[ruby-core:25662] [Feature #1408] 0.1.to_r not equal to (1/10)

Issue #1408 has been updated by Marc-Andre Lafortune.


Sorry to be late to the party on this one.

It is important to remember that a Float is always an approximation.

1.0 has to be understood as 1.0 +/- EPSILON, where the EPSILON is platform dependent. 1.0 is not more equal to 1 than to 1 + EPSILON/2. Indeed, there is no way to distinguish either when they are stored as floats.

To believe that Float#to_s loses data is wrong. If r.to_s returns "1.2", it implies that 1.2 is one of the values in the range of possible values for that floating number. It could have been 1.2000...0006. Or something else. There is no way to know, so #to_s chooses, wisely, to return the simplest value in the range.

There are many rationals that would be encoded as floats the same way. There is no magic way to know that the "exact" value was exactly 12/10 or 5404319552844595/4503599627370496, or anything in between. All have the same representation as a float. There is no reason to believe that the missing (binary) decimals that couldn't be written in space allowed where all 0. Actually, there is reason to believe that they were _probably_ non zero, because fractions that can not be expressed with a finite number of terms in their expansion in a given base all have a recurring expansion. I.e. if the significand does not end with a whole bunch of zeros (rational has finite expansion) then it probably ends with an infinite pattern (say 011011011 in binary, or 333333 in decimal).

For any given float, there is one and only one rational with the smallest denominator that falls in the range of its possible values. It is currently given by Number#rationalize, and I really do not understand why #to_r would return anything else. 

I cannot see any purpose to any other fraction. Moreover, the current algorithm, which returns the middle of the range of possibilities, is platform dependent since the range of possibilities is platform dependent. That makes it even less helpful.

Is there an example where one would want 0.1.to_r to be 3602879701896397/36028797018963968 ?  
Do we really think that 0.1.to_r to be 3602879701896397/36028797018963968 corresponds to the principle of least surprise?
Note that I'm writing that fraction but with a different native double encoding, the fraction would be different.

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