Why the best way to define a function from a performance perspective is to use the expr.Function option. #734

package main

import (
	"strconv"
	"testing"

	"github.com/expr-lang/expr"
)

type EnvStruct struct{}

func (EnvStruct) Atoi(input string) int {
	value, _ := strconv.Atoi(input)
	return value
}

func BenchmarkCustomFunctions(b *testing.B) {
	exprString := `Atoi("42")`

	// Method 1: Add to environment
	envMap := map[string]interface{}{
		"Atoi": func(input string) int {
			value, _ := strconv.Atoi(input)
			return value
		},
	}

	compiledEnvMap, err := expr.Compile(exprString, expr.Env(envMap))
	if err != nil {
		b.Fatal(err)
	}

	// Method 2: Define on struct
	envStruct := EnvStruct{}
	compiledEnvStruct, err := expr.Compile(exprString, expr.Env(envStruct))
	if err != nil {
		b.Fatal(err)
	}

	// Method 3: Using Function option
	atoiFunc := expr.Function(
		"Atoi",
		func(params ...interface{}) (interface{}, error) {
			return strconv.Atoi(params[0].(string))
		},
		strconv.Atoi,
	)
	compiledFunc, err := expr.Compile(exprString, atoiFunc)
	if err != nil {
		b.Fatal(err)
	}

	b.Run("EnvMap", func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			r, _ := expr.Run(compiledEnvMap, envMap)
			if r != 42 {
				b.Errorf("got %v, want 42", r)
			}
		}
	})

	b.Run("EnvStruct", func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			r, _ := expr.Run(compiledEnvStruct, envStruct)
			if r != 42 {
				b.Errorf("got %v, want 42", r)
			}
		}
	})

	b.Run("FunctionOption", func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			r, _ := expr.Run(compiledFunc, nil)
			if r != 42 {
				b.Errorf("got %v, want 42", r)
			}
		}
	})
}