On 1/5/2010 1:31 PM, r0g wrote:
Michi wrote:
On Jan 4, 1:30 pm, Steven D'Aprano
<ste...@remove.this.cybersource.com.au> wrote:
In some, limited, cases you might be able to use the magic return value
strategy, but this invariably leads to lost programmer productivity, more
complex code, lowered readability and usability, and more defects,
because programmers will invariably neglect to test for the special value:
I disagree here, to the extent that, whether something is an error or
not can very much depend on the circumstances in which the API is
used. The collection case is a very typical example. Whether failing
to locate a value in a collection is an error very much depends on
what the collection is used for. In some cases, it's a hard error
(because it might, for example, imply that internal program state has
been corrupted); in other cases, not finding a value is perfectly
normal.
A pattern I have used a few times is that of returning an explicit
success/failure code alongside whatever the function normally returns.
While subsequent programmers might not intuit the need to test for
(implicit) "magic" return values they ought to notice if they start
getting tuples back where they expected scalars...
def foo(x)
if x>0:
return True, x*x
else:
return False, "Bad value of x in foo:",str(x)
ok, value = foo(-1)
if ok:
print "foo of x is", value
else:
print "ERROR:", value
Except that that is a reinvention of try-wheel:
def foo(x):
if x > 0:
return x*x
else:
raise MathError("Bad value of x in foo: %s" % x)
try:
print foo(-1)
except MathError, e:
print "ERROR: System integrity is doubted"
or rather; that is perhaps a good example of when to use 'assert'. If
the domain of foo() is positive integers, calling -1 on foo is a bug in
the caller, not foo().
I have been looking at Haskell recently and the way the pure functional
language handled exceptions and I/O gives me a new distinct "insight"
that exceptions can be thought of as a special return value that is
implicitly wrapped and unwrapped up the call stack until it is
explicitly handled.
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