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<h1 class="chapter"> 10. Error Processing </h1>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#SEC372">10.1 Error Codes</a></td><td> </td><td align="left" valign="top">
</td></tr>
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<p>If an error occurs, Logo takes the following steps. First, if there is an
available variable named <code>ERRACT</code>, Logo takes its value as an
instructionlist and runs the instructions. The operation <code>ERROR</code> may be
used within the instructions (once) to examine the error condition. If the
instructionlist invokes <code>PAUSE</code>, the error message is printed before the
pause happens. Certain errors are <em>recoverable</em>; for one of those errors,
if the instructionlist outputs a value, that value is used in place of the
expression that caused the error. (If <code>ERRACT</code> invokes <code>PAUSE</code> and
the user then invokes <code>CONTINUE</code> with an input, that input becomes the
output from <code>PAUSE</code> and therefore the output from the <code>ERRACT</code>
instructionlist.)
</p>
<p>It is possible for an <code>ERRACT</code> instructionlist to produce an
inappropriate value or no value where one is needed. As a result, the same
error condition could recur forever because of this mechanism. To avoid that
danger, if the same error condition occurs twice in a row from an
<code>ERRACT</code> instructionlist without user interaction, the message
‘<samp>Erract loop</samp>’ is printed and control returns to toplevel. "Without user
interaction" means that if <code>ERRACT</code> invokes <code>PAUSE</code> and the user
provides an incorrect value, this loop prevention mechanism does not take
effect and the user gets to try again.
</p>
<p>During the running of the <code>ERRACT</code> instructionlist, <code>ERRACT</code> is
locally unbound, so an error in the <code>ERRACT</code> instructions themselves will
not cause a loop. In particular, an error during a pause will not cause a
pause-within-a-pause unless the user reassigns the value <code>[PAUSE]</code> to
<code>ERRACT</code> during the pause. But such an error will not return to
toplevel; it will remain within the original pause loop.
</p>
<p>If there is no available <code>ERRACT</code> value, Logo handles the error by
generating an internal <tt>THROW "ERROR</tt>. (A user program can also
generate an error condition deliberately by invoking <code>THROW</code>.) If this
throw is not caught by a <tt>CATCH "ERROR</tt> in the user program, it is
eventually caught either by the toplevel instruction loop or by a pause loop,
which prints the error message. An invocation of <tt>CATCH "ERROR</tt> in a
user program locally unbinds <code>ERRACT</code>, so the effect is that whichever of
<code>ERRACT</code> and <tt>CATCH "ERROR</tt> is more local will take precedence.
</p>
<p>If a floating point overflow occurs during an arithmetic operation, or a
two-input mathematical function (like <code>POWER</code>) is invoked with an illegal
combination of inputs, the ‘<samp>doesn't like</samp>’ message refers to the second
operand, but should be taken as meaning the combination.
</p>
<p>See section <a href="usermanual_11.html#SEC378">erract</a> ,
<a href="usermanual_8.html#SEC335">throw</a> ,
<a href="usermanual_8.html#SEC336">error</a> ,
<a href="usermanual_8.html#SEC334">catch</a> ,
<a href="usermanual_8.html#SEC337">pause</a> ,
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<h2 class="section"> 10.1 Error Codes </h2>
<p>Here are the numeric codes that appear as the first member of the list
output by <code>ERROR</code> when an error is caught, with the corresponding messages.
Some messages may have two different codes depending on whether or not
the error is recoverable (that is, a substitute value can be provided
through the <code>ERRACT</code> mechanism) in the specific context. Some messages are
warnings rather than errors; these will not be caught. Errors 0 and 32 are
so bad that Logo exits immediately.
</p>
<table><tr><td> </td><td><pre class="example"> 0 Fatal internal error <span class="roman">(can't be caught)</span>
1 Out of memory
2 Stack overflow
3 Turtle out of bounds
4 <var>proc</var> doesn't like <var>datum</var> as input <span class="roman">(not recoverable)</span>
5 <var>proc</var> didn't output to <var>proc</var>
6 Not enough inputs to <var>proc</var>
7 <code>proc</code> doesn't like <code>datum</code> as input <span class="roman">(recoverable)</span>
8 Too much inside ()'s
9 You don't say what to do with <var>datum</var>
10 ')' not found
11 <var>var</var> has no value
12 Unexpected ')'
13 I don't know how to <var>proc</var> <span class="roman">(recoverable)</span>
14 Can't find catch tag for <var>throwtag</var>
15 <var>proc</var> is already defined
16 Stopped
17 Already dribbling
18 File system error
19 Assuming you mean IFELSE, not IF <span class="roman">(warning only)</span>
20 <var>var</var> shadowed by local in procedure call <span class="roman">(warning only)</span>
21 Throw "Error
22 <var>proc</var> is a primitive
23 Can't use TO inside a procedure
24 I don't know how to <var>proc</var> <span class="roman">(not recoverable)</span>
25 IFTRUE/IFFALSE without TEST
26 Unexpected ']'
27 Unexpected '}'
28 Couldn't initialize graphics
29 Macro returned <var>value</var> instead of a list
30 You don't say what to do with <var>value</var>
31 Can only use STOP or OUTPUT inside a procedure
32 APPLY doesn't like <var>badthing</var> as input
33 END inside multi-line instruction
34 Really out of memory <span class="roman">(can't be caught)</span>
35 user-generated error message (THROW "ERROR <var>message</var>)
36 END inside multi-line instruction
37 Bad default expression for optional input: <var>expr</var>
38 Can't use OUTPUT or STOP inside RUNRESULT
39 Assuming you meant 'FD 100', not FD100 <span class="roman">(or similar)</span>
40 I can't open file <var>filename</var>
41 File <var>filename</var> already open
42 File <var>filename</var> not open
43 Runlist [<var>expr</var> <var>expr</var>] has more than one expression.
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