Nested Elm JSON decoder failures report missing fields without full path context

The core Json.Decode module in Elm 0.19 does not automatically accumulate a full breadcrumb path in its error messages. When a decoder fails inside a nested andThen or map combinator, the resulting error reports the specific failure (e.g., "Missing key") but does not prepend the keys of the parent objects traversed to reach that point.

Diagnostic Behavior and Limitations

In the standard library, error reporting is designed for simplicity. The diagnostic detail remains consistent across different navigation methods, but the context is localized:

• Json.Decode.field and andThen: If a nested field is missing, the error returned is simply the failure of the innermost decoder. The parent context is discarded.
• Json.Decode.at: While this allows navigating to a specific path, a failure at the destination still reports the local error. It does not automatically generate a string representing the full path traversed.
• Json.Decode.index: Similar to field extraction, index failures report the missing index but not the full object path leading to that array.

Recommended Approach for Path Specificity

To maintain Elm's no-runtime-exception guarantees while improving error specificity, you must manually wrap decoders to append context. Since the core library does not provide a "path-aware" decoder, the standard pattern is to create a helper function that catches a failure and modifies the error message.

Implementation Strategy:

1. Create a wrapper function that takes a label (the field name) and a decoder.
2. Since Json.Decode.Decoder is an opaque type, you cannot directly modify the error string inside the decoder chain without using Json.Decode.oneOf or custom result handling after the decoding process has finished.
3. The most common approach is to use a custom Result-based wrapper that maps over the Error string of the final result.

Practical Verification

To verify this behavior in your current environment, run the following test case:

-- Test JSON: { "user": { "profile": { "name": "Alice" } } }
-- Decoder: field "user" (field "profile" (field "name" string))
-- Action: Remove "name" from the JSON and check the Debug.log output.

You will observe that the output is "Missing key: name" rather than "user.profile.name: Missing key".

Diagnostic Detail Needed: Are you using any third-party decoding libraries, or are you strictly limited to the elm/core implementation?