On Tue, Jan 8, 2019 at 12:47 PM Eric Griffis <ded...@gmail.com> wrote:
> Apologies for the backtracking, but I'm still having trouble understanding
> before/after/between and ordinary function composition. For non-method
> functions, what's the difference?
>
Easy extension of existing code. For example:
(define/contract (process hsh)
(-> (hash/c symbol? any/c) list?)
(for/list ([key (sort (hash-keys hsh) symbol<?)])
(some-func key))
'process' is used a lot in your module and it gets its data from something
that generates hash keys as symbols. Now you'd like to connect this module
to a source that generates hash keys as strings -- maybe a database call.
You can either go in and modify 'process' throughout the module, or you can
modify the new source, or you can write a wrapper that converts the
argument to have symbols as keys. One easy way to do that is:
(require (only-in handy/hash hash-keys->symbols))
(before process hash-keys->symbols)
Or, perhaps you want to tighten the contract of 'process' from "returns a
list" to "returns a list of integers", but only in one module. You can use
with-contract, or you can write a wrapper function, or you can say
(after process (or/c (and/c (listof integer?) identity) (curry
raise-arguments-error 'oops "bad result"))
Or, maybe you want to ensure that, iff you're running in debug mode, all
calls to 'process' will log their inputs and outputs, but you don't want to
modify all the places where 'process' is called in the entire file. How
about this?:
; at top of file
(when is-debug-mode? (around process (lambda (hsh) (log-debug hsh) (define
result (inner hsh)) (log-debug result) result))
...and now you don't need to change it anywhere else.
> Also, I'd never heard of these functions as a Perl dev. A quick search
> turns up Moose. Is that what you're talking about?
>
Yep.
On Tue, Jan 8, 2019 at 2:28 PM Neil Van Dyke <n...@neilvandyke.org> wrote:
>
> [1] However, if `before` in the quoted example is actually only
> syntactic sugar for `define` with `apply`, then it's not mutating the
> original procedure, so I can put down the fire extinguisher. In that
> syntactic sugar case, whether to do it is a very local decision, of each
> module that internally opts to use that sugar. Maybe that sugar makes
> sense, for those particular modules that use it, within whatever
> project/organization maintains that source code.
This. In an ideal world, before/after/around would be parameterized so
that you can make the change only for a defined scope. Still, the intent
was never that it would extend its effects outside the current module.
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