Stepping

  Stepping allows the user to observe the evaluation of an expression one reduction step  at a time. When Donkey is in step mode, the current state of the evaluation process is displayed in the evaluation area.

• Step
  The Step  button allows evaluation of a Scheme expression one step at a time. Hitting Step the first time places a copy of the expression in the current block into the evaluation area. Each subsequent Step applies one reduction rule  to the expression. One may select specific rules to stop at with the Breakpoints menu (Section 4.4). The subexpression that is about to be reduced is displayed in a bold, red font.

  While stepping, the user can select a different block, perform evaluations, and later return to the original block to finish the stepping process. Note that an arbitrary number of blocks may be independently stepped through at the same time.

• Unstep
  Unstep  may only be used if the evaluation area  contains a result obtained from Step or Step All. Unstep backs up to the state previously seen. If at any point you desire to move forward again, Step will work as normal. The number of states you can move back can be set by the History  preference (Section 4.3). If you unstep through the whole history, the next Unstep will return you to the state at which you were before first unstepping.

  While you are stepping and unstepping through history, you are only seeing snapshots of what you have seen before. Consequently, you cannot perform advanced stepping techniques on these expressions. The exception is that you can use the Breakpoints menu (Section 4.4). This will resume normal stepping at the current point, and you can use the advanced stepping features.

• Step All
  Pushing the Step All  button when Donkey is not in step mode is very similar to pushing Eval. The difference is that it maintains history and therefore you can unstep when it has completed. Maintaining this history makes Step All slower than Eval. One common technique is to use Eval  first; if Eval results in a run time error, you should then use Step All to make use of error recovery and Unstep.

  If pressed, during stepping, Step All steps completely through the selected subexpression and replaces it by its value. This allows you to quickly step through subexpressions in which you are not interested.

  One may only use Step All for subexpressions that are ready for evaluation  . That is, they contain the next subexpression to be reduced. This restricts the user from evaluating a subexpression that may not be evaluated in the normal course of stepping.

  For example, in the expression:

      (if (= 2 (+ 1 1))
          (+ 2 2)
          (+ 3 3))

  The next subexpression to be reduced is (+ 1 1) so (= 2 (+ 1 1)), (+ 1 1), or the entire expression can be evaluated but (+ 2 2) and (+ 3 3) cannot since which of these will be evaluated depends on the value of the (unevaluated) condition.

• Break 
  This button only appears while Donkey is processing an Eval, Step, or Step All, and pressing it stops the current evaluation. While the Break button is visible, the rest of Donkey is disabled to any mouse or keyboard events.