Hi John,
By :pre, do you mean function preconditions? eg. (fn [] {:pre [..]}) ?
How is :pre related to metadata and dispatch? AFAICT it's purely for
macroexpansion and
there is no metadata available on the precondition post-macroexpansion.
Thanks,
Ambrose
On Mon, May 27, 2013 at 9:00 PM, John D. Hume <[email protected]>wrote:
> On May 26, 2013 8:53 PM, "Mark Engelberg" <[email protected]>
> wrote:
> >
> > Another possible design choice is to store a domain-testing predicate in
> the function's metadata.
>
> Using metadata would be a much more idiomatic choice than using arity.
> Multiple arities are idiomatically used (like method overloading) to
> default arguments.
>
> An interesting detail James thought of but didn't call out is putting the
> domain check in the :pre metadata of the fn. This is an infrequently used
> Clojure feature, but it's a perfect fit for partial functions. Calling a fn
> with an argument that fails the :pre check will throw an exception, so the
> body can be written to assume it's got good input. Anyone can grab the :pre
> metadata to test for applicability, and anyone who knows clojure will have
> at least passing familiarity with the concept.
>
> Your macro can still pull apart the pattern matching forms to build the
> :pre fn, and it's still easy to do orElse-style composition.
>
> Unrelated: If you haven't already, you may want to read and follow up on
> the May 16 thread "core.match" before going very far with it.
>
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