I recently criticized Swift 3 for allowing many expressions with no side
effects as statements, in this blog post:
https://blogs.synopsys.com/software-integrity/2017/03/24/swift-programming-language-design-part-2/
(search for "that expression"). I've seen some related discussion such as
"Make non-void functions @warn_unused_result by default", but not quite this.
For emphasis, let me include a real example (from open-source software,
simplified) of defective JavaScript code that happens to be legal Swift as well:
var html = "<table class='modification'>" +
"<tbody>" +
trs
"</tbody>" +
"</table>";
DEFECT SPOILER: (There is a
missing '+')
Part of my argument is that people commonly ignore compiler warnings. We see
lots of defective code that would be (or is) caught by compiler warnings but
people don't pay attention.
I have not formulated this into a detailed proposal, but I suspect that because
of Swift's pervasive overloading, implicit constructor calls, etc., it would
involve introducing a new @error_unused_result annotation and using that in
many places in the standard library. I also suggest that user overloads of
traditionally non-side-effecting operators, as well as non-mutating struct
methods, be @error_unused_result by default, or perhaps by mandate. Our
experience also suggests this @error_unused_result feature could also be useful
for ordinary methods of classes, as we find a number of defects where a method
call is expecting a side effect but there is none, because the method is only
useful for its return value.
If you would like to see more defect examples from open-source software (other
languages for the moment), I should be able to dig up some more.
Thanks
--
Peter Dillinger, Ph.D.
Software Engineering Manager, Coverity Analysis, Software Integrity Group |
Synopsys
www.synopsys.com/software
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