Adding to Kim-Ee's point, 1) When you're talking about invertible functions, the idea you're probably reaching for is an isomorphism -- that is, we want the function to have certain nice properties on top of just being a map from a -> b with an inverse map from b -> a. You also want the function to be a bijection, which is captured in the notion of an isomorphism. 2) Iso from Lens composes with the normal function composition operator (.) instead of rappend -- which is a little more convenient. Arjun On Mon, Mar 17, 2014 at 9:28 AM, Kim-Ee Yeoh wrote:

Hi Javran,

1. Have you looked at iso lens? The lens library contextualizes isomorphisms among other interesting maps. Worth looking into.

2. Your code looks nicely idiomatic. You must have worked hard observing models of good haskell.

3. It's easy to declare at the type-level: a->b and b->a. It's just that the types don't say anything about whether they are isos or not. Whereas that's what we want.

Typically when I have such a pair of functions, I lean on quickcheck to give me a rapid verify as I tweak away, e.g.:

prop_encdecOk :: String -> Bool prop_encdecOk xs = xs == (decode . encode $ xs)

-- Kim-Ee

On Mon, Mar 17, 2014 at 2:44 PM, Javran Cheng wrote:

...

Hi,

These days I find the notion of "inverse function" might be useful, the basic idea is to keep a pair of function f and g which are the inverse functions of each other and then manipulate on this pair of functions.

The detail is both on my blog post:

http://javran.github.io/posts/2014-03-17-capture-the-notion-of-invertible-fu...

and also code review:

http://codereview.stackexchange.com/questions/44550/capture-the-notion-of-in...

I think this is an interesting idea and want to share it with you. Advice and comments are welcomed and appreciated since I learn haskell through LYAH and some wiki pages and still not sure about what would be the most idiomatic way of doing it in haskell.

Thanks,

-- Javran (Fang) Cheng

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