From: Tony Arcieri <tony@...>
Date: 2009-05-20T07:23:20+09:00
Subject: Re: Pythonic indentation (or: beating a dead horse)

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Let me get right to the heart of the issue here.  It really comes down to
this:

On Tue, May 19, 2009 at 3:40 PM, J Haas <Myrdred@gmail.com> wrote:

> I think code blocks are cool, and I love Ruby's very flexible
> expressiveness. I dig the way every statement is an expression


These are both incompatible with a Python-style indentation sensitive
syntax.  You can have the Pythonic indent syntax or a purely expression
based grammar with multi-line blocks.  You can't have both.

In Python, all indent blocks are statements.  This is why Python can't have
multi-line lambdas using Python's indent rules: lambdas are only useful as
expressions, but all indent blocks in Python are statements.  The same issue
carries over to blocks, as a good deal of the time you want a method which
takes a block to return a value (e.g. map, inject, filter, sort, grep)

There's quite an interesting interplay of design decisions to make Python's
indent-sensitive grammar work the way it does.  Indent blocks in Python have
no terminator token, whereas every expression in a Ruby-like grammar must be
terminated with ";" or a newline.  This works because Python's expressions
are a subset of its statements, so it can have different rules for
statements versus expressions.

Implicit line joining works in Python because the only syntactic
constructions which can exist surrounded in [...] (...) {...} tokens are
expressions, so you can't put an indent block inside of these.  If you have
an indent-sensitive Ruby with implicit line joining, you limit the
expressiveness of what you can do inside any syntactic constructs enclosed
by these tokens.

If you want to have indent blocks in a purely expression-based grammar, you
need to use a syntax more like Haskell.  I've seen a somewhat Python-looking
language called Logix which uses Haskell's indent rules.  It was created by
Tom Locke, who has since gone on to author Hobo in Ruby, and for what it's
worth now says he prefers Ruby's syntax.  Go figure.

P.S. I tried to make a Ruby-like language with an indentation-sensitive
syntax.  These are the lessons I learned.  I gave up and added an "end"
keyword.

-- 
Tony Arcieri
medioh.com

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