# [isabelle] Problem with pretty printing rationals

Dear all,

`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.)
`
Instead, I now added an axiom
consts show_rat

`axioms show_rat_def[code]: "show_rat (Fract a b) = (if b = 1 then
``show_int a else show_int a @ ''/'' @ show_int b)"
`

`Now, the code-generator is working. However, I do not like to add
``axioms to my theory, so is there a better way around? 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?
`
Thanks a lot,

`René
``
`

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