error — error interface + sentinels

github.com/go-composites/error (Go package Error, imported from the src/ sub-directory) defines the family's error contract and a couple of sentinel implementations. It is a leaf module with no intra-org dependencies.

The Error interface

import Error "github.com/go-composites/error/src"

type Interface interface {
    Message() string
    IsNull() bool
}

Error.New(message string) returns a concrete error whose Message() returns the supplied text and whose IsNull() returns false (it is a real error).

e := Error.New("boom")
e.Message() // "boom"
e.IsNull()  // false

Sentinels

Null error — src/null

import NullError "github.com/go-composites/error/src/null"

e := NullError.New()
e.Message() // "" (empty)
e.IsNull()  // true

The null error represents "no error". It is what result installs as the default error of a freshly constructed Result.

Method-not-implemented error — src/method_not_implemented

import MethodNotImplementedError "github.com/go-composites/error/src/method_not_implemented"

e := MethodNotImplementedError.New("Copy")
e.Message() // "The method Copy is not implemented"
e.IsNull()  // false

A formatted error for stubbed methods. New(name) builds the message "The method <name> is not implemented".

Notes

• IsNull() is the discriminator used elsewhere in the org to tell a real error apart from the "no error" sentinel. result builds on it: result.HasError() is defined as !error.IsNull(), so a result reports an error only when it actually carries a non-null one.