Re: else ladders practice

Bart <bc@freeuk.com> wrote:

On 05/11/2024 12:42, Waldek Hebisch wrote:

Bart <bc@freeuk.com> wrote:

>
Then we disagree on what 'multi-way' select might mean. I think it means
branching, even if notionally, on one-of-N possible code paths.

 
OK.
 

The whole construct may or may not return a value. If it does, then one
of the N paths must be a default path.

 
 
You need to cover all input values.  This is possible when there
is reasonably small number of possibilities.  For example, switch on
char variable which covers all possible values does not need default
path.  Default is needed only when number of possibilities is too
large to explicitely give all of them.  And some languages allow
ranges, so that you may be able to cover all values with small
number of ranges.
 

 
What's easier to implement in a language: to have a conditional need for
an 'else' branch, which is dependent on the compiler performing some
arbitrarily complex levels of analysis on some arbitrarily complex set
of expressions...
 
...or to just always require 'else', with a dummy value if necessary?

Well, frequently it is easier to do bad job, than a good one.  However,
normally you do not need very complex analysis: if simple analysis
is not enough, then first thing to do is to simpliy the program.
And in cases where problem to solve is really hard and program can
not be simplified ("irreducible complexity"), then it is time for
cludges for example in form of default case.  But it should not
be the norm.

Even if you went with the first, what happens if the compiler can't
guarantee that all values of a selector are covered; should it report
that, or say nothing?

Compile time error.

What happens if you do need 'else', but later change things so all bases
are covered; will the compiler report it as being unnecesary, so that
you remove it?

When practical, yes.

Now, C doesn't have such a feature to test out (ie. that is a construct
with an optional 'else' branch, the whole of which returns a value). The
nearest is function return values:
 
  int F(int n) {
      if (n==1) return 10;
      if (n==2) return 20;
  }
 
Here, neither tcc not gcc report that you might run into the end of the
function. It will return garbage if called with anything other than 1 or 2.

Hmm, using gcc-12 with your code in "foo.c":

gcc -Wall -O3 -c foo.c
foo.c: In function ‘F’:
foo.c:4:1: warning: control reaches end of non-void function [-Wreturn-type]
    4 | }
      | ^

 
gcc will say something with enough warning levels (reaches end of
non-void function). But it will say the same here:
 
  int F(unsigned char c) {
      if (c<128) return 10;
      if (c>=128) return 20;
  }

Indeed, it says the same.  Somebody should report this as a bug.
IIUC gcc has all machinery needed to detect that all cases are
covered.

--
                              Waldek Hebisch