List comprehension confusion...

[Lulu of the Lotus-Eaters]

|> Peter Hansen <peter at engcorp.com> wrote previously:
|> I actually kinda agree.  Most people dread lambdas because of what they
|> are (the semantics).  Myself, I only have trouble with their spelling.

|Yeah, it's basically the syntax.  I find the missing return statement
|bothersome, somewhere in the back of my mind, and also the parentheses
|that aren't there around the arguments...

Ah...  I think you haven't quite "got" the gestalt switch yet, Peter.
The reason I use lambdas at all is BECAUSE of the lack of return
statment... that helps express a concept.  Here's an explanation that I
hope might help:

A *pure* function is simply a type of mapping between a domain and a
range.  In mathematics, it is no problem to express infinite domains and
ranges (or just large ones); but in programs it is often convenient to
express a calculable relationship algorithmically rather than
statically.  The thing to understand is that these are not fundamentally
different creatures, just different conveniences.  For example, compare:

    _2xdict = {1:2, 2:4, 3:6}
    _2xfunc = lambda i: i*2
    print _2xdict[2]
    print _2xfunc(2)

Obviously, the dict has a restricted domain compared to the function,
but within their common domain, they do exactly the same thing.

I will often explictly name a function using a lambda form when I want
to emphasize that it really is a pure function, acting as a mapping.  A
mapping does not "return", it just has a correspondence between objects,
hence no 'return' statement.

Of course, it is easily possible to smuggle side-effects into a lambda
form in various ways.  So it's not really true that every lambda is a
pure function.  But as a convention, I find it helpful to use a lambda
as a way of indicating that I am eschewing side effects (or at least
making them "safe" ones).

Yours, Lulu...

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