72 lines
1.4 KiB
Forth
72 lines
1.4 KiB
Forth
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Some consider abort to be the most fundamental building block
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in writing programs. `trail` and `listen` abort execution.
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~~~
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:s:shout (s-) '!!->_ s:prepend '_<-!! s:append s:put nl ;
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{{
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:put-name (a-) fetch d:lookup-xt
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dup n:-zero? [ d:name s:put nl ] &drop choose ;
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---reveal---
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:trail (-0) '1_or_more_0s_left_on_the_stack s:shout
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repeat pop put-name again ;
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}}
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~~~
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`trail` adds at least one 0 ( FALSE flag ) on top of stack.
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```
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:t0 trail ;
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:t1 t0 ;
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:t2 t1 ;
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t2
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```
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Example of `0;` killing a session:
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```
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:t 'before_ s:put 0; 'after s:put nl ;
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#0 t (works
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#1 t (works
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reset t (kills_session
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```
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If this is dangerous, place a `guard` or a `check` before `0;` .
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~~~
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:d:ego (-s) d:last d:name compile:lit ; immediate
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:s:prepend;put s:prepend s:put nl ;
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:s:trail (s-0) '(_s:trail_)__ s:prepend;put trail ;
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:s:listen (s-) '(_s:listen_)__ s:prepend;put listen ;
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:guard (q-) call &trail -if ;
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:check (q-) call &listen -if ;
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:s:guard (sq-) call &drop &s:trail choose ;
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:s:check (sq-) call &drop &s:listen choose ;
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~~~
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`check` is less noisy than `trail` .
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```
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:t0 (q-) 'Doing... s:put nl
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d:ego '_charlie s:append swap s:guard
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'Next... s:put nl ;
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:u0 t0 ;
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:v0 d:ego '_calling_u0 s:append s:put;nl u0 ;
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nl nl
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&TRUE v0 nl
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&FALSE v0 nl
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```
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```
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:t1 (q-) 'Doing... s:put nl
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d:ego '_ckpt s:append swap s:check
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'Next... s:put nl ;
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:u1 t1 ;
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:v1 d:ego '_calling_u1 s:append s:put;nl u1 ;
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nl nl
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&TRUE v1 nl
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&FALSE v1 nl
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```
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