"as" keyword woes

[MRAB]

Aaron Brady wrote:
> On Dec 9, 12:40 pm, MRAB <goo... at mrabarnett.plus.com> wrote:
>> Aaron Brady wrote:
>>> On Dec 9, 8:28 am, MRAB <goo... at mrabarnett.plus.com> wrote:
>>> snip
>>>> In some languages (I think Delphi is one of them - it's been a while!)
>>>> some words which would normally be identifiers have a special meaning in
>>>> certain contexts, but the syntax precludes any ambiguity, and not in a
>>>> difficult way. "as" in Python was one of those.
>>>> I certainly wouldn't want something like PL/I, where "IF", "THEN" and
>>>> "ELSE" could be identifiers, so you could have code like:
>>>>      IF IF = THEN THEN
>>>>          THEN = ELSE;
>>>>      ELSE
>>>>          ELSE = IF;
>>>> Seehttp://en.wikipedia.org/wiki/PL/I_(programming_language).
>>> snip
>>> That is, 'certainly' doesn't change the meaning of your statement
>>> any.  You wouldn't want it, but King George III didn't want the
>>> American Revolution.
>> It's called emphasis.
> 
> I just take you to have meant, then, +1 on excluding keywords from
> identifiers.  You said it the long way though, so I thought I missed
> something deeper, that didn't come across.
> 
IIRC, most computer languages have an LL(1) grammar, which means that 
when they are parsed you need to look at only the next word. If you're 
about to parse a statement and the next word is "IF" then you know it's 
an IF-statement, if it's an identifier then it's either a call or an 
assignment statement (OK, you don't know exactly what kind of statement 
it is at that point, but it works out just fine!).

In the example from PL/I, "IF" could be the start of an IF-statement "IF 
<condition> THEN" or an assignment statement "IF = <expression>". It's a 
bit more tricky for the parser as well as the programmer.

Life is easier if words having special meanings are reserved.

However, that doesn't mean that all special words /must/ be reserved 
(pragmatism beats purity). Sometimes the syntax makes it clear and 
unambiguous, so you can get away with not making it a reserved word. The 
word "as" in Python doesn't need to be reserved because the syntax 
precludes ambiguity, but it's the only such word in the language, so 
it's just tidier to make it reserved too.