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

[HoChihchou]

The best way to define a function from a performance perspective is to use a Function option.

Quoted from documentation（https://expr-lang.org/docs/functions）

This is my benchmark test:

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)
			}
		}
	})
}

And result

goos: darwin
goarch: arm64
cpu: Apple M1 Pro
BenchmarkCustomFunctions/EnvMap-10              24067287                48.26 ns/op
BenchmarkCustomFunctions/EnvStruct-10            3250443               365.2 ns/op
BenchmarkCustomFunctions/FunctionOption-10      19875295                60.49 ns/op
PASS
ok      command-line-arguments  4.534s

goos: darwin
goarch: arm64
cpu: Apple M1 Pro
BenchmarkCustomFunctions/EnvMap-10              23904123                48.59 ns/op
BenchmarkCustomFunctions/EnvStruct-10            3274218               374.6 ns/op
BenchmarkCustomFunctions/FunctionOption-10      19192731                61.28 ns/op
PASS
ok      command-line-arguments  4.460s

The performance of map and function methods are very close, there is no particularly obvious difference, and map method is even faster.

[mdmcconnell]

This is great, very helpful! I've wondered about this since I make heavy use of functions. It's nice to have data.

[antonmedv]

Nice benchmark!

I put a lot of effort to make function calls as fast as possible.

This is still true: The best way to define a function from a performance perspective is to use a Function option.

In your case the map slightly outperforms Function due to a heavy ammount of optimizations. But for general, uncommon func signature results will be different. True for example: func(int, string, int, time.Time, uin16) or any other random funcs signatures.

[HoChihchou]

For such a function func(int, string, int, time.Time, uin16) , will the expr.Function be much faster than the map? Or is it just a little faster?

[antonmedv]

Faster. But on how much - benchmarks are needed.