Indentation and optional delimiters

bearophileHUGS at lycos.com bearophileHUGS at lycos.com
Tue Feb 26 14:36:57 CET 2008


This is the best praise of semantic indentation I have read so far, by
Chris Okasaki:
http://okasaki.blogspot.com/2008/02/in-praise-of-mandatory-indentation-for.html

A quotation:
>Imagine my surprise when I started teaching this language and found the students picking it up faster than any language I had ever taught before. As fond as I am of the language, I'm certainly under no illusions that it's the ultimate teaching language. After carefully watching the kinds of mistakes the students were and were not making, I gradually realized that the mandatory indentation was the key to why they were doing better.<

I have appreciated that article, and I have personally seen how fast
students learn Python basics compared to other languages, but I think
that it's way more than just indentation that makes the Python
language so quick to learn [see appendix].

I used to like indentation-based block delimiting years before finding
Python, and this article tells me that it may be a good thing for
other languages too, despite some disadvantages (but it's little
probable such languages will change, like the D language). Some people
have actually tried it in other languages:
http://people.csail.mit.edu/mikelin/ocaml+twt/
So I may try to write something similar for another language too.

One of the most common complaints about it is this written by James on
that blog:
>I prefer explicit delimiters because otherwise the line wrapping of code by various email programs, web mail, mailing list digesters, newsgroup readers, etc., often results in code that no longer works.<

A possible solution to this problem is "optional delimiters". What's
the path of less resistance to implement such "optional delimiters"?
Is to use comments. For example: #} or #: or something similar.
If you use such pairs of symbols in a systematic way, you have cheap
"optional delimiters", for example:

def insort_right(a, x, lo=0, hi=None):
    if hi is None:
        hi = len(a)
    #}
    while lo < hi:
        mid = (lo + hi) // 2
        if x < a[mid]:
            hi = mid
        #}
        else:
            lo = mid+1
        #}
    #}
    a.insert(lo, x)
#}

It looks a bit ugly, but a script is able to take such code even
flattened:

def insort_right(a, x, lo=0, hi=None):
if hi is None:
hi = len(a)
#}
while lo < hi:
mid = (lo + hi) // 2
if x < a[mid]:
hi = mid
#}
else:
lo = mid+1
#}
#}
a.insert(lo, x)
#}

And build the original Python code (it's possible to do the opposite
too, but it requires a bit more complex script). Such #} may even
become a "standard" (a convention. Not something enforced by the
compiler. What I'd like to see the Python compiler enforce is to raise
a syntax error if a module mixes tabs and spaces) for Python, so then
it's easy to find the re-indenter script when you find flattened code
in some email/web page, etc.

-------------------------------

Appendix:
I believe there can exist languages even faster than Python to learn
by novices. Python 3.0 makes Python even more tidy, but Python itself
isn't the most semantically clear language possible. I have seen that
the widespread reference semantics in Python is one of the things
newbies need more time to learn and understand. So it can be invented
a language (that may be slower than Python, but many tricks and a JIT
may help to reduce this problem) where

a = [1, 2, 3]
b = a
Makes b a copy-on-write copy of a, that is without reference
semantics.
Other things, like base-10 floating point numbers, and the removal of
other complexity allow to create a language more newbie-friendly. And
then I think I too can see myself using such simple to use but
practically useful language for very quick scripts, where running
speed is less important, but where most possible bugs are avoided
because the language semantics is rich but very clean. Is some one
else interested in such language?
Such language may even be a bit fitter than Python for an (uncommon)
practice called "real time programming" where an artist writes code
that synthesizes sounds and music on the fly ;-)

-------------------------------

Bye,
bearophile



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