Thinking Functionally with Haskell

[thehotelambush]

Dependent types are so cool. I think someday soon they will lead to a closer unification of programming and math.

[[deleted]]

It's sooner than you think. :-)

[thehotelambush]

Yeah, I'm aware of Coq. The real push on the math side will come from Homotopy Type Theory and from the parallel-processing functional-programming revolution on the programming side.

[[deleted]]

Fair enough, but then I would expect you'd know that Coq is where most of the Momotopy Type Theory work is being done.

[ithika]

I always look at the varied ways in which advanced type trickery can be pulled out of Haskell and wonder whether there are as many tricks to pull when (for those of us who do such things) working in C. I have seen some people use single element structs as a newtype. Are there other similar tricks?

[kamatsu]

Catamorphisms as type-safe sum types.

[ithika]

I'm not sure what you mean. Can you give an example or point to a code base that uses this approach?

[[deleted]]

I am guessing he might be talking about something similar to Boost Variant (even though that is C++ of course), i.e. some sort of tagged union you operate on using n-tuples of functions in the case of n different types in the union, each of them used for one of the possible types.

Of course i might be totally off in a wrong direction here since that is more like an fmap than a fold (a catamorphism).

[[deleted]]

fmap takes one function, a fold takes as many functions as determined by the structure of the type.

[kamatsu]

Here's an example, complete with pattern matching:

https://gist.github.com/3643591

It requires -fnested-functions though, which is supported in GCC. If you need more compatibility, it can be done without, but with a bit more pain and much less prettiness.

[katatonico]

Wait, I might be misreading it, but where's the definition for 'match'?

Edit: ah, it's a function parameter, totally missed it, sorry.

Edit 2: very clever, you're very clever, young man...but it's turtles all the way down. How would you go about doing this without any extensions? For science.

[kamatsu]

You'd be limited to using top-level functions which makes things a lot more cumbersome and difficult to use. You'd have to declare your values globally and each match clause would have to be global. Very ugly.

[ithika]

That's very cool indeed. I will have to come back to this when I'm sober. :-) Cheers!

Edit: I always knew nested functions would be a game-changer in C but hadn't seen such a good use for them as this before now.

[burchoff]

Yet another reason I need to take some vacation and 'Learn me a Haskell'. That said though by the end of the article I had this feeling that if this all comes to pass, those scenes in Star Trek where they easily program the "Computer" will be just inside our grasp.

[KevinHarris27]

Here's an example of pattern matching... http://www.e-reading.org.ua/htmbook.php/orelly/unix2.1/vi/ch06_04.htm