Re: [isabelle] Problem with pretty printing rationals

The function should be called "normalize :: rat => int * int" and should
also take care of the sign: the denominator should be positive. Since
gcd is there already, it is easy to define directly. Any takers?

I have a solution which (I think) is too long for this mailing list. However, please contact me if you want to have the theory-file.



Alexander Krauss wrote:
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.


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