Return values of function as input arguments to another

Question

If I have

func returnIntAndString() (i int, s string) {...}

And I have:

func doSomething(i int, s string) {...}

Then I can do the following successfully:

doSomething(returnIntAndString())

However, let's say I want to add another argument to doSomething like:

func doSomething(msg string, i int, s string) {...}

Go complains when compiling if I call it like:

doSomething("message", returnIntAndString())

With:

main.go:45: multiple-value returnIntAndString() in single-value context
main.go:45: not enough arguments in call to doSomething()

Is there a way to do this or should I just give up and assign the return values from returnIntAndString to some references and pass msg and these values like doSomething(msg, code, str) ?

Answer 1

It's described here in the spec. It requires the inner function to return the correct types for all arguments. There is no allowance for extra parameters along with a function that returns multiple values.

As a special case, if the return values of a function or method g are equal in number and individually assignable to the parameters of another function or method f, then the call f(g(parameters_of_g)) will invoke f after binding the return values of g to the parameters of f in order. The call of f must contain no parameters other than the call of g, and g must have at least one return value. If f has a final ... parameter, it is assigned the return values of g that remain after assignment of regular parameters.

func Split(s string, pos int) (string, string) {
  return s[0:pos], s[pos:]
}

func Join(s, t string) string {
  return s + t
}

if Join(Split(value, len(value)/2)) != value {
  log.Panic("test fails")
}

If those specific conditions are not met, then you need to assign the return values and call the function separately.