Debugging

Debugging commands are essential to find programming errors. They help with inspecting programs and data, to pause a program, to find out why a program takes too much time, and much more. Logo can pause because of several reasons:

The PAUSE command has been executed
A runtime error has occured
The PAUSE button has been clicked
A STEPped element has triggered a pause

BACKTRACE

Also: BT

Prints the list of nested procedure calls; no inputs.

Syntax

BACKTRACE

Description

BACKTRACE prints the list of currently executing procedures.

BPCLEAR

Deletes all breakpoints; no inputs.

Syntax

BPCLEAR

Description

BPCLEAR deletes all breakpoints in all procedures.

CONTINUE

Also: CO

Ends a pause; no inputs.

Syntax

CONTINUE

Description

CONTINUE ends a pause. A Logo program can pause because it is stepped (see STEP), the PAUSE command has been executed, or if Logo has hit a runtime error (if the PREFS property PREFS/DEBUGGER is set to TRUE).

If this command is used when the debugger is not active, it does nothing.

PAUSE

Pauses a procedure; no inputs.

Syntax

PAUSE

Description

PAUSE temporarily halts the execution of a procedure. PAUSE makes it possible to check variables or change the environment during the execution of a procedure. To resume execution of the procedure, press the Continue button or type CO or CONTINUE. To return to toplevel, press the STOP button or type TOPLEVEL.

If this command is used outside of a procedure, it does nothing.

It is more convenient to set a breakpoint in the editor by clicking the line number of the line where you want Logo to pause.

Examples

TO SQUAREDRAW FORWARD 60 RT 90 PAUSE REPEAT 3 [FD 60 RT 90] END Result: SQUAREDRAW defined SQUAREDRAW Result: Program paused

STEP

Turns on stepping for the given elements; one input.

Syntax

STEP procedurename
STEP [procedurename1 ...]
STEP contents-list

Description

STEP turns the stepping on for the given elements. If the input is a list or a word, stepping is turned on for the given procedures. If the input is a structured contents list, stepping is turned on for the given procedures, names and properties.

When a procedure is being stepped, Logo pauses at the beginning of that procedure and enters debug mode. This is the same as starting the procedure definition with a PAUSE command.

When a name is being stepped, Logo pauses whenever the value of the name is changed, or the name is erased, and enters debug mode. Note that the name must be a global name; local names cannot be stepped.

When a property list is being stepped, Logo pauses whenever any value of the property is changed, or a property is erased, and enters debug mode.

STEPPED

Outputs a structured contents list of all stepped elements; no inputs.

Syntax

STEPPED

Description

STEPPED outputs a structured contents list of all stepped, non-empty elements. For an explanation of stepping see STEP.

STEPPED?

Also: STEPPEDP

Outputs TRUE if the element described by its input is stepped; one input.

Syntax

STEPPED? name-or-list

Description

STEPPED? outputs TRUE if the element described by its input is stepped. If the input is a list or a structured contents list, STEPPED? outputs the stepped state of the first non-empty element of the list.

TRACE

Turns on tracing of the given elements; one input.

Syntax

TRACE procedurename
TRACE [procedurename1 ...]
TRACE contents-list

Description

TRACE turns the tracing on for the given elements. If the input is a list or a word, tracing is turned on for the given procedures. If the input is a structured contents list, tracing is turned on for the given procedures, names, and properties.

When a procedure is being traced, Logo prints a message to the Listener panel every time the procedure is entered or left. When entering the procedure, the call is printed along with the procedure arguments. When the procedure leaves, the message contains the value that the procedure outputs, if any.

When a name is being traced, Logo prints a message to the Listener panel whenever the value for the name is changed along with the new value, or when the value is erased. Also, Logo prints a message if a local variable hides a global variable (or a local variable in a calling procedure) that has the same name.

When a property list is being stepped, Logo prints a message to the Listener panel whenever any value of the property is changed, or a property is erased, and enters debug mode.

Note that the global variable :TRACE takes precedence over the settings of this command. Also, TRACE does not display each line of a procedure as it is being executed, but :TRACE does.

TRACED

Outputs a structured contents list of all traced elements; no inputs.

Syntax

TRACED

Description

TRACED outputs a structured contents list of all traced, non-empty elements. For an explanation of tracing see TRACE.

TRACED?

Also: TRACEDP

Outputs TRUE if the element described by its input is traced; one input.

Syntax

TRACED? name-or-list

Description

TRACED? outputs TRUE if the element described by its input is traced. If the input is a list or a structured contents list, TRACED? outputs the traced state of the first non-empty element of the list.

UNSTEP

Turns off stepping of the given elements; one input.

Syntax

UNSTEP procedurename
UNSTEP [procedurename1 ...]
UNSTEP contents-list

Description

UNSTEP turns the stepping off for the given elements. If the input is a list or a word, stepping is turned off for the given procedures. If the input is a structured contents list, stepping is turned off for the given procedures, names and properties. For an explanation of stepping, see STEP.

UNTRACE

Turns off tracing of the given elements; one input.

Syntax

UNTRACE procedurename
UNTRACE [procedurename1 ...]
UNTRACE contents-list

Description

UNTRACE turns the tracing off for the given elements. If the input is a list or a word, tracing is turned off for the given procedures. If the input is a structured contents list, tracing is turned off for the given procedures, names, and properties. For an explanation of tracing, see TRACE.