Re: [isabelle] Problem with pretty printing rationals



Hi René,

I just encountered the following problem. When writing a pretty printer for rational numbers the standard function I would like to write is:

show_rat (Fract a b) = (if b = 1 then show_int a else show_int a @ ''/'' @ show_int b)"

However, since Fract is a only a code_datatype and not a datatype, Isabelle refused this function definition. (And it must refuse, since Fract 1 2 = Fract 2 4, but show_rat .. = 1/2 != 2/4 = show_rat .. Hence, show_rat is not a function.)

Right. This cannot work.

Are there for example functions like "get_numerator" and "get_denominator" which return unique results independent of whether using (Fract 1 2) or (Fract 2 4) which I overlooked in Rational.thy?

There aren't, but it seems that they should be there, so contributions
are welcome (probably a function that returns (int * int) is best). It
can be defined using THE.

Alex






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