Blog / Go Language Spec

Evaluation of function parameters

December 15, 2023

Calls

If f denotes a generic function, it must be instantiated before it can be called or used as a function value.

We’ve seen this rule applied to methods and other types. It should be standard by now, so I won’t dwell on it.

In a function call, the function value and arguments are evaluated in the usual order. After they are evaluated, the parameters of the call are passed by value to the function and the called function begins execution. The return parameters of the function are passed by value back to the caller when the function returns.

We haven’t gotten to the “order of evaluation” yet, but a preview is in order. TL;DR; function parameters are evaluated in left-to-right order, then the function itself is called with the results. That means given the following function call:

a(b(), c(d(), e()))

The nested functions would be called in the following order:

  1. b
  2. d
  3. e
  4. c
  5. a

Quotes from The Go Programming Language Specification Version of August 2, 2023

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Refresher on methods

In yesterday’s email, I failed to include the example included in the spec, which doesn’t make sense to include otherwise. I’ve updated the web version of the email in case you’re super curious to see what I left out. We’ve already covered these details, but they’re logically mentioned again in the section about function calls, so we’ll cover them here: Calls … A method call x.m() is valid if the method set of (the type of) x contains m and the argument list can be assigned to the parameter list of m.

Special case: Multi-value arguments to functions

You may recall that when we started the section on function calls, we were told about a “special case”… well here it is! Calls … 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.

nil functions

Calls … Calling a nil function value causes a run-time panic. So, calling a nil function causes a panic. Sensible enough. But when would you ever run across a nil function? Let’s look at some examples. This is probably the most obvious example, and perhaps one that popped to your mind: var fn func(int) // fn is of type func(int), but uninitialized, so nil fn(3) // run-time panic But let’s consider another example, which can be a bit more confusing.

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