:** [December 17, 2022, 11:05pm UTC](https://discourse.elm-lang.org/t/elm-common-tests-library-test-results-across-the-ecosystem/8828/1 "2022-12-17T23:05:49Z")

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Hello all,

I just want to announce a small library that provides tests for your data structure implementations.

Right now it’s just `Dict`, but I’d like to extend it to other data structures.

➡ [**`Janiczek/elm-common-tests`**](https://package.elm-lang.org/packages/Janiczek/elm-common-tests/latest/)

* * *

What `CommonTests.isDict` will check and how it works underneath is _comparison_ to `elm/core` Dict behaviour. **We run the same actions on your Dict implementation and on the core Dict, and then check they arrived at the same results.** (a.k.a. model based testing.)

The Dict API is conceptually separated into three categories:

1. **Creation** : `empty`, `singleton`, `fromList`
2. **Updates** : anything that takes a Dict and returns a Dict. `insert`, `map`, `union`, etc.
3. **Queries** : anything that returns a non-Dict. `size`, `member`, `get`, `foldl`, `keys`, `toList`, etc.

All of which are randomized and tested.

* * *

I’ve built on top of @miniBill’s work regarding categorizing the various Dict implementations, and went on to test them with this package. Here are the results:

| Library | Module | Behaves like elm/core Dict? |
| --- | --- | --- |
| edkelly303/elm-any-type-collections | Any.Dict | ✅ |
| edkv/elm-generic-dict | GenericDict | ✅ |
| matzko/elm-opaque-dict | OpaqueDict | ⚠ `update` is buggy \[1\] |
| owanturist/elm-avl-dict | AVL.Dict | ✅ |
| pzp1997/assoc-list | AssocList | ⚠ Uses insertion order \[2\] |
| timo-weike/generic-collections | AutoDict | ✅ |
| timo-weike/generic-collections | ManualDict | ✅ |
| turboMaCk/any-dict | Dict.Any | ✅ |

* * *

**Future work**

The test results above seem to say, we’re doing pretty good! 🙂

But again, these test results check conformance to `elm/core` Dict behaviour. We should check _it_ as well! Some [unit tests](https://github.com/elm/core/blob/master/tests/tests/Test/Dict.elm) do exist already, but we could write some property based tests to perhaps help uncover some more bugs. (Indeed there are [some](https://github.com/elm/core/issues/1013)\[3\].)

If anybody feels compelled to work on that Dict test suite with me (might be a nice intro to property based tests), send me a message somewhere! 🙂 We can brainstorm the properties together and then implement them.\[4\]

* * *

1. More specifically, the paths `Just -> Nothing` and `Nothing -> Just` don’t remove and insert elements respectively. Tracked in a [GitHub issue](https://github.com/matzko/elm-opaque-dict/issues/2). 

2. This is not necessarily a bug, but it’s a _difference_ from elm/core Dict. It might be unexpected behaviour for those depending on Dicts being sorted by keys. 

3. Not that this one in particular could be caught by an unit/property-based test suite: it only shows up with `--optimize`! 

4. I can’t promise PRs to `elm/core` being merged, of course 🙂

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<div class="post-metadata">

**Author:** ![lue-bird](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/lue-bird/32/4678_2.png) [@lue-bird](https://discourse.elm-lang.org/u/lue-bird)\
**Post date:** [December 17, 2022, 11:49pm UTC](https://discourse.elm-lang.org/t/elm-common-tests-library-test-results-across-the-ecosystem/8828/2 "2022-12-17T23:49:22Z")

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List of dicts looks pretty complete. A few other ones I know of (that might not fit the fuzzing format)

- [`escherlies/elm-ix-dict`: `IxDict`](https://package.elm-lang.org/packages/escherlies/elm-ix-dict/1.0.0/IxDict) → key derived from element
- [`joneshf/elm-tagged`: `Tagged.Dict`](https://dark.elm.dmy.fr/packages/joneshf/elm-tagged/latest/Tagged-Dict) → simple wrapper
- [`jjant/elm-dict`: AllDict](https://dark.elm.dmy.fr/packages/jjant/elm-dict/latest/AllDict) → basically just constructs one big function
- [`lue-bird/elm-keysset`: `KeySet` (and `KeysSet`)](https://dark.elm.dmy.fr/packages/lue-bird/elm-keysset/latest/) → key derived from element, no stored function

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<div class="post-metadata">

**Author:** ![Janiczek](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/janiczek/32/4516_2.png) [@Janiczek](https://discourse.elm-lang.org/u/Janiczek)\
**Post date:** [December 17, 2022, 11:57pm UTC](https://discourse.elm-lang.org/t/elm-common-tests-library-test-results-across-the-ecosystem/8828/3 "2022-12-17T23:57:01Z")

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Thank you! I have tried to implement the tests for some of those, but the tests do make some assumptions like being able to hardcode `String` keys and `Int` values, and having a `toList`. I believe I could get around both of these though 🤔

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**Author:** ![rupert](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/rupert/32/1775_2.png) [@rupert](https://discourse.elm-lang.org/u/rupert)\
**Post date:** [December 18, 2022, 11:27am UTC](https://discourse.elm-lang.org/t/elm-common-tests-library-test-results-across-the-ecosystem/8828/4 "2022-12-18T11:27:27Z")

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Interesting. I did something similar when writing tests for my Trie implementation. You can find my version of the Dict interface here:

> <https://github.com/elm-scotland/elm-tries/blob/1.0.1/tests/DictIface.elm#L29>

I had a few more type variables that you, since I also generalized the comparable key type, and needed a load more to generalize operations over multiple Dicts (`diff` and `merge`) with the same key and datum types.

I also added another type variable `x` to make the Dict interface extensible, since I was taking the view that a Trie is a Dict extended with some other operations. [https://github.com/elm-scotland/elm-tries/blob/1.0.1/tests/TrieIface.elm#L14](https://github.com/elm-scotland/elm-tries/blob/1.0.1/tests/TrieIface.elm#L14)

Its a fairly good way to proceed for testing I think, a bit overkill for applications. However with @jhbrown’s type hiding with continuations idea, its possible sometimes to drop many of the type variables. Operations over multiple data structures might not work with that idea though, unless they go through the interface rather than direct to the implementation. In the case of `Dict` that might be ok, since `diff` and `merge` can be implemented in terms of `foldl` and `remove`, but of course not all Dict implementation might want to do it that way.

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<div class="post-metadata">

**Author:** ![system](https://canada1.discourse-cdn.com/flex035/uploads/elm_lang/original/1X/50a05e53677a2c3b47776d7abd0f113eb50193a1.png) [@system](https://discourse.elm-lang.org/u/system)\
**Post date:** [December 28, 2022, 11:27am UTC](https://discourse.elm-lang.org/t/elm-common-tests-library-test-results-across-the-ecosystem/8828/5 "2022-12-28T11:27:46Z")

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