[doc] Document that func (*Call) Return can accept functions

[KrzysztofMadejski]

Functions that computes mock return arguments based on input arguments.

Example: mocking a io.Write function:

mf := mocks.RemoteFile{}
mf.On("Close").Return(tt.sftpErrorFileClose)
mf.On("Write", mock.AnythingOfType("[]uint8")).Return(
	func(input []byte) int { return len(input)}, 
	tt.sftpErrorFileWrite)

[boyan-soubachov]

Given that functions are first-class citizens in Go and a feature of the language, I would think that's implied. I'm not sure this needs documentation given it's a way the language works?

[KrzysztofMadejski]

Your case would make sense if passed function was returned as value. But it is invoked with called mock input arguments and only its return value is returned from mock. Definitely sth to document.

[mrog]

It seems that this behavior has changed. If you try it now, you'll see that the mock returns the function, not the result of calling the function.

package main

import (
	"testing"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/mock"
)

type mockGreeter struct {
	mock.Mock
}

func (m *mockGreeter) Greet(name string) string {
	args := m.Called(name)
	return args.String(0)
}

func TestThing(t *testing.T) {
	thing := new(mockGreeter)
	thing.On("Greet", mock.Anything).Return(func(name string) string { return "Hello " + name })
	assert.Equal(t, "Hello User1", thing.Greet("User1"))
	thing.AssertExpectations(t)
}

=== RUN   TestThing
--- FAIL: TestThing (0.00s)
panic: assert: arguments: String(0) failed because object wasn't correct type: %!s(func(string) string=0x104e97f10) [recovered]
	panic: assert: arguments: String(0) failed because object wasn't correct type: %!s(func(string) string=0x104e97f10)

I really wish there was a way to return the result of a function call, but I don't see how to do it with the current API.

[brackendawson]

@mrog that discussion is #350.

[brackendawson]

There is indeed no need to document this.