[Haskell-cafe] [Haskell] ANN: io-memoize 1.1

Dan Burton danburton.email at gmail.com
Thu Jun 12 19:16:32 UTC 2014

I created io-memoize a few years ago, around the same time that I also
created runmemo <http://hackage.haskell.org/package/runmemo>. I was
interested in "memoization" at the time, and was exploring various
memoization techniques. It was just for theory/fun and had no motivating
use case at the time.

I've been following various pipes/conduit discussions, and one particular
discussion about WAI involved an interest in a "call once" callback. It's
hard to enforce that sort of thing at the compile time without using
hardcore solutions such as linear types or indexed monads, so the obvious
solution is to use something like io-memoize to enforce it at runtime


Something else I've been working on lately is "the observer pattern" in
Haskell. When you subscribe to an observer, it will give you a
"subscription", which is just an action you can run to "unsubscribe"
yourself. Again, there's potential for "once" to be useful here for
implementing the unsubscribe action. If I manage to create something solid
and novel out of this observer pattern stuff, then I'll release it to
hackage as the "observable" package, or something like that.

Coincidentally, Simon pinged me about io-memoize with his use case in
HSpec. So it's something that's been on my mind, but Simon came to me with
a fresh Real Code use case that motivated the recent tweaks and the 1.1

On Thu Jun 12 2014 at 3:28:47 AM, Simon Hengel <sol at typeful.net> wrote:

> Hi Kim-Ee,
> > I may want a one-time only IO action, although so far the need hasn't
> > arisen yet. ;)
> >
> > In the spirit of pilfering while the buns are still hot out of the oven,
> is
> > there a backstory for this package you could possibly sketch out? Some
> > canonical use case maybe?
> I use it to implement beforeAll/beforeAllWith[1] in Hspec[2].  I have
> been reluctant to add them because used in the wrong way they lead to
> bad/order dependent test cases.  But there are some valid use cases.  So
> I decided to finally add them.
> Cheers,
> Simon
> [1] https://github.com/hspec/hspec/pull/173
> [2] http://hspec.github.io/
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