October 22, 2015

Another argument for s-expressions

I found myself back in our Python code base today working with a set of nested dictionary objects. As I set out to look for the equivalent of Clojure's get-in function, I ended up at a nifty solution using reduce. Inspired, I ended up with the following function in our Util module:

def get_in(d, keys, notfound=None):
    """Python implementation of Clojure's get-in function."""
        return reduce(dict.__getitem__, keys, d)
    except TypeError, exceptions.KeyError:
        return notfound

Of course this failed the unit test that I wrote:

class UtilTests(unittest.TestCase):
    def test_get_in(self):
        self.assertFalse(get_in({}, ["keya", "keyb"]), False)

The problem is with the except block of code in the get_in function - I should have passed in the multiple types as a tuple instead of a comma separated list. Implementing that small change causes the failing unit test to succeed.

It's pretty bad that python didn't complain as it was compiling the function definition. What's worse is that I made the mistake and didn't have a clear idea that it was a mistake (until I searched Google for try-except syntax). If python used s-expressions, this type of silly mistake would not have come up.

(define (get-in d keys notfound=None)
        (try (reduce dict.__getitem__ keys d)
        (except '(TypeError exceptions.KeyError)

Notice how it would be impossible to define a list of items to except without representing them as a list. In fact, the only syntax mistake I can really make with an s-expression language is forgetting to terminate an open expression with a closing parentheses. I want to love the ML syntax of Haskell, PureScript, Elm or Python but I'd rather use a handful of parentheses and spend the time thinking about the problem I'm solving.

It's a good thing I decided to write a unit test before shipping. :)

27 Dec 2015: I updated the title of this post to better reflect the fact that in addition to enabling macros, s-expressions also make the syntax rules easy for the humans.

Tags: s-expressions opinion PLT
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Dan Hable