Spell incantation options "mixin"

To allow to compose, manipulate and read spell incantation options afte
the initial creation, two new types will be added for the spell
packages [1]:

- <I> `spell.Options` - A `interface` type as a generic representation
  for `spell.Incantation` options.
- <I> `spell.Mixin` - A `interface` type that allows to compose
  functions that process `spell.Options` of `spell.Incantation`s.
  - `Apply(Options) (Options, error)` - applies generic `spell.Options`
    to `spell.Incantation` options.

[1]: https://pkg.go.dev/github.com/svengreb/wand/pkg/spell

GH-25

pkg/spell/mixin.go

@@ -0,0 +1,13 @@
+// Copyright (c) 2019-present Sven Greb <[email protected]>
+// This source code is licensed under the MIT license found in the LICENSE file.
+
+package spell
+
+// Options is a generic representation for spell incantation options.
+type Options interface{}
+
+// Mixin allows to compose functions that process Options of spell incantations.
+type Mixin interface {
+	// Apply applies generic Options to spell incantation options.
+	Apply(Options) (Options, error)
+}

pkg/spell/spell.go

@@ -30,7 +30,7 @@ type Incantation interface {
 	// Kind returns the Kind of a spell.
 	Kind() Kind
 
-	// Options return the options of a spell.
+	// Options returns the options of a spell.
 	Options() interface{}
 }