Date: Wed, 15 Jul 2009 19:55:15 +1200 From: Greg Ewing greg.ewing@canterbury.ac.nz To: python-ideas@python.org Subject: [Python-ideas] Where-statement (Proposal for function        expressions) Message-ID: 4A5D8B63.4040509@canterbury.ac.nz Content-Type: text/plain; charset=UTF-8; format=flowed

Carl Johnson wrote:

...

My suggestion for future would be language extenders is that they try to solve a broader problem than just the "how do I make callbacks more convenient" problem.

One such extension might be a "where" block. Applied to the current problem:

foo(f) where:     def f(x):       ...

I was just thinking of something similar:

in: foo(f,a) let: def f(x): ... a = ...

(Keywords open to debate.) It's a little verbose, but I dislike it much less than all the proposals that introduce special sigils or do weird things to the syntax of the code. Things I like about it: there's a clear warning (the "in:") right at the start that the statement (or block?) is special and relies on code that comes after; the syntax isn't complex; both lets you provide a name and prevents it leaking into places you don't want it; and the contents of both the "in:" block and the "let:" block are normal Python. I'd expect the behaviour to be something like this: execute the "let:" block in its own nested scope, then execute the "in:" block in the regular local scope but with the symbols defined by the "let:" block available. Does that make some sort of sense?

Most of the things I like are achieved by Greg's "where:" block, the only thing I don't like about it is tacking a keyword onto the end of a statement, which feels a bit awkward to me. That may just be aesthetics. I certainly like the "where:" idea a lot better than the other proposals. It seems to achieve the most utility with the least confusing syntax.