Working With Errors

The Error Type

When using Decode.AsResult.OfParseError, each decoder can fail with a Result Error that holds a variant value representing the structured decode error.

The underlying/primitive decode errors look like:

type failure = [
  | `ExpectedBoolean
  | `ExpectedString
  | `ExpectedNumber
  | `ExpectedInt
  | `ExpectedArray
  | `ExpectedTuple(int) // expected size of
  | `ExpectedObject
  | `ExpectedValidDate
  | `ExpectedValidOption
]

The full error structure (where the base Val errors hold the failure type above) looks like:

type t =
  | Val(failure, Js.Json.t)
  | TriedMultiple(NonEmptyList.t(t))
  | Arr(NonEmptyList.t((int, t)))
  | Obj(NonEmptyList.t((string, objError)))
and objError =
  | MissingField
  | InvalidField(t);

Ultimately, this is saying that a ParseError will be either a Val error, a TriedMultiple error, an Arr error, or an Obj error.

• Val errors contain one of the primitive errors defined above and the JSON that it failed to decode
• TriedMultiple happens when multiple decoders were attempted (e.g. using alt) and it contains a non-empty list of other errors
• Arr errors hold a non-empty list of position/error pairs
• Obj errors hold a non-empty list of fieldname/object-error pairs
  • Object errors happen either because the field is a MissingField, or
  • the field exists but its value couldn't be decoded (InvalidField which recursively contains any of the above errors)

This error structure can be destructured with recursive pattern matching.

Logging

One of the most common reasons to pattern match on the error structure is to log what went wrong while decoding. This is a common enough use case that helpers are provided to do just that:

switch (decode(json)) {
| Ok(v) => ... // do something with your successful value
| Error(err) =>
  Js.log(Decode.ParseError.failureToDebugString(err))
};

The actual output of failureToDebugString will depend on the decoder and the JSON value you pass to it, but the string output of that function could look something like:

Failed to decode array:
    At position 3: Failed to decode object:
        Field "foo" is required but was not present
        Field "bar" had an invalid value: Failed to decode array:
            At position 0: Expected int but found []