WikiSpec

Let's go through the first test case in WikiSpec.elm commit step by step:

Test Description: The test is named "Decode JSON into Page" and verifies the decoding of JSON into a Page object.

Test Function: The test function takes no arguments (\() ->). This is a placeholder for any test data that might be passed to the test case.

Variables: Inside the test function, we define three variables:

JSON String: The jsonString variable is assigned the value of rawData, which contains the JSON representation of a Page object.

Expected Page: The expectedPage variable is initialized with a Page object that matches the structure of the decoded JSON. In this case, it has an empty story list and an empty journal list.

Decoding: The decoded variable is assigned the result of decoding the jsonString using Decode.decodeString and the pageDecoder decoder.

Assertion: The Expect.equal function is used to compare the decoded result with the expected Page object (Ok expectedPage). If the decoded result matches the expected value, the test passes; otherwise, it fails.

By running this test case, we can verify that the JSON decoding produces the expected Page object.

Note: As long as the Create New Page Test page is not created, we call it a Ghost Page, a page that has been created for the browser but not yet saved anywhere. As long as the page has not been saved, the ID changes every time the page is accessed.

In the spirit of TDD, we need to update the failing (RED) test case first before modifying the Wiki module. commit

Based on the updated JSON response, we can modify the test case to expect the new Page structure with the error message in the story field. Here's the updated test case code:

We perform our tests.

gives us:

To resolve the naming error, we need to import the Story type from the Wiki module in our test file. Here's the updated code for WikiSpec.elm:

elm-test gives us:

Exposing Page(..) instead of Page and Story has the advantage of making all the constructors of the Page type available outside the Wiki module. This means that code in other modules can directly use the constructors without explicitly importing them.

The (..) syntax is for exposing variants of a custom type. It cannot be used with a type alias like `Page` though.

What would be the consequences of representing Page as a custom type with the (..) syntax for the implementation of our JSON decoder/encoders?

Representing Page as a custom type with the (..) syntax would impact the implementation of JSON decoders and encoders for Page in the following ways: (chat.openai)

1. Decoder: When decoding JSON into a custom type with (..) syntax, you need to provide a decoder for each variant explicitly. In the case of Page, if you have different variants like NotFoundPage, ContentPage, ErrorPage, etc., you would need to define separate decoders for each variant. This allows you to handle different JSON structures corresponding to each variant of Page.

2. Encoder: Similarly, when encoding a custom type with (..) syntax, you would need to provide an encoder for each variant. This involves converting the data in each variant to the appropriate JSON representation.

3. Matching JSON structures: The JSON structure you receive from an API or other data source needs to align with the defined variants of the custom type. Each variant of Page should have a corresponding JSON structure that can be decoded into that variant. If the JSON structure doesn't match any of the defined variants, decoding would fail.

4. Handling unknown JSON structures: If you want to handle unknown or unexpected JSON structures, you would need to include a catch-all variant in your Page type to handle those cases. This variant would be used when the JSON structure doesn't match any of the explicitly defined variants.

Overall, representing Page as a custom type with (..) syntax for JSON decoding and encoding provides more fine-grained control over the mapping between JSON and Elm types. It allows you to handle different JSON structures and encode/decode them into specific variants of Page. However, it also increases the complexity of the implementation as you need to define decoders and encoders for each variant and ensure the JSON structures align with the defined variants. (chat.openai)

Show me a catch-all variant in our Page type.

Certainly! Here's an example of how you can include a catch-all variant in the Page type to handle unknown or unexpected JSON structures:

In this example, the Page type has multiple variants: ContentPage, NotFoundPage, ErrorPage, and UnknownPage. The UnknownPage variant is the catch-all variant that is used when the JSON structure doesn't match any of the explicitly defined variants.

The pageDecoder function uses Decode.oneOf to try multiple decoders in sequence. If the JSON structure matches the expected format for a ContentPage, it is decoded using contentDecoder and wrapped in the ContentPage variant. If the structure matches the NotFoundPage or ErrorPage format, the corresponding variants are used. Finally, if the structure doesn't match any of the expected formats, it is decoded into a Decode.Value and wrapped in the UnknownPage variant.

By including the UnknownPage variant, you can handle unknown JSON structures and still have a representation for them within the Page type. (chat.openai)

I am not sure if this is a good idea for our Page type. But maybe we can apply this idea to our Journal type? Note: The "journal" collects story edits. Because type future is a variant of story edits, type create would be next, type edit next, type move next and so on and so forth. Since this list is not finite: would such a catch all variant be helpful for our journal type? What would our code look like then?

If you want to handle unknown story edit types in the Journal type, you can indeed include a catch-all variant. Here's an example of how you can modify the Journal type and the decoder to accommodate this:

In this example, the Journal type includes variants for specific story edit types such as CreateJournal, EditJournal, and MoveJournal. The UnknownJournal variant is used as a catch-all variant to handle unknown story edit types.

The journalDecoder function uses Decode.oneOf to try multiple decoders in sequence. If the JSON structure matches the expected format for a specific story edit type, it is decoded into the corresponding variant. If the structure doesn't match any of the expected formats, it is decoded into a Decode.Value and wrapped in the UnknownJournal variant.

See also Understanding Machines and Model Free Method Zoo:

By including the UnknownJournal variant, you can handle unknown story edit types and still have a representation for them within the Journal type. (chat.openai)

Sounds promising. Refactor type Journal with Create, Edit, Move etc.

What does Unknown Decode.Value mean? Explain.

In the refactored Journal type, the Unknown variant represents a story edit type that is not known or recognized. It serves as a catch-all variant to handle unexpected or unrecognized story edit types encountered during decoding.

The Decode.Value type in Elm's Json.Decode module represents a JSON value that has been decoded but its specific type is not known. It can hold any valid JSON data structure such as objects, arrays, strings, numbers, booleans, or null. By using Decode.value as the decoder for the Unknown variant, we can capture the decoded JSON value as a Decode.Value type when the story edit type is not recognized.

The Decode.value function is a decoder that decodes any JSON value without providing any specific type information. It essentially treats the decoded JSON value as an opaque value without any additional decoding or interpretation. This allows you to handle unexpected or unknown story edit types gracefully by capturing the decoded JSON value and handling it appropriately in your code.

By using the Unknown variant with Decode.value, you can handle cases where the story edit type is not known or defined, providing flexibility and extensibility to handle future additions or changes to the story edit types without causing decoding errors or breaking the code. (chat.openai)

⇒ WikiSpec Journal ⇒ WikiSpec Story ⇒ WikiSpec: test: GREEN commit (pageDecoder) ⇒ WikiSpec Encoders