Unsafe Operations

An unsafe operation is one which may be applied to inappropriate arguments, thus raising an error. Unsafe operations are a natural starting point for static debugging. MrSpidey highlights these unsafe operations via font and color changes, as follows:

• Any primitive operation that may be applied to inappropriate arguments, thus raising a run-time error, is highlighted in red (or underlined on monochrome screens). Conversely, primitive operations that never raise errors are shown in green.
• Any function that may be applied to an incorrect number of arguments is highlighted by displaying the lambda keyword in red (again, underlined on monochrome screens).
• Any application expression where the function position may return a non-function is highlighted by displaying the enclosing parentheses in red (or underlined).

Figure 2.1 contains examples of these three different kinds of unsafe operations. The tab key moves the focus forward to the next unsafe operation, and the shift-tab key moves the focus backward to the previous unsafe operation. These keystrokes make it easy to inspect the unsafe operations in a program.

  
Figure 2.1: Identifying unsafe operations