Trace

Usage

• Trace[expr] generates a list of all expressions used in the evaluation of expr.
• Trace[expr, form] includes only those expressions which match form.
• Trace[expr, s] includes all evaluations which use transformation rules associated with the symbol s.

Notes

• In general, form in Trace[expr, form] is compared both with each complete expression that is evaluated, and with the tag associated with any transformation rule used in the evaluation.

• Trace[expr, lhs -> rhs] picks out expressions which match lhs, then replaces them with rhs in the list returned.

• All expressions in the list returned by Trace are wrapped in HoldForm.

• Trace returns a set of nested lists. Each individual list corresponds to a single evaluation chain, which contains the sequence of forms found for a particular expression. The list has sublists which give the histories of subsidiary evaluations.

• Example: Trace[2 3 + 4]

• The following options can be given:

MatchLocalNames	True	whether to allow x to stand for x$nnn
TraceAbove	False	whether to show evaluation chains which contain the chain containing form
TraceBackward	False	whether to show expressions preceding form in the evaluation chain
TraceDepth	Infinity	how many levels of nested evaluations to include
TraceForward	False	whether to show expressions following form in the evaluation chain
TraceOff	None	forms within which to switch off tracing
TraceOn	_	forms within which to switch on tracing
TraceOriginal	False	whether to look at expressions before their heads and arguments are evaluated

• During the execution of Trace, the settings for the form argument, and for the options TraceOn and TraceOff, can be modified by resetting the values of the global variables $TracePattern, $TraceOn and $TraceOff, respectively.

• See Section 2.6.4 and Section 2.6.11.

• See also: TraceDialog, TracePrint, TraceScan, EvaluationMonitor.

• New in Version 2.

Further Examples