Builder Pattern in Go only with functions

main.go

package main

import "fmt"

type StructuredMessage struct {
	Kind    string
	Details map[string]any
}

type BuildFunc[T any] func() *T
type ApplyFunc[T any] func(string, map[string]any) (ApplyFunc[T], BuildFunc[T])

func NewErrorBuilder(kind string) (ApplyFunc[StructuredMessage], BuildFunc[StructuredMessage]) {
	var applyF ApplyFunc[StructuredMessage]
	target := &StructuredMessage{Kind: kind, Details: make(map[string]any)}

	buildF := func() *StructuredMessage {
		return target
	}

	applyF = func(kind string, detail map[string]any) (ApplyFunc[StructuredMessage], BuildFunc[StructuredMessage]) {
		target.Details[kind] = detail
		return applyF, buildF
	}

	return applyF, buildF
}

func main() {
	with, build := NewErrorBuilder("messages")

	with, build = with("error", map[string]any{"type": "unrecoverable"})

	r := build()
	fmt.Printf("Result: %+v\n", r)
	// OUT:
	// Result: &{Kind:messages Details:map[error:map[type:unrecoverable]]}
}