Re: [isabelle] int vs. of_nat



The reasons are historical. Function "int" belongs to the original construction of the integers. When (a few years ago) I reworked the numeric types to take advantage of type classes, I realised that a polymorphic "of_nat" function could easily be defined. Naturally most developments should use of_nat rather than int, but function "int" appears to be necessary to develop the integers in the first place and to establish the type class axioms.

Note that "of_nat" has a single polymorphic definition. There is a similar function of_int, and we could easily define of_rat too, all using type classes. On the other hand, "real" is merely an overloaded constant, which is far less attractive.

Larry


On 8 Jun 2007, at 00:22, Brian Huffman wrote:

Isabelle's standard library has two different coercions from the naturals to the integers: IntDef.thy defines "int::nat=>int" which is monomorphic, and
Nat.thy defines "of_nat::nat=>'a" which coerces from naturals into an
arbitrary semiring. At type "nat=>int" they both mean exactly the same thing (see lemma IntDef.int_eq_of_nat). The only difference is that they are each
set up with different sets of simp rules.

My question is, why do we have both? If any of you have chosen to use one over
the other, which one did you choose, and why?

Unless there is a compelling reason to keep both, I would propose to either make "int" an abbreviation for "of_nat::nat=>int", or just eliminate it altogether. Also, if users prefer the way the simplifier works with "int",
then the simp rules for "of_nat" should be changed accordingly.

I could also ask the same thing about the overloaded function "real::'a=>real" from the HOL-Complex library, which is redundant with "of_nat::nat=>real" and "of_int::int=>real" at those types. Do we need separate constants for these?






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