BERKELEY LOGO: ERROR PROCESSING

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# 10. Error Processing

> 10.1 Error Codes |  |

If an error occurs, Logo takes the following steps. First, if there is an available variable named ERRACT, Logo takes its value as an instructionlist and runs the instructions. The operation ERROR may be used within the instructions (once) to examine the error condition. If the instructionlist invokes PAUSE, the error message is printed before the pause happens. Certain errors are "recoverable"; 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 ERRACT invokes PAUSE and the user then invokes CONTINUE with an input, that input becomes the output from PAUSE and therefore the output from the ERRACT instructionlist.)

It is possible for an ERRACT 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 ERRACT instructionlist without user interaction, the message "Erract loop" is printed and control returns to toplevel. "Without user interaction" means that if ERRACT invokes PAUSE and the user provides an incorrect value, this loop prevention mechanism does not take effect and the user gets to try again.

During the running of the ERRACT instructionlist, ERRACT is locally unbound, so an error in the ERRACT 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 [PAUSE] to ERRACT during the pause. But such an error will not return to toplevel; it will remain within the original pause loop.

If there is no available ERRACT value, Logo handles the error by generating an internal `THROW "ERROR`. (A user program can also generate an error condition deliberately by invoking THROW.) If this throw is not caught by a `CATCH "ERROR` 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 `CATCH "ERROR` in a user program locally unbinds ERRACT, so the effect is that whichever of ERRACT and `CATCH "ERROR` is more local will take precedence.

If a floating point overflow occurs during an arithmetic operation, or a two-input mathematical function (like POWER) is invoked with an illegal combination of inputs, the ``doesn't like'` message refers to the second operand, but should be taken as meaning the combination.

See section erract , throw , error , catch , pause , continue .

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## 10.1 Error Codes

Here are the numeric codes that appear as the first member of the list output by ERROR 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 ERRACT 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.

      0	Fatal internal error (can't be caught)
      1	Out of memory
      2	Stack overflow
      3	Turtle out of bounds
      4	PROC doesn't like DATUM as input (not recoverable)
      5	PROC didn't output to PROC
      6	Not enough inputs to PROC
      7	PROC doesn't like DATUM as input (recoverable)
      8	Too much inside ()'s
      9 	You don't say what to do with DATUM
     10	')' not found
     11	VAR has no value
     12	Unexpected ')'
     13	I don't know how to PROC (recoverable)
     14	Can't find catch tag for THROWTAG
     15	PROC is already defined
     16	Stopped
     17	Already dribbling
     18	File system error
     19	Assuming you mean IFELSE, not IF (warning only)
     20	VAR shadowed by local in procedure call (warning only)
     21	Throw "Error
     22	PROC is a primitive
     23	Can't use TO inside a procedure
     24	I don't know how to PROC (not recoverable)
     25	IFTRUE/IFFALSE without TEST
     26	Unexpected ']'
     27	Unexpected '}'
     28	Couldn't initialize graphics
     29	Macro returned VALUE instead of a list
     30	You don't say what to do with VALUE
     31	Can only use STOP or OUTPUT inside a procedure
     32	APPLY doesn't like BADTHING as input
     33	END inside multi-line instruction
     34	Really out of memory (can't be caught)

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This document was generated by Brian Harvey on October, 2 2002 using texi2html