On 16/08/2019 12:36 p.m., Hugh Parsonage wrote:
I was initially pretty shocked by the result in this question:
https://stackoverflow.com/questions/57527434/when-do-i-need-parentheses-around-an-if-statement-to-control-the-sequence-of-a-f
Briefly, the following returns 0, not 3 as might be expected:
if (TRUE) {
0
} else {
2
} + 3
At first I thought it the question was simply one of syntax
precedence, but I believe the result is too surprising to not warrant
note in the documentation of Control. I believe the documentation
should highlight that the `alt.expr` is demarcated by a semicolon or
newline and the end of a *statement*, not a closing brace per se.
Perhaps in the paragraph starting 'Note that it is a common mistake to
forget to put braces...' it should end with. "Note too that it is the
end of a *statement*, not a closing brace per se, that determines
where `alt.expr` ends. Thus if (cond) {0} else {2} + 2 means if (cond)
{0} else {2 + 2} not {if (cond) {0} else {2}} + 2."
I agree this is surprising, and should perhaps be pointed out in the
docs, but I don't think your suggestion is quite right. { 2 } + 3 is a
legal expression. It doesn't have to be the end of a statement that
limits the alt.expr, e.g. this could be one big statement:
{ if (TRUE) {
0
} else {
2
} + 3 }
What ends alt.expr is a token that isn't collected as part of alt.expr,
not just a semicolon (which separates statements) or a newline. I don't
know how many of those there are, but the list would include at least
semicolon, newline, }, ), ], and maybe others.
Duncan Murdoch
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