2533: Generic functions that take integral arguments should work the same way as their prelude counterparts

Simon Peyton-Jones simonpj at microsoft.com
Tue Aug 26 05:20:09 EDT 2008

| >> I've actually long wondered about this: why don't more functions use
| >> Nat where it'd make sense? It can't be because Nat is hard to define -
| >> I'd swear I've seen many definitions of Nat (if not dozens when you
| >> count all the type-level exercises which include one).
| >
| > Because naive definitions are dog-slow and fast definitions are anything
| > but easy to use?

I doubt that even GHC is going to optimise
        data Nat = Z | S Nat
        data Nat = N Int#
(with appropriate checks) anytime soon.

I think the main reason that the latter (which can easily be implemented as a library) is not more widely used is that it's tiresomely incompatible with functions that produce Ints.  Also perhaps if length :: [a] -> Nat, then computing the difference between two lengths (length xs - length ys) could produce a runtime error.

But these are all matters of taste, software engineering, and inertia (legacy code). It'd be a worthwhile exercise to try making a version of the standard libs using Nat where it's appropriate, and see how convenient or otherwise it turns out to be.


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