doc: params.ExtraPayloadGetter comments and improved readability

params/example.libevm_test.go

@@ -1,3 +1,12 @@
+// In practice, everything in this file except for the Example() function SHOULD
+// be a standalone package, typically called `extraparams`. As long as this new
+// package is imported anywhere, its init() function will register the "extra"
+// types, which can be accessed via [extraparams.FromChainConfig] and/or
+// [extraparams.FromRules]. In all other respects, the [params.ChainConfig] and
+// [params.Rules] types will act as expected.
+//
+// The Example() function demonstrates how the `extraparams` package might be
+// used from elsewhere.
 package params_test
 
 import (
@@ -9,8 +18,12 @@ import (
 	"github.com/ethereum/go-ethereum/params"
 )
 
-// TODO: explain why this isn't in an init()
+// In practice this would be a regular init() function but nuances around the
+// testing of this package require it to be called in the Example().
 func initFn() {
+	// This registration makes *all* [params.ChainConfig] and [params.Rules]
+	// instances respect the payload types. They do not need to be modified to
+	// know about `extraparams`.
 	getter = params.RegisterExtras(params.Extras[ChainConfigExtra, RulesExtra]{
 		NewRules: constructRulesExtra,
 	})
@@ -18,30 +31,41 @@ func initFn() {
 
 var getter params.ExtraPayloadGetter[ChainConfigExtra, RulesExtra]
 
-func FromChainConfig(c *params.ChainConfig) *ChainConfigExtra {
-	return getter.FromChainConfig(c)
-}
-
-func FromRules(r *params.Rules) *RulesExtra {
-	return getter.FromRules(r)
-}
-
-type ChainConfigExtra struct {
-	MyForkTime *uint64 `json:"myForkTime"`
-}
-
-type RulesExtra struct {
-	IsMyFork bool
-}
-
+// constructRulesExtra acts as an adjunct to the [params.ChainConfig.Rules]
+// method. Its primary purpose is to construct the extra payload for the
+// [params.Rules] but it MAY also modify the [params.Rules].
 func constructRulesExtra(c *params.ChainConfig, r *params.Rules, cEx *ChainConfigExtra, blockNum *big.Int, isMerge bool, timestamp uint64) *RulesExtra {
 	return &RulesExtra{
 		IsMyFork: cEx.MyForkTime != nil && *cEx.MyForkTime <= timestamp,
 	}
 }
 
-func ExampleRegisterExtras() {
-	initFn() // TODO: explain
+// ChainConfigExtra can be any struct. Here it just mirrors a common pattern in
+// the standard [params.ChainConfig] struct.
+type ChainConfigExtra struct {
+	MyForkTime *uint64 `json:"myForkTime"`
+}
+
+// RulesExtra can be any struct. It too mirrors a common pattern in
+// [params.Rules].
+type RulesExtra struct {
+	IsMyFork bool
+}
+
+// FromChainConfig returns the extra payload carried by the ChainConfig.
+func FromChainConfig(c *params.ChainConfig) *ChainConfigExtra {
+	return getter.FromChainConfig(c)
+}
+
+// FromRules returns the extra payload carried by the Rules.
+func FromRules(r *params.Rules) *RulesExtra {
+	return getter.FromRules(r)
+}
+
+// This example demonstrates how the rest of this file would be used from a
+// *different* package.
+func ExampleExtraPayloadGetter() {
+	initFn() // Outside of an example this is unnecessary as the function will be a regular init().
 
 	const forkTime = 530003640
 	jsonData := fmt.Sprintf(`{
@@ -51,31 +75,33 @@ func ExampleRegisterExtras() {
 		}
 	}`, forkTime)
 
-	// ChainConfig now unmarshals any JSON field named "extra" into a pointer to
-	// the registered type, which is available via the ExtraPayload() method.
+	// Because [params.RegisterExtras] has been called, unmarshalling a JSON
+	// field of "extra" into a [params.ChainConfig] will populate a new value of
+	// the registered type. This can be accessed with the [FromChainConfig]
+	// function.
 	config := new(params.ChainConfig)
 	if err := json.Unmarshal([]byte(jsonData), config); err != nil {
 		log.Fatal(err)
 	}
 
-	fmt.Println(config.ChainID) // original geth fields work as expected
+	fmt.Println("Chain ID", config.ChainID) // original geth fields work as expected
 
-	ccExtra := FromChainConfig(config)
+	ccExtra := FromChainConfig(config) // extraparams.FromChainConfig() in practice
 	if ccExtra != nil && ccExtra.MyForkTime != nil {
-		fmt.Println(*ccExtra.MyForkTime)
+		fmt.Println("Fork time", *ccExtra.MyForkTime)
 	}
 
 	for _, time := range []uint64{forkTime - 1, forkTime, forkTime + 1} {
 		rules := config.Rules(nil, false, time)
-		rExtra := FromRules(&rules)
+		rExtra := FromRules(&rules) // extraparams.FromRules() in practice
 		if rExtra != nil {
 			fmt.Printf("%+v\n", rExtra)
 		}
 	}
 
 	// Output:
-	// 1234
-	// 530003640
+	// Chain ID 1234
+	// Fork time 530003640
 	// &{IsMyFork:false}
 	// &{IsMyFork:true}
 	// &{IsMyFork:true}