Symbols as parameters?

Andre Engels andreengels at gmail.com
Thu Jan 21 09:18:32 CET 2010


On Thu, Jan 21, 2010 at 8:43 AM, Martin Drautzburg
<Martin.Drautzburg at web.de> wrote:
> Hello all,
>
> When passing parameters to a function, you sometimes need a paramter
> which can only assume certain values, e.g.
>
>        def move (direction):
>                ...
> If direction can only be "up", "down", "left" or "right", you can solve
> this by passing strings, but this is not quite to the point:
>
>        - you could pass invalid strings easily
>        - you need to quote thigs, which is a nuisance
>        - the parameter IS REALLY NOT A STRING, but a direction
>
> Alternatively you could export such symbols, so when you "import *" you
> have them available in the caller's namespace. But that forces you
> to "import *" which pollutes your namespace.
>
> What I am really looking for is a way
>
>        - to be able to call move(up)
>        - having the "up" symbol only in the context of the function call
>
> So it should look something like this
>
> ... magic, magic ...
> move(up)
> ... unmagic, unmagic ...
> print up
>
> This should complain that "up" is not defined during the "print" call,
> but not when move() is called. And of course there should be as little
> magic as possible.
>
> Any way to achieve this?

It probably can be done, but in my opinion even it can, it would be
ugly. I would just define "up" as some global constant - I don't see
why it would be a problem that it's defined when you don't need it. In
fact, I think it would be beneficial - you can then have
direction-valued variables, so that you can (for example) implement
"go the same direction I went last time". If that kind of thing
happens more often, it might be useful to have a Direction class, to
be able to easily program in things like "the opposite of up is down"
and "right from right is back".



-- 
André Engels, andreengels at gmail.com



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