Errors[This slide adapted from one by John Werth.]

Several kinds of errors are possible:

• Lexical: x := y ~ z
  The character ~ is not allowed in Pascal.
• Syntactic: x := y z
  There is no operator between y and z.
• Semantic: x := y mod 3.14
  The operator mod requires integer arguments.

The seriousness of errors can vary:

• Diagnostic: not necessarily an error, but might be:
  x == 3.14 may not be a meaningful comparison.
• Error: definitely an error; code generation will be aborted, but compilation may continue.
• Fatal error: so bad that the compiler must stop immediately.

Cascading errors occur when one real error causes many reported errors, e.g. forgetting to declare a variable can cause an error at each use.