Custom error type

We're using Haskell to write one of our services, and we're using Aeson for input-json parsing.

However, the API of the platform as a whole _must_ expose standardised validation errors like my example:

```javascript
{
"errors": [
{
"code": "ERR.SOMEFIELD.INVALID",
"field": "foo",
"type": "field",
"message": "Expecting an 'int' but got 'string'"
},
{
"code": "ERR.PLAN.INVALID_NAME",
"type": "field",
"field" : "user.plan",
"message": "All plan names must start with a 'urn:plan'" // nullable
}
],
"service": "SomeServiceIdentifier/version"
}
```
I see that in Aeson this is not possible, since the `Error` data constructor only uses `String` for its output errors. Also, my understanding is that accumulating JSON errors is achieved using `(<*>+)` which is quite cool.

What I'd like to understand is how we achieve some sort of a custom error type that can be represented like the example above, if possible at all. The naive approach would be to use `fail` with some sort of standardized string and then parse the custom error fields out of it, but that is rather unsatisfactory.

Is there a way to achieve this?

#

Edit: I'd like to contribute to this, but I'm quite new to Haskell.