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Prefix Operators in Haskell

Posted on January 12, 2014 by oubiwann


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I wanted to name this post something a little more catchy, like "McCarthy's Dream: Haskell as Lisp's M-Expressions" but I couldn't quite bring myself to do it. If s-expressions had greater support in Haskell, I would totally have gone for it, but alas, they don't.

However, there is still reason to celebrate: many Haskell operators do support prefix notation! This was a mind-blower to me, since I hadn't  heard about this until last night...

At the Data Day Texas conference this past Saturday, O'Reilly/S trataConf had a fantastic booth. Among the many cool give-aways they were doing, I obtained a coupon for a free ebook and another for 50% off. Upon returning home and having made my free book decision, I was vacillating between an OCaml book and the famous Haskell one. I've done a little Haskell in the past but have never touched OCaml, so I was pretty tempted.

However, something amazing happened next. I stumbled upon a page that was comparing OCaml and Haskell, which led to another page... where Haskell prefix notation was mentioned. I know many Haskellers who might read this would shrug, or say "yeah, we know", but this was quite a delightful lit tle discovery for me :-)

I don't remember the first page I found, but since then, I've come across a couple more resources:
That's pretty much it, though. (But please let me know if you know of or find any other resources!)

As such, I needed to do a lot more exploration. Initially, I was really excited and thought I'd be able to convert all Haskell forms to s-expressions (imports, lets, etc.), but I quickly found this was not the case. But the stuff that did work is pretty cool, and I saved it in a series of gists for your viewing pleasure :-)

Addition

The first test was pretty simple. Finding success, I thought I'd try something I do when using a Lisp/Scheme interpreter as a calculator. As you can see below, that didn't work (the full traceback is elided). Searching on Hoogλe got me to the answer I was looking for, though. Off to a good start:


More Operators

I checked some basic operators next, and a function. Everything's good so far:


Lists

Here are some basic list operations, including the ever-so-cool list difference operator, (\). Also, I enjoyed the cons (:) so much that I made a t-shirt of it :-) (See the postscript below for more info.)


List Comprehensions

I'm not sure if one can do any more prefixing than this for list comprehensions:

< br />Functions

Same thing for functions; nothing really exciting to see. (Btw, these examples were lifted from LYHGG.)


Function Composition

Using the compose operator in prefix notation is rather... bizarre :-) It looks much more natural as a series of compositions in a lambda. I also added a mostly-standard view of the compose operator for comparison:


Monads

I've saved the best for last, an example of the sort of thing I need when doing matrix operations in game code, graphics, etc. The first one is standard Haskell...

/>Wow. Such abstract.So brains. 

Note that to make the prefix version anywhere close to legible, I added whitepsace. (If you paste this in ghci, you'll see a bunch of prompts, and then the result. If you're running in the Sublime REPL, be sure to scroll to the right.)

And that pretty much wraps up what I did on Sunday afternoon :-)

Other Resources
Post Script

If you're in the San Antonio / Austin area (or SF; I'll be there in March) and want to go in on a t-shirt order, let me know :-) We need at least six for the smallest order (tri-blend Next Level; these are a bit pricey, but they feel GREAT). I'll collect payment before I make an order.


Author oubiwann
Date January 12, 2014
Time 20:47:08
Category
Tags books clojure ddtx fp functional-programming haskell languages lisp m-expressions ocaml programming s-expressions scheme syntax texas
Line Count 1
Word Count 849
Character Count 8277
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