Go Academy · Lesson

Dynamic Function Calls

Calling functions and setting values via reflect

Lesson 3 of 413 steps

Dynamic Function Calls is a free Go Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, and your progress syncs across the web and the CoddyKit app. The Go Academy course includes 4 lessons in total.

reflect.Value.Call

Value.Call(args) calls the function represented by the Value with the given arguments (as a []reflect.Value) and returns the results as a []reflect.Value.

fn := reflect.ValueOf(add)
results := fn.Call([]reflect.Value{
    reflect.ValueOf(2),
    reflect.ValueOf(3),
})
fmt.Println(results[0].Int()) // 5

Argument type checking

Ensure argument types match the function signature before calling. Mismatched types cause a panic.

t := reflect.TypeOf(fn)
if t.NumIn() != len(args) { panic("wrong arg count") }
for i, arg := range args {
    if !reflect.TypeOf(arg).AssignableTo(t.In(i)) { panic("type mismatch") }
}

Variadic functions

Call a variadic function with Value.Call (spreading the slice) or Value.CallSlice (passing the variadic args as a slice value):

sum := reflect.ValueOf(func(ns ...int) int {
    total := 0
    for _, n := range ns { total += n }
    return total
})
args := reflect.ValueOf([]int{1,2,3})
results := sum.CallSlice([]reflect.Value{args})

reflect.MakeFunc

reflect.MakeFunc creates a new function at runtime with a given type and a wrapper body. Used to generate adapter functions for interfaces.

t := reflect.TypeOf(func(int) string { return "" })
fn := reflect.MakeFunc(t, func(args []reflect.Value) []reflect.Value {
    return []reflect.Value{reflect.ValueOf(strconv.Itoa(args[0].Interface().(int)))}
})

Calling methods by name

Retrieve a method by name and call it:

v := reflect.ValueOf(myObj)
m := v.MethodByName("Process")
if m.IsValid() {
    m.Call(nil)
}

Checking if method exists

Always check m.IsValid() before calling — MethodByName returns the zero Value if the method is not found, and calling the zero Value panics.

Dynamic dispatch use cases

Dynamic function calls are used in: plugin systems (call functions by string name), test frameworks (call TestXxx functions), RPC dispatchers (map method names to handlers), and decorators.

Performance

reflect.Call is ~10-50× slower than a direct call. Cache the reflect.Value of frequently called functions; avoid in hot loops.

Type-safe wrapper

Wrap dynamic calls in a typed helper that performs the reflect machinery once during startup and calls the cached Value repeatedly:

type Processor func(context.Context, []byte) ([]byte, error)
// Build at startup, reuse at request time

Error return handling

If the function returns an error as its last result, extract and check it:

results := fn.Call(args)
if !results[len(results)-1].IsNil() {
    return results[len(results)-1].Interface().(error)
}

Alternatives to reflection

If the set of functions is known at compile time, use a map of function values (map[string]func(...)) instead of reflection — it is safer and faster.

handlers := map[string]func(ctx context.Context, req []byte) []byte{
    "greet": greetHandler,
}

Quick Check

What does reflect.Value.Call return?

Answer it in the interactive lesson

Recap: Dynamic Function Calls

Key points:

Value.Call([]reflect.Value) invokes the function; returns []reflect.Value
Check m.IsValid() before calling MethodByName result
MakeFunc creates a new function with a given type at runtime
Prefer map[string]func over reflection for known function sets

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Yes — any 3 lessons of the Go Academy learning path, including “Dynamic Function Calls”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Go Academy course includes 4 lessons in total.

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