quick Forth question

Richard A. Smith richard at laptop.org
Mon Jan 25 12:29:43 EST 2010


On 01/25/2010 10:59 AM, Daniel Drake wrote:
> Can anyone help me with a tiny Forth script? Can never quite get my
> head around the language.
>
> I'm trying to set up an if-else based on whether a mfg tag exists (or
> whether writing a mfg tag succeeded or not)
>
> I'm trying:
>
>      add-tag ak 0 catch if 2drop ." Laptop already activated" cr then

add-tag is look ahead word (so it can parse your argument) which doesn't 
work like you expect and catch needs a reference to what you are trying 
to run.  Plus if the tag does not exist then add-tag is going to reboot. 
  I'm not sure exactly how to make it work using add-tag.

I'll have to ask Mitch.

> But, if ak already exists, it simply says:
>      Tagname already exists
> ...rather than executing my conditional code.
>
> Also experimented with find-tag but couldn't figure it out.

find-tag is much better. find-tag leaves success or failure on the stack 
and if its true then has the string value of the tag.

The following will do what you want

" ak" find-tag if 2drop ." Laptop already activated" cr then

-- 
Richard A. Smith  <richard at laptop.org>
One Laptop per Child


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