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digitalmars.D.bugs - [Issue 6843] New: Function to check whether std.conv.to will succeed

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=6843

           Summary: Function to check whether std.conv.to will succeed
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: jmdavisProg gmx.com



PDT ---
In some cases, std.conv.to will complain at compile time that it can't do a
particular conversion, but in others it depends on runtime values. For
instance, to!int("hello world") will fail at runtime. Currently, the only way
to know if such a conversion will succed is to call std.conv.to and then catch
the ConvException when it fails. However, unfortunately, exception handling is
_very_ expensive in D (as in an expression which throws an exception is more
than 100x slower than one that doesn't), so in cases where there's a good
chance that std.conv.to will fail (or when you just want to check whether it
will succeed or not) are _very_ expensive.

I would very much like to see a function in std.conv.to which returned whether
std.conv.to will succeed or not so that it will be efficient to check whether
std.conv.to will succeed or not. And actually, because it's not particularly
efficient to call such a function and then call std.conv.to (since that would
do the conversion twice - though that's still far more efficient than testing
with std.conv.to, since exceptions are so expensive), the function should
probably give the converted value as well if it succeeded.

For instance, we could have

bool tryTo(T, S)(S toConvert) {...}
bool tryTo(T, S)(S toConvert, out T result) {...}

The first version checks whether the conversion will succeed (probably doing
the full conversion) and returns whether it succeeded or not, whereas the
second version does the full conversion and not only returns whether it
succeeded or not, but it sets result to the result of the conversion (or T.init
upon failure - though out should do that automatically if result is never set
within the function).

We should probably have the same for std.conv.parse as well.

bool tryParse(Target, Source)(ref Source s) {...}
bool tryParse(Target, Source)(ref Source s, out Target result) {...}

It's probably not quite as useful for parse (particularly since tryParse would
still need to consume s in the first overload to be consistent), but it would
make parsing more efficient when parsing failures are common.

Regardless, tryTo would make dealing with std.conv.to _much_ more efficient in
cases where conversion failures are common.

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next sibling parent d-bugmail puremagic.com writes:
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PDT ---
Oh, and std.conv.to and std.conv.parse would probably use tryTo and tryParse
respectively internally in order to reduced code duplication, but I suppose
that that's up whoever implements them.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs eml.cc



This seems a kind of dupe of bug 6840

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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PDT ---

template which wraps whether std.conv.to succeeds by catching the exception.
This is highly inefficient and ends up creating a new type in the deal, which
is messier IMHO.

What I'm suggesting here is that a new function be created which doesn't risk a
ConvException being thrown at all, completely avoiding the huge inefficiency of
catching the thrown exception.

And if we were to do something like maybeTo, I'd be tempted to find a way to
generalize it beyond std.conv.to to encompass any function which returns a
value and could have thrown an exception (probably by taking the function as
aliased template argument) rather than making it specific to std.conv.to. But
that's so insanely inefficient, that I wouldn't be in hurry to add something
like that to Phobos.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 template which wraps whether std.conv.to succeeds by catching the exception.
 This is highly inefficient and ends up creating a new type in the deal, which
 is messier IMHO.
 
 What I'm suggesting here is that a new function be created which doesn't risk a
 ConvException being thrown at all, completely avoiding the huge inefficiency of
 catching the thrown exception.


You have missed this part of that bug 6840:


 2) This simple implementation of maybeTo contains a try-catch, but there is no
 need to actually throw and catch an exception in all wrong conversion cases.
 Example: when Tin is string and Tout is int, maybeTo is free to use something
 lighter, like a C function that produces no exceptions, instead of to!(). I
 think this allows the compiler to perform some extra optimizations.


If you create a conversion function that doesn't throw, then it is useful to
create a maybeTo :-)

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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PDT ---
And this enhancement request is for a conversion function which doesn't throw.
And once you have such a function, I see no need for maybeTo, but that's a
separate argument. Regardless, maybeTo _without_ a separate conversion function
which doesn't throw is too inefficient to even consider IMHO.

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prev sibling parent d-bugmail puremagic.com writes:
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PDT ---
Another possibility would be a function which did the same thing as std.conv.to
except that it returned success, and the actual result was returned via an out
parameter. That's even more similar to maybeTo, but again, a key difference is
that there is zero throwing going on whatsoever and std.conv.to would not be
benig wrapped, since the key thing we need here IMHO is to avoid throwing for
efficiency's sake. This would probably require breaking out many of the toImpl
functions so that they use this new function and then throw on failure (in
order to avoid code duplication).


problem though.

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Sep 06 2013