[Ironruby-core] Automatic conversion of bool to int

Dino Viehland dinov at exchange.microsoft.com
Fri Jan 11 12:17:40 EST 2008

And regarding your applicable targets question: Yes, infact there's two ways to specify the preference, hopefully that'll go back down to 1 someday :).  The ActionBinder provides the conversion helpers and has two methods related to this.  The first is the PreferConvert method where you're passed two types and you select which one you want.  The 2nd (and more likely to survive long term) is SelectBestConversionFor where you get the actual type passed in, and the two types that are candidates for method selection, as well as the current NarrowingLevel.  Currently we first consult SelectBestConversionFor and the last thing we do is check PreferConvert if we still haven't figured out the best method.

This is done on a parameter by parameter basis and so all of the parameters need to be better than the parameters for another method otherwise we'll have an ambiguous match.

From: ironruby-core-bounces at rubyforge.org [mailto:ironruby-core-bounces at rubyforge.org] On Behalf Of Tomas Matousek
Sent: Friday, January 11, 2008 9:16 AM
To: ironruby-core at rubyforge.org
Subject: Re: [Ironruby-core] Automatic conversion of bool to int

The conversions are wrong. In fact, they are quire arbitrary (the code is in fact partly copied from IronPython). We have an item on our todo list to reimplement them from scratch.


From: ironruby-core-bounces at rubyforge.org [mailto:ironruby-core-bounces at rubyforge.org] On Behalf Of Peter Bacon Darwin
Sent: Friday, January 11, 2008 5:10 AM
To: ironruby-core at rubyforge.org
Subject: [Ironruby-core] Automatic conversion of bool to int

In irb we have:

1 + true
=> TypeError: true can't be coerced into Fixnum

In rbx we have:

1 + true
=> 2

There are the following methods for + in Fixnum:

       public static object Add(int self, int other)
public static double Add(int self, double other)
public static object Add(CodeContext/*!*/ context, object self, object other)

I expected that passing a bool into + operator would have ended up at the third overload, which would give the correct behaviour when it tries to coerce on the bool.  But it actually gets mapped to the first overload and there is an implicit conversion of true to 1.  You can see this in the resulting AST dump:

    (.bound $arg0),
        (Object)(.bound $arg1),

After some digging I found this method in Ruby.Runtime.Converter:

        private static bool HasImplicitNumericConversion(Type fromType, Type toType) {
            if (fromType.IsEnum) return false;

            if (fromType == typeof(BigInteger)) {
                if (toType == typeof(double)) return true;
                if (toType == typeof(float)) return true;
                if (toType == typeof(Complex64)) return true;
                return false;

            if (fromType == typeof(bool)) {
                if (toType == typeof(int)) return true;
                return HasImplicitNumericConversion(typeof(int), toType);


Here you can see that bool is automatically converted to an int.  Is this correct behaviour?  I appreciate that we can take any non-nil value as true and nil as false but not the other way round.

ASIDE: In fact all three methods are added as "Applicable Targets" but the int version gets there first.  Is there a heuristic for the order in which overloads are considered?

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