CATCH and THROW I
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| Leo:
| Last time we discussed Signed and Unsigned. Today, I'd like you to tell me more about THROW. I know THROW expects a number on the data stack. When this number is zero, nothing happens. Other values cause Forth to abort the program, sometimes after printing "error: message such and such." For example, the result of -14 THROW is
Compile-only word (message # -14)
I'm sure there is more to tell about this mechanism.
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| Theo:
| Certainly. But let's take a detour first. You already know about EXECUTE. What do you expect the following does?
17 ' DUP EXECUTE .S
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| Leo:
| Easy. ' (Tick) finds DUP, and EXECUTE runs DUP's code. It does the same as a simple DUP.
[rtn] ( 17 17 ) ok
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| Theo:
| Indeed. One moment, let me define a word to ... O.K., you can tell me what EX does:
: EX BEGIN DEPTH 0> WHILE DROP REPEAT ;
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| Leo:
| In case DEPTH is not zero, numbers are dropped off the stack. Thus EX cleans up the stack.
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| Theo:
| O.K. Here's Puzzle # 1. What does this do:
EX 1 ' DUP CATCH .S
Let me add that in this case CATCH has almost the same effect as EXECUTE. The difference is that CATCH additionally places a zero on the stack. Deduce the answer before pressing [rtn] .
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