---
package: prelude
version: "0.4.0-beta"
source: errorable.kex
title: Errorable
entities:
  - { kind: trait, name: "Errorable" }
  - { kind: record, name: "ParseError" }
---

# Errorable

## trait `Errorable`

`Errorable`: the trait for values that describe a failure.

Implemented by error types that carry a human-readable message, so a generic handler can display or log any error without knowing its concrete type or its structured fields.

```kex
foul report(e: Errorable) -> Void do
  IO.printError("error: ${e.message}")
end
```

### Required methods

#### `message`

```kex
message : String
```

A human-readable description of what went wrong.

Written for a person reading output, not for a program to match on: branch on the error's own type or fields for that.

**Returns**: the failure message

**Examples**

```kex
IO.printError("error: ${e.message}")
```



## record `ParseError`

A parse failure, with everything needed to report it or to carry on from it.

Answered by `Integer.parse`, `Float.parse` and `Number.parse` when the input is not what they expected. Reach for those over `to(Integer)` exactly when this detail matters: `to` answers a plain `None`.

```kex
Integer.parse("12x")
# => Error(ParseError { input: "12x", position: 2, value: 12,
#                       message: "trailing characters after integer",
#                       rest: "x" })
```

The `position` and `rest` are what make a hand-written scanner possible: the error says exactly where it stopped and what was left.

**Fields**

  - `input` : [String](string.md#make-string)
  - `position` : [Integer](number.md#make-integer)
  - `value` : Any
  - `message` : [String](string.md#make-string)
  - `rest` : [String](string.md#make-string)