github.com/reiver/go-opt -> sourcecode.social/reiver/go-opt

LICENSE

@@ -1,4 +1,4 @@
-Copyright (c) 2017 Charles Iliya Krempeaux <charles@reptile.ca> :: http://changelog.ca/
+Copyright (c) 2017 Charles Iliya Krempeaux :: http://changelog.ca/
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.md

@@ -1,14 +1,56 @@
 # go-opt
 
-Package **opt** provides alternatives to Go's built-in types (bool, float64, int64, string, time.Time)
-that also let you express a "lack of a value"; for the Go programming language;
-these are similar to "option types" or "maybe types" in some other programming languages.
-
-(Code for this was generated using https://github.com/reiver/gogen-optiontype )
+Package **opt** implements an **optional-type**, for the Go programming language.
 
+In other programming languages, an **optional-type** might be know as: a **option-type**, or a **maybe-type**.
 
 ## Documention
 
-Online documentation, which includes examples, can be found at: http://godoc.org/github.com/reiver/go-opt
+Online documentation, which includes examples, can be found at: http://godoc.org/sourcecode.social/reiver/go-opt
 
-[![GoDoc](https://godoc.org/github.com/reiver/go-opt?status.svg)](https://godoc.org/github.com/reiver/go-opt)
+[![GoDoc](https://godoc.org/sourcecode.social/reiver/go-opt?status.svg)](https://godoc.org/sourcecode.social/reiver/go-opt)
+
+## Examples
+
+Here is an example **optional-type** that can hold a string:
+```go
+import "sourcecode.social/reiver/go-opt"
+
+//
+
+var op opt.Optional[string] // the default value is nothing.
+
+// ...
+
+if opt.Nothing[string]() == op {
+	fmt.Println("nothing")
+}
+
+// ...
+
+op = opt.Something("Hello world! 👾")
+
+// ...
+
+switch op {
+case op.Nothing[string]():
+	//@TODO
+case op.Something("apple"):
+	//@TODO
+case op.Something("banana"):
+	//@TODO
+case op.Something("cherry"):
+	//@TODO
+default:
+	//@TODO
+}
+
+// ...
+
+value, found := op.Get()
+if found {
+	fmt.Println("VALUE:", value)
+} else {
+	fmt.Println("nothing")
+}
+```

doc.go

@@ -1,43 +0,0 @@
-/*
-Package opt has a number of sub-packages provides alternatives to Go's built-in types (bool, float64, int64, string, time.Time)
-that also let you express a "lack of a value"; for the Go programming language; these are similar to "option types" or "maybe types"
-in some other programming languages.
-
-Example:
-
-	var s optstring.NullableType
-	
-	// ...
-	
-	switch s {
-	case optstring.NoneNullable():
-		//@TODO
-	case optstring.Null():
-		//@TODO
-	case optstring.SomeNullable("Hello world!"):
-		//@TODO
-	case optstring.SomeNullable("apple banana cherry"):
-		//@TODO
-	default:
-		//@TODO
-	}
-
-Example:
-
-	var s optstring.Type
-	
-	// ...
-	
-	switch s {
-	case optstring.None():
-		//@TODO
-	case optstring.Some("Hello world!"):
-		//@TODO
-	case optstring.Some("apple banana cherry"):
-		//@TODO
-	default:
-		//@TODO
-	}
-*/
-package opt
-

go.mod

@@ -0,0 +1,3 @@
+module sourcecode.social/reiver/go-opt
+
+go 1.18

map.go

@@ -0,0 +1,47 @@
+package opt
+
+// Map applies the function ‘fn’ to the value inside of the optional-type ‘op’, if the optional-type ‘op’ is holding something, and returns it as a new optional-type.
+// If the optional-type ‘op’ is holding nothing, then Map also returns nothing.
+//
+// For example:
+//
+//	var op opt.Optional[string] = opt.Something("Hello world!")
+//	
+//	var result opt.Optional[string] = opt.Map(op, strings.ToUpper)
+//	
+//	// result == opt.Something[string]("HELLO WORLD!")
+//	
+//	// ...
+//	
+//	var op2 opt.Optional[string] = opt.Nothing[string]()
+//	
+//	var result2 opt.Optional[string] = opt.Map(op, strings.ToUpper)
+//	
+//	// result2 == opt.Nothing[string]()
+//
+// Or also, for example:
+//
+//	fn := func(s string) int {
+//		return len(s)
+//	}
+//	
+//	var op opt.Optional[string] = opt.Something("Hello world!")
+//	
+//	var result opt.Optional[int] = opt.Map(op, fn)
+//	
+//	// result == opt.Something[int](12)
+//	
+//	// ...
+//	
+//	var op2 opt.Optional[string] = opt.Nothing[string]()
+//	
+//	var result2 opt.Optional[int] = opt.Map(op, fn)
+//	
+//	// result2 == opt.Nothing[int]()
+func Map[T1 any, T2 any](op Optional[T1], fn func(T1)T2) Optional[T2] {
+	if !op.something {
+		return Nothing[T2]()
+	}
+
+	return Something(fn(op.value))
+}

map_test.go

@@ -0,0 +1,69 @@
+package opt_test
+
+import (
+	"sourcecode.social/reiver/go-opt"
+
+	"testing"
+)
+
+func TestMap_stringToUint(t *testing.T) {
+
+	tests := []struct{
+		Optional opt.Optional[string]
+		Expected opt.Optional[int]
+	}{
+		{
+			Optional: opt.Nothing[string](),
+			Expected: opt.Nothing[int](),
+		},
+
+
+
+		{
+			Optional: opt.Something(""),
+			Expected: opt.Something[int](0),
+		},
+
+
+
+		{
+			Optional: opt.Something("once"),
+			Expected: opt.Something[int](4),
+		},
+		{
+			Optional: opt.Something("twice"),
+			Expected: opt.Something[int](5),
+		},
+		{
+			Optional: opt.Something("thrice"),
+			Expected: opt.Something[int](6),
+		},
+		{
+			Optional: opt.Something("fource"),
+			Expected: opt.Something[int](6),
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		fn := func(s string) int {
+			return len(s)
+		}
+
+		mapped := opt.Map(test.Optional, fn)
+
+		{
+			expected := test.Expected
+			actual   := mapped
+
+			if expected != actual  {
+				t.Errorf("For test #%d, the actual value is not what was expected.", testNumber)
+				t.Logf("EXPECTED: (%T) %#v", expected, expected)
+				t.Logf("ACTUAL:   (%T) %#v", actual, actual)
+				t.Logf("OPTIONAL: (%T), %#v", test.Optional, test.Optional)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+	}
+}

optional.go

@@ -0,0 +1,209 @@
+package opt
+
+import (
+	"fmt"
+)
+
+// Optional represents an optional value.
+//
+// In other programming languages this is know as: an option type, or a maybe type.
+//
+// For example:
+//
+//	var op opt.Optional[string] = opt.Something("once twice thrice fource")
+//	
+//	// ...
+//	
+//	value, found := op.Get()
+//	if found {
+//		fmt.Println("value:", value)
+//	} else{
+//		fmt.Println("nothing")
+//	}
+//
+// Also, for example:
+//
+//	var op opt.Optional[uint8] = opt.Something[uint8](101)
+//	
+//	// ...
+//	
+//	value, found := op.Get()
+//	if found {
+//		fmt.Println("value:", value)
+//	} else{
+//		fmt.Println("nothing")
+//	}
+type Optional[T any] struct {
+	value T
+	something bool
+}
+
+// Nothing returns an optional-type with nothing in it.
+//
+// For example, here is an optional-type that can hold a string with nothing in it:
+//
+//	var op opt.Optional[string] = opt.Nothing[string]()
+//
+// Note that the default value for any optional-type is nothing.
+// So the following code it equivalent to it:
+//
+//	var op opt.Optional[string]
+//
+// Also, for example, here is an optional-type that can hold a uint8 with nothing in it:
+//
+//	var op opt.Optional[uint8] = opt.Nothing[uint8]()
+//
+// Again, note that the default value for any optional-type is nothing.
+// So the following code it equivalent to it:
+//
+//	var op opt.Optional[uint8]
+func Nothing[T any]() Optional[T] {
+	var nothing Optional[T]
+
+	return nothing
+}
+
+// Something returns a optional-type with something in it.
+//
+// For example, here is an optional-type with the string "once twice thrice fource" in it:
+//
+//	var op opt.Optional[string] = opt.Something("once twice thrice fource")
+//
+// And, for example, here is an optional-type with the uint8 101 in it:
+//
+//
+//	var op opt.Optional[uint8] = opt.Something(uint8(101))
+func Something[T any](value T) Optional[T] {
+	return Optional[T]{
+		something:true,
+		value:value,
+	}
+}
+
+// Filter returns itself if it is holding something and ‘fn’ applied to its value returns true.
+// Else it return nothing.
+//
+// For example:
+//
+//	fn := func(value int) bool {
+//		return 0 == (value % 2)
+//	}
+//	
+//	// ...
+//	
+//	var op1 opt.Optional[int] = opt.Something[int](10)
+//	
+//	op1 = op1.Filter(fn)
+//	
+//	// ...
+//	
+//	var op2 opt.Optional[int] = opt.Something[int](11)
+//	
+//	op2 = op2.Filter(fn)
+//	
+//	// ...
+//	
+//	var op3 opt.Optional[int] = opt.Nothing[int]()
+//	
+//	op3 = op3.Filter(fn)
+func (receiver Optional[T]) Filter(fn func(T)bool) Optional[T] {
+	if !receiver.something {
+		return Nothing[T]()
+	}
+
+	if !fn(receiver.value) {
+		return Nothing[T]()
+	}
+
+	return receiver
+}
+
+// Get returns the value inside of the optional-type if it is holding something.
+//
+// Example usage:
+//
+//	var op opt.Optional[string]
+//	
+//	// ...
+//	
+//	value, found := op.Get()
+//	
+//	if found {
+//		fmt.Println("VALUE:", value)
+//	} else {
+//		fmt.Println("nothing")
+//	}
+func (receiver Optional[T]) Get() (T, bool) {
+	return receiver.value, receiver.something
+}
+
+// GoString makes it that when the fmt.Fprintf(), fmt.Printf(), and fmt.Sprintf() family of functions
+// renders this type with the %#v verb, that it will be easier to understand.
+//
+// For example:
+//
+//	var op opt.Optional[string] = opt.Something("once twice thrice fource")
+//	
+//	// ...
+//	
+//	fmt.Printf("op = %#v", op)
+//
+//	// Output:
+//	// op = opt.Something[string]("once twice thrice fource")
+//
+// Also, for example:
+//
+//	var op opt.Optional[uint8] = opt.Nothing[uint8]()
+//	
+//	// ...
+//	
+//	fmt.Printf("op = %#v", op)
+//
+//	// Output:
+//	// op = opt.Nothing[uint8]()
+func (receiver Optional[T]) GoString() string {
+	if !receiver.something {
+		return fmt.Sprintf("opt.Nothing[%T]()", receiver.value)
+	}
+
+	return fmt.Sprintf("opt.Something[%T](%#v)", receiver.value, receiver.value)
+}
+
+// WhenNothing will call ‘fn’ when ‘receiver’ is holding nothing.
+//
+// It will not call ‘fn’ when ‘receier’ is hold something.
+//
+// For example:
+//
+//	var op opt.Optional = opt.Nothing[string]
+//	
+//	// ...
+//
+//	op.WhenNothing(func(){
+//		//@TODO
+//	})
+func (receiver Optional[T]) WhenNothing(fn func()) {
+	if !receiver.something {
+		fn()
+	}
+}
+
+// WhenSomething will ‘fn’ when ‘receiver’ is holding something.
+// The value that ‘receiver’  is holding will be passed as a parameter to the function ‘fn’.
+//
+// It will not call ‘fn’ when ‘receiver’ is hold nothing.
+//
+// For example:
+//
+//	var op opt.Optional = opt.Something("once twice thrice fource")
+//	
+//	// ...
+//
+//	op.WhenSomething(func(value string){
+//		//@TODO
+//	})
+func (receiver Optional[T]) WhenSomething(fn func(T)) {
+	if receiver.something {
+		fn(receiver.value)
+	}
+}

optional_filter_test.go

@@ -0,0 +1,61 @@
+package opt_test
+
+import (
+	"sourcecode.social/reiver/go-opt"
+
+	"testing"
+)
+
+func TestOptional_Filter_int(t *testing.T) {
+
+	tests := []struct{
+		Optional opt.Optional[int]
+		Expected opt.Optional[int]
+	}{
+		{
+			Optional: opt.Nothing[int](),
+			Expected: opt.Nothing[int](),
+		},
+
+
+
+		{
+			Optional: opt.Something[int](-2),
+			Expected: opt.Something[int](-2),
+		},
+		{
+			Optional: opt.Something[int](-1),
+			Expected: opt.Nothing[int](),
+		},
+		{
+			Optional: opt.Something[int](0),
+			Expected: opt.Something[int](0),
+		},
+		{
+			Optional: opt.Something[int](1),
+			Expected: opt.Nothing[int](),
+		},
+		{
+			Optional: opt.Something[int](2),
+			Expected: opt.Something[int](2),
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		fn := func(value int) bool {
+			return 0 == (value % 2)
+		}
+
+		actual   := test.Optional.Filter(fn)
+		expected := test.Expected
+
+		if expected != actual {
+			t.Errorf("For test #%d, the actual value is not what was expected.", testNumber)
+			t.Logf("EXPECTED: %#v", expected)
+			t.Logf("ACTUAL:   %#v", actual)
+	/////////////// CONTINUE
+			continue
+		}
+	}
+}

optional_get_test.go

@@ -0,0 +1,380 @@
+package opt_test
+
+import (
+	"sourcecode.social/reiver/go-opt"
+
+	"testing"
+)
+
+func TestOptional_Get_string(t *testing.T) {
+
+	tests := []struct{
+		Optional opt.Optional[string]
+		ExpectedValue     string
+		ExpectedSomething bool
+	}{
+		{
+			Optional: opt.Nothing[string](),
+			ExpectedValue: "",
+			ExpectedSomething: false,
+		},
+
+
+
+		{
+			Optional: opt.Something(""),
+			ExpectedValue:          "",
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something("once twice thrice fource"),
+			ExpectedValue:          "once twice thrice fource",
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something("😈"),
+			ExpectedValue:          "😈",
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something("۰۱۲۳۴۵۶۷۸۹"),
+			ExpectedValue:          "۰۱۲۳۴۵۶۷۸۹",
+			ExpectedSomething: true,
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		value, something := test.Optional.Get()
+
+		{
+			expected := test.ExpectedSomething
+			actual   := something
+
+			if expected != actual {
+				t.Errorf("For test #%d, the actual something-flag is not what was expected.", testNumber)
+				t.Logf("EXPECTED FLAG: %t", expected)
+				t.Logf("ACTUAL   FLAG: %t", actual)
+				t.Logf("OPTIONAL: %#v", test.Optional)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+
+		{
+			expected := test.ExpectedValue
+			actual   := value
+
+			if expected != actual {
+				t.Errorf("For test #%d, the actual value is not what was expected.", testNumber)
+				t.Logf("EXPECTED VALUE: %q", expected)
+				t.Logf("ACTUAL   VALUE: %q", actual)
+				t.Logf("OPTIONAL: %#v", test.Optional)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+	}
+}
+
+func TestOptional_Get_int8(t *testing.T) {
+
+	tests := []struct{
+		Optional opt.Optional[int8]
+		ExpectedValue     int8
+		ExpectedSomething bool
+	}{
+		{
+			Optional: opt.Nothing[int8](),
+			ExpectedValue: 0,
+			ExpectedSomething: false,
+		},
+
+
+
+		{
+			Optional: opt.Something(int8(-127)),
+			ExpectedValue:          int8(-127),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-126)),
+			ExpectedValue:          int8(-126),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-125)),
+			ExpectedValue:          int8(-125),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-124)),
+			ExpectedValue:          int8(-124),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-123)),
+			ExpectedValue:          int8(-123),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-122)),
+			ExpectedValue:          int8(-122),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-121)),
+			ExpectedValue:          int8(-121),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-120)),
+			ExpectedValue:          int8(-120),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-9)),
+			ExpectedValue:          int8(-9),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-8)),
+			ExpectedValue:          int8(-8),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-7)),
+			ExpectedValue:          int8(-7),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-6)),
+			ExpectedValue:          int8(-6),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-5)),
+			ExpectedValue:          int8(-5),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-4)),
+			ExpectedValue:          int8(-4),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-3)),
+			ExpectedValue:          int8(-3),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-2)),
+			ExpectedValue:          int8(-2),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(-1)),
+			ExpectedValue:          int8(-1),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(0)),
+			ExpectedValue:          int8(0),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(1)),
+			ExpectedValue:          int8(1),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(2)),
+			ExpectedValue:          int8(2),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(3)),
+			ExpectedValue:          int8(3),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(4)),
+			ExpectedValue:          int8(4),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(5)),
+			ExpectedValue:          int8(5),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(6)),
+			ExpectedValue:          int8(6),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(7)),
+			ExpectedValue:          int8(7),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(8)),
+			ExpectedValue:          int8(8),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(9)),
+			ExpectedValue:          int8(9),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(120)),
+			ExpectedValue:          int8(120),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(121)),
+			ExpectedValue:          int8(121),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(122)),
+			ExpectedValue:          int8(122),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(123)),
+			ExpectedValue:          int8(123),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(124)),
+			ExpectedValue:          int8(124),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(125)),
+			ExpectedValue:          int8(125),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(126)),
+			ExpectedValue:          int8(126),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(int8(127)),
+			ExpectedValue:          int8(127),
+			ExpectedSomething: true,
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		value, something := test.Optional.Get()
+
+		{
+			expected := test.ExpectedSomething
+			actual   := something
+
+			if expected != actual {
+				t.Errorf("For test #%d, the actual something-flag is not what was expected.", testNumber)
+				t.Logf("EXPECTED FLAG: %t", expected)
+				t.Logf("ACTUAL   FLAG: %t", actual)
+				t.Logf("OPTIONAL: %#v", test.Optional)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+
+		{
+			expected := test.ExpectedValue
+			actual   := value
+
+			if expected != actual {
+				t.Errorf("For test #%d, the actual value is not what was expected.", testNumber)
+				t.Logf("EXPECTED VALUE: %q", expected)
+				t.Logf("ACTUAL   VALUE: %q", actual)
+				t.Logf("OPTIONAL: %#v", test.Optional)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+	}
+}
+
+func TestOptional_Get_uint8(t *testing.T) {
+
+	tests := []struct{
+		Optional opt.Optional[uint8]
+		ExpectedValue     uint8
+		ExpectedSomething bool
+	}{
+		{
+			Optional: opt.Nothing[uint8](),
+			ExpectedValue: 0,
+			ExpectedSomething: false,
+		},
+
+
+
+		{
+			Optional: opt.Something(uint8(0)),
+			ExpectedValue:          uint8(0),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(uint8(1)),
+			ExpectedValue:          uint8(1),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(uint8(2)),
+			ExpectedValue:          uint8(2),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(uint8(254)),
+			ExpectedValue:          uint8(254),
+			ExpectedSomething: true,
+		},
+		{
+			Optional: opt.Something(uint8(255)),
+			ExpectedValue:          uint8(255),
+			ExpectedSomething: true,
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		value, something := test.Optional.Get()
+
+		{
+			expected := test.ExpectedSomething
+			actual   := something
+
+			if expected != actual {
+				t.Errorf("For test #%d, the actual something-flag is not what was expected.", testNumber)
+				t.Logf("EXPECTED FLAG: %t", expected)
+				t.Logf("ACTUAL   FLAG: %t", actual)
+				t.Logf("OPTIONAL: %#v", test.Optional)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+
+		{
+			expected := test.ExpectedValue
+			actual   := value
+
+			if expected != actual {
+				t.Errorf("For test #%d, the actual value is not what was expected.", testNumber)
+				t.Logf("EXPECTED VALUE: %q", expected)
+				t.Logf("ACTUAL   VALUE: %q", actual)
+				t.Logf("OPTIONAL: %#v", test.Optional)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+	}
+}

optional_gostring_test.go

@@ -0,0 +1,191 @@
+package opt_test
+
+import (
+	"sourcecode.social/reiver/go-opt"
+
+	"fmt"
+
+	"testing"
+)
+
+func TestOptional_GoString(t *testing.T) {
+
+	tests := []struct{
+		Value any
+		Expected string
+	}{
+		{
+			Value:                           "",
+			Expected: `opt.Something[string]("")`,
+		},
+		{
+			Value:                           "once twice thrice fource",
+			Expected: `opt.Something[string]("once twice thrice fource")`,
+		},
+		{
+			Value:                           "apple banana cherry",
+			Expected: `opt.Something[string]("apple banana cherry")`,
+		},
+
+
+
+		{
+			Value:                   uint8 (0x0),
+			Expected: `opt.Something[uint8](0x0)`,
+		},
+		{
+			Value:                   uint8 (0x1),
+			Expected: `opt.Something[uint8](0x1)`,
+		},
+		{
+			Value:                   uint8 (0x2),
+			Expected: `opt.Something[uint8](0x2)`,
+		},
+		{
+			Value:                   uint8 (0xfe),
+			Expected: `opt.Something[uint8](0xfe)`,
+		},
+		{
+			Value:                   uint8 (0xff),
+			Expected: `opt.Something[uint8](0xff)`,
+		},
+
+
+
+		{
+			Value:                   uint16 (0x0),
+			Expected: `opt.Something[uint16](0x0)`,
+		},
+		{
+			Value:                   uint16 (0x1),
+			Expected: `opt.Something[uint16](0x1)`,
+		},
+		{
+			Value:                   uint16 (0x2),
+			Expected: `opt.Something[uint16](0x2)`,
+		},
+		{
+			Value:                   uint16 (0xfe),
+			Expected: `opt.Something[uint16](0xfe)`,
+		},
+		{
+			Value:                   uint16 (0xff),
+			Expected: `opt.Something[uint16](0xff)`,
+		},
+		{
+			Value:                   uint16 (0x100),
+			Expected: `opt.Something[uint16](0x100)`,
+		},
+		{
+			Value:                   uint16 (0x101),
+			Expected: `opt.Something[uint16](0x101)`,
+		},
+		{
+			Value:                   uint16 (0x102),
+			Expected: `opt.Something[uint16](0x102)`,
+		},
+		{
+			Value:                   uint16 (0xfffe),
+			Expected: `opt.Something[uint16](0xfffe)`,
+		},
+		{
+			Value:                   uint16 (0xffff),
+			Expected: `opt.Something[uint16](0xffff)`,
+		},
+
+
+
+		{
+			Value:                   struct { A string; B int }{A:"joeblow",B:7},
+			Expected: `opt.Something[struct { A string; B int }](struct { A string; B int }{A:"joeblow", B:7})`,
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		op := opt.Something(test.Value)
+		gostring := op.GoString()
+
+
+		{
+			expected := test.Expected
+			actual   := gostring
+
+			if expected != actual {
+				t.Errorf("For test #%d, the actual go-string is not what was expected.", testNumber)
+				t.Logf("EXPECTED: %q", expected)
+				t.Logf("ACTUAL:   %q", actual)
+				t.Logf("VALUE-TYPE: %T", test.Value)
+				t.Logf("VALUE: %#v", test.Value)
+	//////////////////////// CONTINUE
+				continue
+			}
+		}
+	}
+}
+
+func TestOptional_GoString_nothing(t *testing.T) {
+
+	tests := []struct{
+		Optional fmt.GoStringer
+		Expected string
+	}{
+		{
+			Optional:  opt.Nothing[string](),
+			Expected: `opt.Nothing[string]()`,
+		},
+
+
+
+		{
+			Optional:  opt.Nothing[int8](),
+			Expected: `opt.Nothing[int8]()`,
+		},
+		{
+			Optional:  opt.Nothing[int16](),
+			Expected: `opt.Nothing[int16]()`,
+		},
+		{
+			Optional:  opt.Nothing[int32](),
+			Expected: `opt.Nothing[int32]()`,
+		},
+		{
+			Optional:  opt.Nothing[int64](),
+			Expected: `opt.Nothing[int64]()`,
+		},
+
+
+
+		{
+			Optional:  opt.Nothing[uint8](),
+			Expected: `opt.Nothing[uint8]()`,
+		},
+		{
+			Optional:  opt.Nothing[uint16](),
+			Expected: `opt.Nothing[uint16]()`,
+		},
+		{
+			Optional:  opt.Nothing[uint32](),
+			Expected: `opt.Nothing[uint32]()`,
+		},
+		{
+			Optional:  opt.Nothing[uint64](),
+			Expected: `opt.Nothing[uint64]()`,
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		expected := test.Expected
+		actual   := test.Optional.GoString()
+
+		if expected != actual {
+			t.Errorf("For test #%d, the actual go-string value is not what was expected.", testNumber)
+			t.Logf("EXPECTED GO-STRING: %q", expected)
+			t.Logf("ACTUAL   GO-STRING: %q", actual)
+			t.Logf("TYPE: %T", test.Optional)
+	/////////////// CONTINUE
+			continue
+		}
+	}
+}

optional_whennothing_test.go

@@ -0,0 +1,139 @@
+package opt_test
+
+import (
+	"sourcecode.social/reiver/go-opt"
+
+	"testing"
+)
+
+func TestOptional_WhenNothing(t *testing.T) {
+
+
+	tests := []struct{
+		Optional interface{WhenNothing(func())}
+	}{
+		{
+			Optional: opt.Nothing[string](),
+		},
+
+
+
+		{
+			Optional: opt.Nothing[int](),
+		},
+		{
+			Optional: opt.Nothing[int8](),
+		},
+		{
+			Optional: opt.Nothing[int16](),
+		},
+		{
+			Optional: opt.Nothing[int32](),
+		},
+		{
+			Optional: opt.Nothing[int64](),
+		},
+
+
+
+		{
+			Optional: opt.Nothing[uint](),
+		},
+		{
+			Optional: opt.Nothing[uint8](),
+		},
+		{
+			Optional: opt.Nothing[uint16](),
+		},
+		{
+			Optional: opt.Nothing[uint32](),
+		},
+		{
+			Optional: opt.Nothing[uint64](),
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		var worked bool = false
+
+		test.Optional.WhenNothing(func(){
+
+			worked = true
+		})
+
+		if !worked {
+			t.Errorf("For test #%d, the call to the method did not seem work.", testNumber)
+			t.Logf("WORKED: %t", worked)
+			t.Logf("OPTIONAL: (%T) %#v", test.Optional, test.Optional)
+	//////////////// CONTINUE
+			continue
+		}
+	}
+}
+
+func TestOptional_WhenNothing_something(t *testing.T) {
+
+
+	tests := []struct{
+		Optional interface{WhenNothing(func())}
+	}{
+		{
+			Optional: opt.Something[string]("once twice thrice fource"),
+		},
+
+
+
+		{
+			Optional: opt.Something[int](-1),
+		},
+		{
+			Optional: opt.Something[int8](-101),
+		},
+		{
+			Optional: opt.Something[int16](-10101),
+		},
+		{
+			Optional: opt.Something[int32](-1010101),
+		},
+		{
+			Optional: opt.Something[int64](-101010101),
+		},
+
+
+
+		{
+			Optional: opt.Something[uint](1),
+		},
+		{
+			Optional: opt.Something[uint8](101),
+		},
+		{
+			Optional: opt.Something[uint16](10101),
+		},
+		{
+			Optional: opt.Something[uint32](1010101),
+		},
+		{
+			Optional: opt.Something[uint64](101010101),
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		var worked bool = false
+
+		test.Optional.WhenNothing(func(){
+
+			worked = true
+		})
+
+		if worked {
+			t.Errorf("For test #%d, the call to the method worked, but it should not have.", testNumber)
+			t.Logf("WORKED: %t", worked)
+			t.Logf("OPTIONAL: (%T) %#v", test.Optional, test.Optional)
+	//////////////// CONTINUE
+			continue
+		}
+	}
+}

optional_whensomething_test.go

@@ -0,0 +1,446 @@
+package opt_test
+
+import (
+	"sourcecode.social/reiver/go-opt"
+
+	"testing"
+)
+
+func TestOptional_WhenSomething_string(t *testing.T) {
+
+
+	tests := []struct{
+		Optional interface{WhenSomething(func(string))}
+		Expected                              string
+	}{
+		{
+			Optional: opt.Something[string](""),
+			Expected:                       "",
+		},
+		{
+			Optional: opt.Something[string]("once twice thrice fource"),
+			Expected:                       "once twice thrice fource",
+		},
+		{
+			Optional: opt.Something[string]("apple banana cherry"),
+			Expected:                       "apple banana cherry",
+		},
+		{
+			Optional: opt.Something[string]("😈"),
+			Expected:                       "😈",
+		},
+		{
+			Optional: opt.Something[string]("۰۱۲۳۴۵۶۷۸۹"),
+			Expected:                       "۰۱۲۳۴۵۶۷۸۹",
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		var worked bool = false
+		var value string = "-<([-RaNdOmNeSs])>-"
+
+		test.Optional.WhenSomething(func(v string){
+
+			worked = true
+			value  = v
+		})
+
+		if !worked {
+			t.Errorf("For test #%d, the call to the method did not seem work.", testNumber)
+			t.Logf("WORKED: %t", worked)
+			t.Logf("OPTIONAL: (%T) %#v", test.Optional, test.Optional)
+	/////////////// CONTINUE
+			continue
+		}
+
+		{
+			expected := test.Expected
+			actual   := value
+
+			if expected != actual {
+			t.Errorf("For test #%d, the actual value is not what was expected", testNumber)
+			t.Logf("EXPECTED: %q", expected)
+			t.Logf("EXPECTED: %q", actual)
+			t.Logf("WORKED: %t", worked)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+	}
+}
+
+func TestOptional_WhenSomething_stringNothing(t *testing.T) {
+
+	var worked bool = false
+
+	opt.Nothing[string]().WhenSomething(func(v string){
+
+		worked = true
+	})
+
+	if worked {
+		t.Errorf("The call to the method worked, but it should not have.")
+		t.Logf("WORKED: %t", worked)
+//////////////// RETURN
+		return
+	}
+}
+
+func TestOptional_WhenSomething_int8(t *testing.T) {
+
+
+	tests := []struct{
+		Optional interface{WhenSomething(func(int8))}
+		Expected                              int8
+	}{
+		{
+			Optional: opt.Something[int8](-127),
+			Expected:                     -127,
+		},
+		{
+			Optional: opt.Something[int8](-126),
+			Expected:                     -126,
+		},
+		{
+			Optional: opt.Something[int8](-125),
+			Expected:                     -125,
+		},
+		{
+			Optional: opt.Something[int8](-124),
+			Expected:                     -124,
+		},
+		{
+			Optional: opt.Something[int8](-123),
+			Expected:                     -123,
+		},
+		{
+			Optional: opt.Something[int8](-122),
+			Expected:                     -122,
+		},
+		{
+			Optional: opt.Something[int8](-121),
+			Expected:                     -121,
+		},
+		{
+			Optional: opt.Something[int8](-120),
+			Expected:                     -120,
+		},
+		{
+			Optional: opt.Something[int8](-9),
+			Expected:                     -9,
+		},
+		{
+			Optional: opt.Something[int8](-8),
+			Expected:                     -8,
+		},
+		{
+			Optional: opt.Something[int8](-7),
+			Expected:                     -7,
+		},
+		{
+			Optional: opt.Something[int8](-6),
+			Expected:                     -6,
+		},
+		{
+			Optional: opt.Something[int8](-5),
+			Expected:                     -5,
+		},
+		{
+			Optional: opt.Something[int8](-4),
+			Expected:                     -4,
+		},
+		{
+			Optional: opt.Something[int8](-3),
+			Expected:                     -3,
+		},
+		{
+			Optional: opt.Something[int8](-2),
+			Expected:                     -2,
+		},
+		{
+			Optional: opt.Something[int8](-1),
+			Expected:                     -1,
+		},
+		{
+			Optional: opt.Something[int8](0),
+			Expected:                     0,
+		},
+		{
+			Optional: opt.Something[int8](1),
+			Expected:                     1,
+		},
+		{
+			Optional: opt.Something[int8](2),
+			Expected:                     2,
+		},
+		{
+			Optional: opt.Something[int8](3),
+			Expected:                     3,
+		},
+		{
+			Optional: opt.Something[int8](4),
+			Expected:                     4,
+		},
+		{
+			Optional: opt.Something[int8](5),
+			Expected:                     5,
+		},
+		{
+			Optional: opt.Something[int8](6),
+			Expected:                     6,
+		},
+		{
+			Optional: opt.Something[int8](7),
+			Expected:                     7,
+		},
+		{
+			Optional: opt.Something[int8](8),
+			Expected:                     8,
+		},
+		{
+			Optional: opt.Something[int8](9),
+			Expected:                     9,
+		},
+		{
+			Optional: opt.Something[int8](120),
+			Expected:                     120,
+		},
+		{
+			Optional: opt.Something[int8](121),
+			Expected:                     121,
+		},
+		{
+			Optional: opt.Something[int8](122),
+			Expected:                     122,
+		},
+		{
+			Optional: opt.Something[int8](123),
+			Expected:                     123,
+		},
+		{
+			Optional: opt.Something[int8](124),
+			Expected:                     124,
+		},
+		{
+			Optional: opt.Something[int8](125),
+			Expected:                     125,
+		},
+		{
+			Optional: opt.Something[int8](126),
+			Expected:                     126,
+		},
+		{
+			Optional: opt.Something[int8](127),
+			Expected:                     127,
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		var worked bool = false
+		var value int8 = -101
+
+		test.Optional.WhenSomething(func(v int8){
+
+			worked = true
+			value = v
+		})
+
+		if !worked {
+			t.Errorf("For test #%d, the call to the method did not seem work.", testNumber)
+			t.Logf("WORKED: %t", worked)
+			t.Logf("OPTIONAL: (%T) %#v", test.Optional, test.Optional)
+	//////////////// CONTINUE
+			continue
+		}
+
+		{
+			expected := test.Expected
+			actual   := value
+
+			if expected != actual {
+			t.Errorf("For test #%d, the actual value is not what was expected", testNumber)
+			t.Logf("EXPECTED: %d", expected)
+			t.Logf("EXPECTED: %d", actual)
+			t.Logf("WORKED: %t", worked)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+	}
+}
+
+func TestOptional_WhenSomething_int8Nothing(t *testing.T) {
+
+	var worked bool = false
+
+	opt.Nothing[int8]().WhenSomething(func(v int8){
+
+		worked = true
+	})
+
+	if worked {
+		t.Errorf("The call to the method worked, but it should not have.")
+		t.Logf("WORKED: %t", worked)
+//////////////// RETURN
+		return
+	}
+}
+
+func TestOptional_WhenSomething_uint8(t *testing.T) {
+
+
+	tests := []struct{
+		Optional interface{WhenSomething(func(uint8))}
+		Expected                              uint8
+	}{
+		{
+			Optional: opt.Something[uint8](0),
+			Expected:                      0,
+		},
+		{
+			Optional: opt.Something[uint8](1),
+			Expected:                      1,
+		},
+		{
+			Optional: opt.Something[uint8](2),
+			Expected:                      2,
+		},
+		{
+			Optional: opt.Something[uint8](3),
+			Expected:                      3,
+		},
+		{
+			Optional: opt.Something[uint8](4),
+			Expected:                      4,
+		},
+		{
+			Optional: opt.Something[uint8](5),
+			Expected:                      5,
+		},
+		{
+			Optional: opt.Something[uint8](6),
+			Expected:                      6,
+		},
+		{
+			Optional: opt.Something[uint8](7),
+			Expected:                      7,
+		},
+		{
+			Optional: opt.Something[uint8](8),
+			Expected:                      8,
+		},
+		{
+			Optional: opt.Something[uint8](9),
+			Expected:                      9,
+		},
+		{
+			Optional: opt.Something[uint8](120),
+			Expected:                      120,
+		},
+		{
+			Optional: opt.Something[uint8](121),
+			Expected:                      121,
+		},
+		{
+			Optional: opt.Something[uint8](122),
+			Expected:                      122,
+		},
+		{
+			Optional: opt.Something[uint8](123),
+			Expected:                      123,
+		},
+		{
+			Optional: opt.Something[uint8](124),
+			Expected:                      124,
+		},
+		{
+			Optional: opt.Something[uint8](125),
+			Expected:                      125,
+		},
+		{
+			Optional: opt.Something[uint8](126),
+			Expected:                      126,
+		},
+		{
+			Optional: opt.Something[uint8](127),
+			Expected:                      127,
+		},
+		{
+			Optional: opt.Something[uint8](250),
+			Expected:                      250,
+		},
+		{
+			Optional: opt.Something[uint8](251),
+			Expected:                      251,
+		},
+		{
+			Optional: opt.Something[uint8](252),
+			Expected:                      252,
+		},
+		{
+			Optional: opt.Something[uint8](253),
+			Expected:                      253,
+		},
+		{
+			Optional: opt.Something[uint8](254),
+			Expected:                      254,
+		},
+		{
+			Optional: opt.Something[uint8](255),
+			Expected:                      255,
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		var worked bool = false
+		var value uint8 = 101
+
+		test.Optional.WhenSomething(func(v uint8){
+
+			worked = true
+			value = v
+		})
+
+		if !worked {
+			t.Errorf("For test #%d, the call to the method did not seem work.", testNumber)
+			t.Logf("WORKED: %t", worked)
+			t.Logf("OPTIONAL: (%T) %#v", test.Optional, test.Optional)
+	//////////////// CONTINUE
+			continue
+		}
+
+		{
+			expected := test.Expected
+			actual   := value
+
+			if expected != actual {
+			t.Errorf("For test #%d, the actual value is not what was expected", testNumber)
+			t.Logf("EXPECTED: %d", expected)
+			t.Logf("EXPECTED: %d", actual)
+			t.Logf("WORKED: %t", worked)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+	}
+}
+
+func TestOptional_WhenSomething_uint8Nothing(t *testing.T) {
+
+	var worked bool = false
+
+	opt.Nothing[uint8]().WhenSomething(func(v uint8){
+
+		worked = true
+	})
+
+	if worked {
+		t.Errorf("The call to the method worked, but it should not have.")
+		t.Logf("WORKED: %t", worked)
+//////////////// RETURN
+		return
+	}
+}

string/gen_optiontype.go

@@ -1,248 +0,0 @@
-package optstring
-
-/*
- * CODE GENERATED AUTOMATICALLY WITH https://github.com/reiver/gogen-optiontype
- * THIS FILE SHOULD NOT BE EDITED BY HAND
- */
-
-import (
-	"bytes"
-	"database/sql/driver"
-	"encoding/json"
-	"errors"
-	"fmt"
-)
-
-var (
-	errNilReceiver  = errors.New("Nil Receiver")
-	errNone         = errors.New("optstring.None()")
-	errNoneNullable = errors.New("optstring.NoneNullable()")
-	errNull         = errors.New("optstring.Null()")
-)
-
-func (receiver *NullableType) Scan(src interface{}) error {
-	if nil == receiver {
-		return errNilReceiver
-	}
-
-	if nil == src {
-		*receiver = Null()
-		return nil
-	}
-
-	switch t := src.(type) {
-	case NullableType:
-		*receiver = t
-		return nil
-	case Type:
-		switch t {
-		case None():
-			*receiver = NoneNullable()
-		default:
-			datum, err := t.String()
-			if nil != err {
-				return fmt.Errorf("Problem unwrapping %T: (%T) %v", t, err, err)
-			}
-			*receiver = SomeNullable(datum)
-		}
-		return nil
-	case string:
-		*receiver = SomeNullable(t)
-		return nil
-	case []byte:
-		s := string(t)
-		*receiver = SomeNullable(s)
-		return nil
-	default:
-		return fmt.Errorf("Cannot scan something of type %T into an %T.", src, *receiver)
-	}
-}
-
-func (receiver NullableType) String() (string, error) {
-	if NoneNullable() == receiver {
-		return "", errNoneNullable
-	}
-	if Null() == receiver {
-		return "", errNull
-	}
-
-	return receiver.value, nil
-}
-
-type NullableType struct {
-	loaded bool
-	null   bool
-	value  string
-}
-
-func (receiver NullableType) MarshalJSON() ([]byte, error) {
-	if NoneNullable() == receiver {
-		return nil, errNoneNullable
-	}
-	if Null() == receiver {
-		return json.Marshal(nil)
-	}
-
-	return json.Marshal(receiver.value)
-}
-
-func (receiver NullableType) WhenNone(fn func()) {
-	if NoneNullable() == receiver {
-		fn()
-	}
-}
-
-func (receiver NullableType) WhenNull(fn func()) {
-	if Null() == receiver {
-		fn()
-	}
-}
-
-func (receiver NullableType) WhenSome(fn func(string)) {
-	if NoneNullable() != receiver && Null() != receiver {
-		fn(receiver.value)
-	}
-}
-
-func (receiver *NullableType) UnmarshalJSON(b []byte) error {
-	if nil == receiver {
-		return errNilReceiver
-	}
-
-	if 0 == bytes.Compare(b, []byte{'n','u','l','l'}) {
-		*receiver = Null()
-		return nil
-	}
-
-	var target string
-
-	if err := json.Unmarshal(b, &target); nil != err {
-		return err
-	}
-
-	*receiver = SomeNullable(target)
-
-	return nil
-}
-
-func (receiver NullableType) Value() (driver.Value, error) {
-	if NoneNullable() == receiver {
-		return nil, errNoneNullable
-	}
-	if Null() == receiver {
-		return nil, nil
-	}
-
-	return receiver.value, nil
-}
-
-func NoneNullable() NullableType {
-	return NullableType{}
-}
-
-func SomeNullable(value string) NullableType {
-	return NullableType{
-		value:  value,
-		loaded: true,
-	}
-}
-
-func Null() NullableType {
-	return NullableType{
-		null:   true,
-		loaded: true,
-	}
-}
-
-func (receiver *Type) Scan(src interface{}) error {
-	if nil == receiver {
-		return errNilReceiver
-	}
-
-	switch t := src.(type) {
-	case Type:
-		*receiver = t
-		return nil
-	case string:
-		*receiver = Some(t)
-		return nil
-	case []byte:
-		s := string(t)
-		*receiver = Some(s)
-		return nil
-	default:
-		return fmt.Errorf("Cannot scan something of type %T into an %T.", src, *receiver)
-	}
-}
-
-func (receiver Type) String() (string, error) {
-	if None() == receiver {
-		return "", errNone
-	}
-
-	return receiver.value, nil
-}
-
-type Type struct {
-	loaded bool
-	value  string
-}
-
-func (receiver Type) MarshalJSON() ([]byte, error) {
-	if None() == receiver {
-		return nil, errNone
-	}
-
-	return json.Marshal(receiver.value)
-}
-
-func (receiver *Type) UnmarshalJSON(b []byte) error {
-	if nil == receiver {
-		return errNilReceiver
-	}
-
-	if 0 == bytes.Compare(b, []byte{'n','u','l','l'}) {
-		return fmt.Errorf("Cannot unmarshal %q into %T.", string(b), *receiver)
-	}
-
-	var target string
-
-	if err := json.Unmarshal(b, &target); nil != err {
-		return err
-	}
-
-	*receiver = Some(target)
-
-	return nil
-}
-
-func (receiver Type) WhenNone(fn func()) {
-	if None() == receiver {
-		fn()
-	}
-}
-
-func (receiver Type) WhenSome(fn func(string)) {
-	if None() != receiver {
-		fn(receiver.value)
-	}
-}
-
-func (receiver Type) Value() (driver.Value, error) {
-	if None() == receiver {
-		return nil, errNone
-	}
-
-	return receiver.value, nil
-}
-
-func None() Type {
-	return Type{}
-}
-
-func Some(value string) Type {
-	return Type{
-		value:  value,
-		loaded: true,
-	}
-}

string/optiontype.go

@@ -1,3 +0,0 @@
-package optstring
-
-//go:generate gogen-optiontype --pkg=optstring --type=string --one-file --no-tests

then.go

@@ -0,0 +1,44 @@
+package opt
+
+// Then applies the function ‘fn’ to the value inside of the optional-type ‘op’, if the optional-type ‘op’ is holding something, and returns the resulting  optional-type.
+// If the optional-type ‘op’ is holding nothing, then Then also returns nothing.
+//
+// For example:
+//
+//	fn := func(s string) opt.Optional[byte] {
+//		
+//		if len(s) < 2 {
+//			return opt.Nothing[byte]()
+//		}
+//		
+//		return opt.Something[byte](s[1])
+//	}
+//	
+//	var op opt.Optional[string] = opt.Something("Hello world!"")
+//	
+//	var result opt.Optional[byte] = opt.Then(op, fn)
+//	
+//	// result == opt.Something[byte]('e')
+//	
+//	// ...
+//	
+//	var op2 opt.Optional[string] = opt.Something[string]("X")
+//	
+//	var result2 opt.Optional[byte] = opt.Then(op, fn)
+//	
+//	// result2 == opt.Nothing[byte]()
+//	
+//	// ...
+//	
+//	var op2 opt.Optional[string] = opt.Nothing[string]()
+//	
+//	var result2 opt.Optional[byte] = opt.Then(op, fn)
+//	
+//	// result2 == opt.Nothing[byte]()
+func Then[T1 any, T2 any](op Optional[T1], fn func(T1)Optional[T2]) Optional[T2] {
+	if !op.something {
+		return Nothing[T2]()
+	}
+
+	return fn(op.value)
+}

then_test.go

@@ -0,0 +1,88 @@
+package opt_test
+
+import (
+	"sourcecode.social/reiver/go-opt"
+
+	"testing"
+)
+
+func TestThen_stringToUint(t *testing.T) {
+
+	tests := []struct{
+		Optional opt.Optional[string]
+		Expected opt.Optional[byte]
+	}{
+		{
+			Optional: opt.Nothing[string](),
+			Expected: opt.Nothing[byte](),
+		},
+
+
+
+		{
+			Optional: opt.Something(""),
+			Expected: opt.Nothing[byte](),
+		},
+
+
+
+		{
+			Optional: opt.Something("A"),
+			Expected: opt.Nothing[byte](),
+		},
+		{
+			Optional: opt.Something("B"),
+			Expected: opt.Nothing[byte](),
+		},
+		{
+			Optional: opt.Something("C"),
+			Expected: opt.Nothing[byte](),
+		},
+
+
+
+		{
+			Optional: opt.Something("once"),
+			Expected: opt.Something[byte]('n'),
+		},
+		{
+			Optional: opt.Something("twice"),
+			Expected: opt.Something[byte]('w'),
+		},
+		{
+			Optional: opt.Something("thrice"),
+			Expected: opt.Something[byte]('h'),
+		},
+		{
+			Optional: opt.Something("fource"),
+			Expected: opt.Something[byte]('o'),
+		},
+	}
+
+	for testNumber, test := range tests {
+
+		fn := func(s string) opt.Optional[byte] {
+			if len(s) < 2 {
+				return opt.Nothing[byte]()
+			}
+
+			return opt.Something[byte](s[1])
+		}
+
+		thened := opt.Then(test.Optional, fn)
+
+		{
+			expected := test.Expected
+			actual   := thened
+
+			if expected != actual  {
+				t.Errorf("For test #%d, the actual value is not what was expected.", testNumber)
+				t.Logf("EXPECTED: (%T) %#v", expected, expected)
+				t.Logf("ACTUAL:   (%T) %#v", actual, actual)
+				t.Logf("OPTIONAL: (%T), %#v", test.Optional, test.Optional)
+	/////////////////////// CONTINUE
+				continue
+			}
+		}
+	}
+}