[Python-ideas] Statements vs Expressions... why?

[Cliff Wells]

On Wed, 2008-09-10 at 15:57 -0600, Adam Olsen wrote:
> On Wed, Sep 10, 2008 at 3:18 PM, Cliff Wells <cliff at develix.com> wrote:
> > Further, I feel that this limitation forces programmers into using hacks
> > and magic or overly spread-out code, which itself leads to readability
> > concerns.  Having used Python for around a decade, I'm quite aware of
> > the fact that you can happily write tons and tons of nice code with
> > Python in its current state.  However, because of the direction of my
> > work (developing a internal DSL in Python) I've suddenly become aware of
> > this glass ceiling.  I'd bumped into it before back when I was doing a
> > lot of GUI development, but blamed it on lambda rather than realizing
> > that it wasn't lambda so much as what I am bringing up now.
> 
> Python is not intended for DSLs.  Really, don't do it.  Python is for
> python code.  

The DSL I work on *is* Python code.  And really, this is the first time
I've heard anyone assert anything like this.  Python is a
general-purpose language.  It's not VBA ;-)  

DSL's are an extremely useful concept.  To summarily dispatch the whole
of them with such an assertion is pretty much bolstering my argument:
you've just asserted that Python is inherently limited in scope.  

> If you want another language, write your own parser.  I
> hear lisp is simple to parse, and has no annoying statements to hold
> you back!

Ah, except Python is the language I like in every way, *except* for this
one particular wart.   Really, had I not entered new programming domains
and tried to take Python with me, I'd probably never have had a
complaint.   

Also, external parsers defeat the entire reasoning behind internal DSL's
(taking advantage of an established VM/compiler, requiring users to
learn a new syntax in addition to their primary programming language).

> Seriously though, there is an advantage to basing so much on
> statements rather than expressions.  We're specialized for one
> statement per line, which is the most common case,

Clearly it's the most common case in existing Python code since nothing
else is allowed.   But frankly, even Javascript doesn't follow this
idiom anymore.   Expression-oriented languages have seen a rebirth for
good reasons (although I admit I'm none-to-fond of many of them, for
various reasons).

>  and it allowed us
> to have extraneous semicolons, braces, or whatever.  

Not following this.  You mean to *not* have extraneous syntax?

> Readability benefits, style consistency benefits.

I strongly disagree.  The artificial distinction between statements and
expressions is the definition of *inconsistent*.  Why do we have two
versions of the "if" conditional?  Why do we have "for" loops *and* list
comprehensions?  They express the same ideas, but the limitations in one
required growing the language another direction.   In short, we could
have had a single, more powerful construct in place of two lesser
constructs and simultaneously had less syntax to memorize and more
consistency across the board.

> Now there are some use cases that suffer here, including the one you
> just gave: defining a dispatch dict with the functions inline.  The
> best I could do is define the dict first, then stick a decorator on
> each function to register them.  That's still ugly though.  A creative
> solution is needed, but none come to mind.

That's because there is none.  And this is my fundamental problem: it's
not so much that it's hard to do in Python, it's that you *cannot* do it
in Python.  No amount of creativity, time, or experience will help, and
this is disappointing.

I won't pretend that any example we might toss up in here won't appear
contrived, but there are definite cases where readability can be
substantially enhanced with such structures.

> An example where this has happened before is the with-statement, which
> is spectacularly successful IMO.  Now, you may notice it could have
> been done in a library rather than core syntax if generic anonymous
> blocks were allowed — so what?  The library is still part of the
> language!  It's still something that has to be learned.  And the
> syntax would be substantially uglier using a generic mechanism, rather
> than the specialized with-statement syntax.

The "so what" is that it could *only* be implemented by the core devs.
It was not possible for an average (or even above-average) Python
programmer to write such a library, whereas it *could* have been had the
language not prohibited it.  

Regards,
Cliff