[go-nuts] Evaluation order in one liner with return + append

On Fri, Jul 25, 2014 at 2:44 PM, Nathan Clark wrote:

I have a simple function I'm using to parse command line arguments. Given
an option name (e.g. "--foo"), if it finds a match, it returns both the
subsequent option value and the argument array with those two elements
removed. When I do this with the following one liner:

return true, args[i+1], append(args[:i], args[i+2:]...)

I'm getting the argument at index [i+1] AFTER the removal of the two
elements, which is not what I intended. The following fixes the issue:

value := args[i+1]
return true, value, append(args[:i], args[i+2:]...)

Are arguments to return evaluated from right to left in general, or is
this a special case with `append`?

Thanks,

Nathan

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On Friday, July 25, 2014 6:11:22 PM UTC-4, kortschak wrote:

That is because the args[x+1] is not a function call.

See here for your expected behaviour:

http://play.golang.org/p/X2twv1IuHJ

And in the spec[1] (last paragraph).

... when evaluating the operands of an expression, assignment, or return
statement, all function calls, method calls, and communication
operations are evaluated in lexical left-to-right order.

For example, in the (function-local) assignment

y[f()], ok = g(h(), i()+x[j()], <-c), k()

the function calls and communication happen in the order f(), h(), i(),
j(), <-c, g(), and k(). However, the order of those events compared to
the evaluation and indexing of x and the evaluation of y is not
specified.

[1]http://golang.org/ref/spec#Order_of_evaluation: