# Error [![CircleCI](https://circleci.com/gh/well-ironed/error.svg?style=svg)](https://circleci.com/gh/well-ironed/error)
Errors modeled as data.
## Installation
```elixir
def deps do
[
{:error, "~> 0.1.0"}
]
end
```
## Creation
Errors come in two kinds:
1. `:domain` errors, which are part of your business model, or *Domain* in DDD
parlance. You can use the map in the second argument position to supply
whatever extra details you find useful:
```
Error.domain(:invalid_username, %{
provided_username: "alex",
unmet_requirement: "Must start with capital letter"}
})
```
2. `:infra` errors, which represent failures of the infrastructure or
computation substrate. Similarly, you can supply a map as the second
argument, with arbitrary details of your choosing.
```
Error.infra(:db_down, %{retried_count: 5})
```
## Usage
Errors should be created at the place where they occur. Domain errors can be
later converted to user-facing messages, in the presentation logic.
Infra errors, on the other hand, most often translate to a "500 error" and the
only piece of information that an end-user needs to know is that "Something
went wrong". At the same time, you want to log these errors extensively so
that you can investigate the issue later.
Errors provide a convenient `to_map` function, so that your view logic doesn't
need to know about the `Error` type.
## Pattern-matching
If you'd like to pattern-match on errors, you will find that Dialyzer rejects
your attempts, complaining that the Error.InfraError and Error.DomainError
structs are opaque. The way to get around this without breaking opacity is by
importing the guard `is_error`, or the specific guards `is_infra_error` and
`is_domain_error`.
```
# DIALYZER WILL REJECT THIS
case something() do
{:ok, %MyItem{} = i} -> handle(i)
{:error, %DomainError{} = e} -> Error.reason(e)
{:error, :some_atom} -> other_path()
end
# DIALYZER WILL ACCEPT THIS
import Error, only: [is_error: 1]
case something() do
{:ok, %MyItem{} = i} -> handle(i)
{:error, e} when is_error(e) -> Error.reason(e)
{:error, :some_atom} -> other_path()
end
```
## Source
Hosted on [github](https://github.com/well-ironed/error).